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| ŠUMARSKI LIST 1-2/1966 str. 107 <-- 107 --> PDF |
THE OCCURRENCE AND MEANING OF HETEROSIS IN HYBRIDS WITHIN AND BETWEEN SPECIES« (Introductory paper) by MIRKO VIĐAKOVIC Forestry Faculty, Zagreb and Institute for Conifers, Jastrebarsko, Yugoslavia. INTRODUCTION Much has been written about the concept of heterosis, and there are various opinions about it, but we believe that in a general way everybody agree if we say that by heterosis we mean the phenomenon occurring when the hybrid is more productive than either of its parents. The location of the test is important to the evaluation of this effect. Duffiel d and Snyde r (15) discussed this matter and stated that it is possible to obtain different results, depending on whether the hybrid is tested in the area of occurrence of one or the other parent or in an intermediate area. As an example, the authors mentioned the hybrid Pinus monticola x (P. monticola x P. strobus), A similar situation is encountered in the hybrid Pinus contorta var. latifolia x P. banksiana (42,45). Wrigh t reported in his exhaustive work on hybridization between and within species that it is well known that the phenomenon of heterosis may be evident in one place and not in another (61). Paule y (40), on the other hand, assumed that often what is understood by the phenomenon of heterosis in Fi in the poplar hybrids is in fact a favourable action of the daylength and not the effect of heterosis. Such interpretation was given by Rohme d e r and Schönbach (48), as well as by other authors. From this it follows — as stated by Duffield and Snyder — that when determining heterosis it is necessary to indicate the origin of the parents and the locality where the hybrids where tested. Besides, in forest tree species it may happen that one hybrid does not show vigor in its early development, while in the later stages of development it exhibits a vigorous growth. This is the case in the hybrid between Pinus ponderosa and P. engelmannii (13). From the standpoint of forestry the phenomenon of hybrid vigour is significant even if it does not fulfill the strict sense of the heterosis concept, if the hybrid shows a combination of a satisfactory growth rate and other qualities such as resistance to diseases or extreme climatic conditions, or at least grows better on a certain site than either parent. * The author wishes to express his thanks to Professor Howard B. Kriebel who has read and corrected the manuscript. 105 |
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HYBRIDS BETWEEN SPECIES PRODUCED SPONTANEOUSLY IN NATURE A fairly large number of interspecific hybrids have been described wich belong to a common genus, and which originated spontaneously. Of these hybrids the greatest number belong to the genera Picea, Pinus, Larix, Abies, Populus, Quercus, Aesculus and Acer. Many spontaneous hybrids came into being in arboreta, parks and tree alleys. If in these places there is only one tree of a species, it crosses with the other species. Spontaneous hybrids can also originate in the natural range of two species, in places where these two species are in contact. In this country Fukare k and his co-workers (17, 18) found spontaneous hybrids between Acer obtusatum and A. pseudoplatanus and between A. Heldreichii and A. pseudoplatanus. In hybridization between species taking place in nature there can occur either introgression or hybrid swarms. Introgression is found in a number of interspecific hybrids of the genus Picea, or in Populus nigra in cases when P. deltoides was introduced. Hybrid swarms, as stated by Wright , are an evident example in the American species of the genus Aesculus. Hybridization between species sometimes leads to only a small number of isolated intermediate types or to introgression or hybrid swarms. This may be the result of limited crossability, differences in flowering time, or nature and amount of selection pressure. Hybrids originating spontaneously have promted tree breeders to attempt to produce them artificially. Observations of growth rate and habit of such spontaneous hybrids can greatly help the tree breeder in planning and carrying out hybridization. An example is Populus nigra in Europe. It appears that Salix alba presents a similar case. Spontaneous hybrids between this species and Salix fragilis appear to have a better growth than the pure parental species. Thus improvement of willows in this sense is presently in progress. PRODUCTION OF HYBRIDS DISPLAYING HETEROSIS One of the most important tasks in interspecific hybridization is obtaining the Fi generation displaying heterosis. Therefore no financial means should be spared if one is to succeed in this. When planning such a project it is necessary to bear in mind all the previous knowledge about the species crossability, so that we may not experience total failure and disappointment. Therefore the work program should be developed gradually according to plan. According to Wrigh t (61) there are 4 stages of work leading to the final goal. First comes the determination of the species which are mutually crossable, then what relationship exists between the species to be crossed. It is fairly well established that success in crossing related species is usually comparatively good, while in distant ones it is usually negative. In the crossing of related species whose geographical distributions are separated, heterosis can be expected in the progeny. Valuable contributions concerning problems of relationship and interspecific hybridization in the genus Pinus were given by D u f f i e 1 d (12) and C r i t c h f i e 1 d (9). Also, the work cf S a y 1 o r (50) is important because it explains the relationships of individual pine species on the basis of karyotype analysis. On the basis of an analysis of spontaneous |
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hybrids between Abies concolor and A. grandis, Gath y (19) concluded that there is a relationship between these two species and A. loviana Wright (58) has dealt with these problems in spruce. For a successful crossing the structure and number of chromosomes are important. In a general way the rule is valid that species with a similar structure and equal number of chromosomes cross more easily than species that are dissimilar in these respects. However, we also know cases where the number of chromosomes is different, and yet some of them can cross with each other as in the genus Betula, in which there are species with 28, 56, 70 and 84 chromosomes. The second step is the production of a small number of hybrids, verification by repetition of the experiment and the testing of the hybrid. When producing a small number of hybrid plants it is not always necessary to pay attention to the particular trees which are chosen for parent pairs. If the because various genotypes and especially different races may give positive result is negative, one should attempt to take different trees for parents, results. The third working step is to ascertain the possibility for mass production of hybrids. Mass production of hybrids depends in the first place upon the character of the parental species. It is well known that the pines yield large quantities of seeds, but in interspecific hybridization this is usually not the case. At present the hybrid P. rigida x P. taeda is mass produced in Korea under the direction of H y u n (22). In the genus Larix, mass interspecific hybridization can also be carried out in seed plantations, as was recommended by S. C. L a r s e n and performed in Denmark. In many species of the genus Populus and in the genus Salix there are no difficulties, because they can be propagated readily by cuttings and rooted cuttings. In Denmark — according to L a r s e n´s scheme — the hybrid of Populus tremula x P. tremuloides is also mass produced. The fourth step is to determine what trees answer best for parental pairs. The results are often different if different trees arc used in the same interspecific crossing, because each species has its own range of variation. It has been established that various biotypes belonging to two species may give different results. Therefore such a crossing ought to be carried out with several parental pairs belonging to different biotypes and races in order to attain the desired purpose. The phenomenon of heterosis is well known in many interspecific hybrids. In this report we do not wish to give a detailed survey of individual hybrids but consider it necessary to comment in short on the more important genera. Considering that investigations of the various species of different genera, and even of the various species within one genus, are in different stages of progress, it is difficult to give with certainty a clear appraisal of the success achieved so far. Neiher do we intend to give a special evaluation of each combination, because these investigations are in progress and will be reported by the investigators themselves. Our only objective is to show by examples what has been achieved to date, and that from these achievements the meeting can draw definite conclusions. Data about many interspecific hybrids were taken from the following literature: 10, 14, 46 and 62. |
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HETEROSIS Genus Pinus: Is exhibited in the hybrid P. strobus x griffithii, which is also resistant to blister rust. The hybrid P. densiflora x P. thunbergii also has hybrid vigor; data are not available on its rate of growth in Japan. Other hybrids with this characteristic include P. nigra x P. resinosa, P. thunbergii x P. nigra, P. ponderosa x P. engelmannii, P. ponderosa x P. montezumae, P. echinata x P. rigida, P. contorta ssp. latifolia x P. banksiana. M o u 1 o- p o u 1 o s and Bassiotis (37) established that the hybrid betwen P. halepcnsis and P. brutia grows faster than either parent at the age of 9 year?. Genus Abies: In Bavaria two hybrids display intensive luxuriant growth (47). A cross Abies Veitchü x A. alba, 11 years old, is 205% taller than its female parent and 301% taller than its male parent. A hybrid A. concolor x A. Veitchii is taller than its female and male parent by 162% and 204% respectively. Rohmede r stresses that the intensive growth of the hybrid not only depends on the combination of suitable species but also on the individuals used in crossing. Besides, in hybrids of fir high resistance to cold is more important than growth rate. In the genus Picea the hybrids displaying heterosis are: P. glauca x P. engelmannii and the reciprocal, and the hybrid between the species P. glaucaand P. jezoensis. According to our investigations (56), the cross between Omorika spruce and Sitka spruce surpasses Omorika spruce in growth at the age of 5 years. According to Langne r (33), a 2-year-old hybrid between these two species exhibits a heterotic effect, while the older hybrids from a previous experiment do not show this character. Genus Larix: The best known is the hybrid between L. decidua and L. leptolepis, which was first established in Scotland. Many papers have been published dealing with the hybridization of these two species. Thus for instance in Denmark, L a r s e n (34, 35) carried out extensive investigations on hybridization between the mentioned two species. In Germany we may mention Dimpflmeier (11), Langner (3,0 32) and Go the (20). In Yugoslavia Vidakovi ć (53, 54) works on the hybridization of larch. In all cases it was shown that the hybrid larch is strongly heterotic, and in addition is resistant to the canker. The hybrid between the Siberian and Japanese Larch is interesting for the Scandinavian countries and the U.S.S.R. Albenski j (1, 2, 3) established the phenomenon of heterosis in this hybrid as well as in the hybrid between the European and Japanese Larch. According to Rohmede r (47), the hybrids L. decidua x L. gmelini, and L. decidua x L. gmelini var. principis Ruprechtii also exhibit the phenomenon of heterosis. In the genus Pseudotsuga, C h i n g (8) obtained by crossing P. taxifolia x P. macrocarpa hybrid plants which at the age of 2 years were larger than their parents. The genus Populus has the well known characteristics that may of its species are easy to cross and that the pure species as well as the hybrids are easily propagated from cuttings. Many natural crosses are well known in poplars. Thus, for instance, several very valuable clones such as 1-214 and 1-156 were selected in Italy from spontaneous crossess between P. deltoides and P. nigra. These clones are successfully grown in Italy and the neighbouring countries. We can be certain that these clones possess hybrid vigor |
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if we compare them with pure P. deltoides and P. nigra. In Europe, the cross betwoen P. tremula and P. tremuloides is also very promising. This hybrid displays a vigorous growth, especially in northern Europe. It is assumed that in this case difference in latitude of origin plays a greater role than the heterosis Phenomenon itself, by extension of the growing season in comparison with that of the mother´s parents (26, 40). A good growth rate is also exhibited by the crosses P. trickocarpa x P. nigra, and P. trichocarpa x P. deltoides. The genus Quercus is very difficult to study from the aspect of hybridization and therefore has not been extensively investigated. Piatnitsk y (41) established the existence of heterosis in the hybrid Q. cerris x Q. suber. In the genus Eucalyptus heterosis was evidenced in two cases of interspecies crossing (60). In the genus Betula, the heterotic effect appears to exist in the hybrid B. papyrifera x B. lenta. Johnsso n (25) states that the hybrids B. verrucosa x B. japonica, B. verrucosa x B. papyrifera, and B. pubescens x B. papyrifera exceed the average stem heights of the parental species at seven years of age. Besides these examples of interspacific hybrids exhibiting the phenomenon of heterosis, we consider it necessary to mention those hybrids which surpass in growth rate one of their parents or are resistant to diseases. It was established that the hybrid between Pinus nigra and P. densiflora grows at a slower rate than P. densiflora but much faster than P. nigra. We also carried out crossing of P. nigra x P. densiflora and found that two-year-old hybrid plants did not always grow faster than the Pinus nigra mother tree; the growth rate depends upon the individual tree taken as a mother tree. The hybrid between Pinus nigra and P. silvestris appears to grow faster than Austrian pine but more slowly than Scots Pine (52). The hybrids P. thunbergii x P. densiflora, P. thunbergil x P. taiwanensis, P. thunbergii x P. yunnanensis, P. thunbergii x P. massoniana, P. thunbergii x P. nigra austriaca, P. densiflora x P. thunbergii, P. densiflora x P. silvestris, P. nigra austriaca x P. thunbergii, P. nigra austriaca x P. tabulaeformis, and P. ponderosa x P. latifolia grow more rapidly than the first-named female parent The hybrid P. echinata x P. taeda grows faster in California than shortleai pine (P. echinata). The hybrid P. rigida x P. taeda outgrows one parent in some localities and in other localities the other parent. The hybrid between P. monticola and P. strobus is significant not only because it surpasses one of its parents in growth rate in several localities, but also because by the selection of parental partners for this combination it is possible to obtain a blister rust-resistant progeny (5, 6). With regard to resistance to blister rust, the hybrid between P. strobus and P. pence is also promising (16). A spontaneous hybrid between Picea abies and P. sitchensis shows at the age of 17 years 25% faster growth than Sitka spruce, and has been much less affected by cold than the latter (47). The hybrid between Fraxinus excelsior and F. americana likewise exhibits at 12 years of age a better growth rate than F. americana. In addition, this hybrid displays great survival capacity (47). A 1 b e n s k i j (2, 4) crossed Acer negundo with A. platanoides, and Acer negundo with A. saccharinum. In both instances he obtained progenies more vigorous than the female parent. 109 |
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In the interspecific hybridization in elms, Albenskij also obtained progenies exhibiting a better increment than the female parent. These crosses were Ulmus pinnatoramosa x U. laevis. OTHER POSSIBILITIES FOR OBTAINING HETEROSIS IN HYBRIDS a) Inbreeding The inbreeding method, well-known in the improvement of allogamous agricultural plants, has the objective of obtaining heterosis in the final crossing of two inbred lines. Since most forest tree species are allogamous, this method can also be used in forest trees. This is feasible because it is an established fact that it suffices to produce the Ii to IK generations. However, there is a difficulty in cases of allogamous forest tree species which are totally self-sterile. There is still insufficient information on this problem as indicated by the fact that there are conflicting reports in the literature on some species. Also, self-fertilization varies from tree to tree (43, 44). Total self-sterility has been estalished in Castanea mollissima, Liriodendron tulipifera, and in several species of the genus Eucalyptus. Almost complete self- sterility has been established in Alnus glutinosa, Larix decidua, Larix leptolepis, Picea sitchensis, Pinus strobus, Pinus silvestris, Pinus nigra, and others. Species such as Pinus monticola and Picea abies have a varying degree of sterility and inbreeding depression. The self-fertile species include Alnus incana, and also several species of the genus Eucalyptus. The inbreeding depression in these species varies from very little to almost 50%. It is also important to mention that the seed obtained by self-fertilization is of poor germinability, its progenies show a poor growth rate, and single individuals often display an abnormal habit. Previous improvement work carried out by this method has generally been in the first stage, i.e. the production of Ii and I» generations. There are several examples which show the general responses to inbreeding. L a n g 1 e t (29) showed that in Norway spruce inbreeding decreased the percentage of germination. Plants in the first year of growth already have a depression which in later years slightly exceeds 50%. This inbreeding depression was also ascertained in other species, such as Pinus silvestris, Pinus Mugo and Larix decidua. Jovanović and Tucović (28) obtained from a monoecious tree of Black poplar an Ii-generation which did not show inbreeding depression in the first two years of life. S y r a c h L a r s e n and his co-workers have already obtained the la-generation in European larch. A further stage of this procedure is the mutual crossing of inbred lines with the aim of obtaining heterotic progenies. If we carry out improvement by simultaneous inbreeding of two different races or provenances, the possibility of obtaining a degree of heterosis is in our opinion greater than if we perform this work within one race or provenance. L a n g n e r (31) also recommends the use of different provenances for improvement by inbreeding. Thus, for instance, N i 1 s s o n (38) obtained an intensive heterotic effect in crossing one Swedish provenance cf Norway spruce with one continental provenance. The author explains that the heterosis phenomenon occurred because naturally-existing inbreeding depression within each provenance, resulting in heterosis in the progenies. |
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b) Multispecies hybrids This kind of hybrids may also exhibit the phenomenon of heterosis. In Placerville the hybrids (P. peuce x P. strobus) x P. monticola were produced which at the age of 11 years surpass in height the indigenous P. monticola. The same Institute also produced the hybrid (P. ponderosa x P. apacheca) x P montezumae, which grows even faster than the hybrid P. ponderosa x P. montezumae. We have obtained the trispecies hybrid P. silvestris x (P. densiflora x P. nigra) displaying a better growth rate than the Austrian pine A natural multispecies hybrid is P. nigra austriaca x (P. thunbergii x densi flora), which grows faster than its male parent. In the genus Castanea the trispecies hybrid C. mollissima x (C. crenata x C. dentata) is known which according to Wright , is very promising because of apparent resistance to Chestnut canker, from which the American chestnut suffers. As can be seen from the exmaples, this working method can also be very useful. Therefore when planning a long-term interspecific hybridization programme it is necessary to take these possibilities into consideration, particularly since we already prossess a large number of hybrids produced by the crossing of two different species. c) Application of irradiation We consider it necessary to mention still another possible method of producing heterotic plants. Geneticists engaged in the study of Drosophila have established that heterosis is often associated with deletions, inversions and translocations. In such a case one chromosome of the pair is completely normal, while the other originating from the other parent differs structurally from the first. For this reason, Osborn e (39) proposed that only one parent should be irradiated before crossing in order to be able in this way to induce heterosis. He believes that this method can be used with precision in long- lived forest trees. By irradiating the pollen and pollinating the nofiirradiated (normal) female gametophyte we should obtain heterozygous, heterotic plants. We should carry out selection among these plants in the course of cultivation. On the basis of the previously mentioned assumption, Rudolp h (4ß) made an experiment with Picea glauca. He tentatively concluded that irradiation of the pollen had a stimulating effect on seed set and viability. Pollen irradiated with 600 r also gave heavier seeds. The investigatios which we are conducting in this direction with Austrian and Scots pine show that plants of greater height and stem diameter can be obtained if the pollen is irradiated with gamma rays before pollination. The fallowing data are presented for the purpose of illustration: 3-year-old plants of Scots pine obtained through controlled pollination with non-irradiated pollen had an average height of 21.11 cm. and a stem thickness of 8.22 mm.; plants originating from pollen irradiated with 400 and 500 r had average heights of 23.8 and 26.78 cm., while their stems were 9.38 and 11.14 mm. in diameter respectively. HYBRIDIZATION BETWEEN RACES In his paper, in the title itself of the chapter on the interracial hybridization, Wrigh t (61) states rightly that this subject has been neglected. So far, there has been a very limited amount of work conducted on the crossing 111 |
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of races within one species, although in many cases good results can be expected from such work. It is well known that in agricultural plants it is possible by crossing various races to obtain heterotic hybrids. The same can also be achieved in forest tree species. Naturally, a necessary prerequisite to this is the determination of the races, varieties and subspecies occurring in a species. The more clearly separate the species and taxonomic units, the greater is the probability of obtaining by crossing them a progeny which v/ill have a good growth rate and even exhibit hybrid vigour. Thus, Johnsso n (27) in Sweden, by crossing aspen of a southern provenance with that of a northern provenance, obtained progenies which at the age of ten years were more vigorous than either parent. This possibility is also confirmed by the results of van V 1 o t e n (57) and Schönbach (51) after crossing various provenances of aspen. An instrucitve example of the value of the crossing of lower taxonomic- units within a species exists also in this country (Istria). In a plantation of Pinus nigra we observed that great differences exist in the morphological characteristics of individual plants. Some of their characteristics can be found in Corsican pine, others in Austrian pine, while a number of them are intermediate. There are no written data on the history of this culture but it is most probable that the seeds from which these plants were raised were collected from a mixed plantation of Austrian and Corsican pines. From visual observation it is apparent that the intermediate plants — which we assume to be hybrids of Austrian and Corsican pines — exhibit greater stem heights and diameters b.h. than Austrian pine. L a n g n e r (31) explained the heterosis in interracial crossing of Norway spruce. The race from the north grows in its native environment under long- day conditions. With respect to the daylength required for its growth, it is homozygous and possesses a genetic constitution of the LLkk-type. The southern race grows under shorter-day conditions, and in this character its genetic constitution is of the HKK-type. If these two races are crossed, they give rise to progenies of the highest possible LIKk-heterozygosity. Such an explanation is acceptable, although we krow that growth is conditioned by several factors, and thus its interpretation is much more complex. We are of the opinion that for the time being it is most important to pay attention to the heterosis phenomenon, and as far as possible to guide hybridization in this direction. Interracial hybridization can have its value also if we do not obtain a heterotic progeny. By crossing two races and the combination of their characters it is possible to obtain progenies possessing the desired characteristics. Norway spruce in Sweden is such a case; the cross between the Swedish and German races grows better than the native race, which is just as cold-resistant as the indigenous Swedish race. DIFFICULTIES IN INTERSPECIES HYBRIDIZATION AND METHODS OF OVERCOMING THEM We often meet with difficulties in hybridization between species. A great number of combinations in crossing give completely negative results. We consider it necessary to say something about this problem. One should search for the causes of negative results in the relationship of the species crossed, |
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the morphology of the flowers, the time of flowering, and the incompatibility* of species. It is understandable that species that are genetically distant from each other usually cannot be crossed. In such a case the difficulties are very great and much effort will be required to surmount them, if they can be surmounted at all. If there is an essetial difference in the structure of flowers, the crossing generally cannot be performed with success. Likewise, if there is a difference in the flowering time between two species, there is difficulty in hybridization, but the overcoming of these difficulties is much easier. In such a case it is nacessary to speed up or retard the time of flowering of male or female flowers so that the pollination can be carried out in due time. The most suitable method consists of storing the pollen and using it when the female flowers are ready for pollination. The difficulties are also great, in incompatible species but we are of the opinion that in some cases they can be surmounted. In order to determine the phase of development after pollination is the pollen, egg cell, zygote or proembryo, it is necessary to carry out embryological investigations as presented in the works of Buc h hol z (7), McWilliam (36), Ha gm an and M i k kola (21), and Hyun and Yim (23). Such investigations should indicate to us how far the pollen tube has penetrated, wheter fertilization has occured, and what happened to the zygote after fertilization. When we get a clear idea about these processes in the crossing of two incompatible species, we shall perhaps be able to overcome more easily the difficulties due to resistence to crossing. In addition, in the crossing of incompatible species it is necessary to work out suitable methods of hybridization. We consider the method of preliminary vegetative approach and the bringing of the stigma fragments of father tree on to the stigma of mother tree pistils interesting, as recommended by J a b 1 o k o v (24). According to our incomplete investigations (55) of the hybridization of incompatible species, pollination of female flowers of one species with the irradiated polen of another species provides a possible new way of solving this problem. In this connexion it is interesting to point out — although the results are incomplete — that after pollination of Austrian pine with Scots pine pollen irradiated with 800 and 1000 r the ovule shows a degeneration in fewer cases than when the pollen of Scots pine was not irradiated. At the end of this report I will take the liberty to make a suggestion. Previous work on hybridization within and especially between species gave conspicuous results. Many heterotic hybrids were obtained. Today we possess much information on different cross combinations. Our experience and theoretical knowledge about the possibilities of crossing two species are constantly improving, and regardless of the fact that many problems still lie ahead, mass production of hybrids deserves full consideration, as Righte r * By incompatibility we mean the inability of pollen to fertilize the egg cell, and if it is fertilized, the inability of the zygote to survive, or the subsequent degeneration of the proembryo. |
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(44) points out. No less important is the problem of hybridization between races with the aim of obtaining heterotic progenies. Therefore we believe that it would be useful to establish organized international collaboration on questions of interspecific and interracial hybridization. PRIDOLAZAK I VAŽNOST HETEROZISA KOD UNUTARVRSNIH I MEDU VRSNIH HIBRID A UVOD O pojmu heterozisa mnogo je pisano i o njemu postoje različita mišljenja, ali vjerujemo da će se uglavnom svi složiti ako kažemo da pod heterozisom podrazumijevamo u užem smislu pojavu kada je hibrid produktivniji od oba roditelja. Za ocjenu toga efekta važno je gdje se hibrid testira. Duffiel d i Snyde r (15) raspravljaju o tome te iznose da se mogu dobiti različiti rezultati prema tome da li se hibrid testira u području pridolaska obaju roditelja ili pak u intermedijarnom području. Kao primjer autori navode hibrid Pinus monticola x (P. monticola x P. strobus). Slična je situacija i s hibridom Pinus contorta var. latifolia x P. banksiana (42, 45). Wrigh t (61) u svom iscrpnom radu o hibridizaciji između vrsta i unutar vrsta izlaže da je vrlo dobro poznato, da pojava heterozisa može biti evidentna na jednom mjestu, a ne mora na drugom. Paule y (40) pak pretpostavlja da je često ono što se predmnijeva pod pojavom heterozisa u Fi generaciji kod hibrida topola u stvari povoljno djelovanje duljine dana, a ne efekt heterozisa. Takvo tumačenje daju i Rohmeđer i Schönbach (48) kao i drugi autori. Iz toga proizlazi — kao što navode Duffiel d i Snyde r — da se za određivanje pojave heterozisa mora naznačiti porijeklo roditelja i mjesto testiranja hibrida. Osim toga kod šumskog drveća može biti takav slučaj da jedan hibrid u početku svog razvoja ne pokazuje luksuriranje, a u kasnijim stadijima razvoja luksurira. Ovakav slučaj imamo na primjer kod hibrida između Pinus ponđerosa i P. engelmannii (13). Sa šumarskog stajališta pojava luksuriranja kod nekog hibrida ima svoje značenje premda u užem smislu ne odgovara pojmu heterozisa i to u slučaju ako hibrid ima zadovoljavajući rast te pokazuje ostale kvalitete kao npr. otpornost na bolesti, na ekstremne klimatske prilike ili bar bolje uspijeva na nekom staništu od oba roditelja (15). MEĐUVRSNI HIBRIDI NASTALI SPONTANO U PRIRODI Opisan je priličan broj međuvrsnih hibrida koji pripadaju istom rodu, a nastali su spontano. Od tih hibrida najveći broj pripada rodovima Picea, Pinus, Larix, Abies, Populus, Quercus, Aesculus i Acer. Mnogi spontani hibridi nastali su u arboretumima, parkovima i drvoredima. Ako na tim mjestima postoji samo jedno stablo jedne vrste, ono se križa s drugom vrstom. Spontani hibridi mogu nastati i u prirodnom području rasprostranjenja dviju vrsta i to na mjestima gdje se te dvije vrste dotiču. Kod nas su Fukare k i njegovi suradnici (17, 18) našli spontane hibride između Acer obtusatum i A. pseuđoplatanus te A. heldreichii i A. pseuđoplatanus. Kod hibridizacije između vrsta koja se dešava u prirodi može doći do introgresije ili hibridnih »rojeva« (hybrid swarms). Pojavu introgresije nailazimo kod nekih međuvrsnih hibrida roda Picea ili kod Populus nigra u Evropi u slučajevima kad je unesen P. đeltoides. Očiti primjer hibridnih »rojeva« imamo, kako donosi Wright , kod američkih vrsta roda Aesculus. Objašnjenje zašto hibridizacija između vrsta vodi koji puta samo malom broju izoliranih intermedijarnih tipova ili pak introgresiji odnosno hibridnom »roju« je prema Wright u u slijedećem: ponajprije jer mogućnost križanja može biti ograničena, drugo razlike u vremenu cvatnje te treće priroda i stupanj selektivnog pritiska. Hibridi koji su nastali spontano ponukali su oplemenjivače da ih pokušaju proizvesti i umjetnim putem. Pored toga takvi spontani hibridi svojim rastom i oblikom mogu vrlo mnogo pomoći oplemenjivaču pri planiranju i izvođenju radova na hibridizaciji. Takav primjer imamo s Populus nigra u Evropi. Čini nam se da je sli 114 |
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čan slučaj i sa Salix alba. Spontani hibridi između ova vrste i SalJx fragilis imaju, izgleda, bolji rast od čistih roditeljskih vrsta. Sada je u toku oplemenjivanje vrbe u tom pravcu. PRODUKCIJA HIBRIDA S POJAVOM HETEROZISA Kod međuvrsne hibridizacije jedan je od najvažnijih zadataka dobivanje Fi generacije s pojavom heterozisa. Zbog toga ne treba žaliti za uloženim sredstvimatrudom ako se to ne uspije odmah postići. S druge strane kod planiranja takog rada treba imati na umu sva dosadašnja znanja o mogućnostima križanja vrsta kako ne bismo doživjeli potpuni neuspjeh i razočaranje. Zbog toga izvođenje radova treba organizirati planski i postupno. Prema Wrigh t u (61) postoje do konačnog cilja četiri faze rada. Prvo je ustanovljivanje koje se vrste mogu međusobno križati te kakva srodstvena veza mora postojati između vrsta koje se križaju. Nije tačno utvrđeno, ali se može reći da je uspjeh kod srodnih vrsta relativno dobar, a kod udaljenih rezultat je obično negativan. Kod križanja srodnih vrsta kod kojih su areali odijeljeni heterozis se može očekivati u potomstvu. O problemima srodstva i međuvrsne hibridizacije kod roda Pinus vrijedni prilozi su Duffielda (12) i Critchfielda (9). U tom pogledu važan je i rad Saylor a (50) koji na osnovi kariotip — analize objašnjava srođstvenu vezu kod pojedinih vrsta borova. Ga th y (19) je na temelju analize spontanih hibrida između Abies concolor i A. granđis zaključio o srodstvenoi vezi tih dviju vrsta i A. lowiana. O tim problemima kod smrče pisao je Wright (58). Za uspjeh križanja važna je struktura i broj hromosoma. Općenito vrijedi pravilo da se vrste koje imaju sličnu strukturu i jednaki broj hromosoma lakše mogu s uspjehom križati. No, znamo i takve slučajeve kod kojih je broj hromosoma različit, kao što je npr. slučaj kod roda Betula gdje imamo kod vrsta 28, 56, 70 i 84 hromosoma, a ipak se neke od njih mogu međusobno križati. Drugi korak je proizvodnja malog broja hibrida te potvrđivanje putem ponavljanja pokusa i samo testiranje hibrida. Kod proizvodnje maloga broja hibridnih biljaka nije potrebno da se uvijek pazi koja stabla odabiremo za roditeljske parove. U slučaju kada je rezultat negativan, treba pokušati da se uzmu za roditelje različita stabla budući da različiti genotipovi, a pogotovo različite rase mogu dati pozitivne rezultate. Treća faza rada je utvrđivanje mogućnosti masovne proizvodnje hibrida. Masovna proizvodnja hibrida na prvom mjestu ovisi o karakteru roditeljskih vrsta. Poznato je da vrste borova daju veliku količinu sjemena, ali kod međuvrsne hibridizacije obično nije tako. Možemo reći da se zasada masovno proizvodi hibrid P. rigiđa x P. taeđa u Koreji pod rukovodstvom Hyu n (22). Kod roda Larix masovna međuvrsna hibriđizacija može se provesti i u sjemenskim plantažama kako je preporučio S. C. Larse n i kako1 se to sprovodi u Danskoj. Kod mnogih vrsta roda Populus, a i kod roda Salix ne postoje poteškoće budući da se lako razmnažaju iz reznica ili korjenjacima. U Danskoj se prema ideji S. C. L arsen a masovno proizvodi i hibrid Populus tremula x P. tremuloiđes. Četvrti korak bio bi utvrđivanje koja bi stabla najbolje odgovarala za roditeljske parove. Činjenica je da su rezultati često različiti ako se kod istoga međuvrsnog križanja koriste različita stabla. To je zbog toga jer svaka vrsta ima svoju amplitudu variranja. Ustanovljeno je da različni biotipovi koji pripadaju dvjema vrstama mogu dati različite rezultate. Zbog toga se takvo križanje mora provoditi s više roditeljskih parova koji pripadaju različitim biotipovima i rasama, kako bi se došlo do željenog cilja. Pojava heterozisa poznata je kod mnogih međuvrsnih hibrida. U ovom referatu ne želimo donijeti detaljan prikaz o pojedinim hibridima, ah smatramo da je potrebno ukratko prikazati važnije rodove. Budući da su istraživanja kod različitih rodova, pa i unutar jednog roda kod različitih vrsta u različitim fazama razvoja, teško se može sa sigurnošću dati jasna ocjena o dosadašnjem uspjehu. No, mi nismo namjeravali da za svaku kombinaciju dajemo posebne ocjene budući da su ta istraživanja u toku, a iznijet će ih sami istraživači. Nama je samo cilj da na primjerima prikažemo što je dosada postignuto i da iz toga ovaj skup izvede određene zaključke. Podatke o mnogim međuvrsnim hibridima uzeli smo iz slijedeće raspoložive literature: 10, 14, 46, 59, 60 i 62. |
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HETBROZIS Rod Pinus: luksuriranje pokazuju hibridi P. strobus x P. griffithii koji je pored toga i otporan na rđu, zatim P. đensiflora x P. thunbergii za koji nema podataka o brzini rasta u Japanu, te P. nigra x P. resinosa, P. thunbergii x P. nigra, P. ponderosa x P. engelmannii, P, ponderosa x P. montesumae, P. echinata x P. rigiđa, P. contorta ssp. latifolia x P. banksiana. Moulopoulcs i Bassiotis (37) ustanovili su da hibrid između P. halepensis i P. brutia kod starosti od 9 godina raste brže od oba roditelja. Rod Abies: U Bavarskoj (47) pokazuju dva hibrida jele snažni efekt luksuriranja. Križanac A. Veitchii x A. alba, star 11 godina, za 205l%> je veći od majčinskog roditelja, a za 30r%> od očinskog roditelja. Hibrid A. concolor x A. Veitchii je za 162°´o veći od majčinskoga, a za 204l8/o od očinskog roditelja. Rohmede r naglašava da snažan rast hibrida ne ovisi samo o kombinaciji prikladnih vrsta, nego također o individuima koji se koriste za križanje. Kod hibrida jele je osim toga od brzine rasta važnija osobina visoka otpornost na hladnoću. Kod roda Picea hibridi s pojavom heterozisa su: Picea glauca x P. engelmannii i obratno, te hibrid između vrsta P. glauca i P. jezoensis. Prema našim istraživanjima (56) križanac između omorike i sitkanske smrče nadmašuje omoriku u rastu kod starosti od 5 godina. Prema Langner u (33) dvogodišnji hibrid između te dvije vrste ima pojavu heterozisa dok svi stariji hibridi iz ranijeg pokusa ne pokazuju to svojstvo. Rod Larix: najpoznatiji je hibrid između L. decidua i L. leptolepis koji je najprije ustanovljen u Škotskoj. U vezi s hibridizacijom tih dviju vrsta publicirano je dosta radova. Tako je npr. u Danskoj Larse n (34, 35) vršio opsežna istraživanja na hibridizaciji između dvije navedene vrste. U Njemačkoj možemo spomenuti Dimpflmeiera (11), Langnera (30, 32), i Go the (20). U Jugoslaviji je V idakovi ć (53, 54) radio na međuvrsnoj hibridizaciji ariša. U svim slučajevima pokazalo se da hibridni ariš ima jaku pojavu heterozisa, a pored toga otporan je i na rak. Za Skandinavske zemlje i SSSR interesantan je i hibrid između sibirskoga i japanskog ariša. Albenski j (1, 2, 3) je utvrdio pojavu heterozisa kod toga hibrida kao i kod hibrida između evropskoga i japanskog ariša. Prema Rohmeder u (47) hibridi L. decidua x L. gmelini i L. decidua x L. gmelini var. Principis Ruprechtii imaju isto tako pojavu luksuriranja. Kod roda Pseudotsuga dobio je Chin g (8) križanjem P. taxifolia x P. macrocarpa hibridne biljke koje su kod starosti od dvije godine u prosjeku bile veće nego roditelji. i | . j jp.aj Rod Populus poznat je po tome jer se mnoge vrste međusobno lako križaju, a što se tiče čistih vrsta kao i hibrida, lako se mogu razmnažati putem reznica ili korjenjaka. Kod topola poznati su mnogi prirodni križanci. Tako je u Italiji od spontanih križanaca između Populus deltoides i P. nigra selekcionirano nekoliko vrlo vrijednih klonova kao npr. I—214 i 1-154 koji se uspješno uzgajaju u Italiji i susjednim zemljama. Sigurno je da ti klonovi imaju efekt luksuriranja ako ih usporedimo s čistim vrstama P. deltoides i P. nigra. U Evropi je vrlo perspektivan i križanac između P. tremula te P. tremuloides. Taj hibrid pokazuje naročito u sjevernoj Evropi bujan rast. Pretpostavlja se da je ovdje bila hitnija razlika u geografskoj širini roditelja koja se odražava u mogućnosti produžetka vegetacije hibrida u usporedbi s majčinskim roditeljem, nego sama pojava heterozisa (26, 40). Dobar rast imaju i križanci P. trichocarpa x P. nigra i P. trichocarpa x P. deltoides. Rod Quercus je s aspekta hibridizacije veoma težak i zbog toga se na njemu radilo relativno malo. Piatnitsk y (41) je ustanovio da hibrid Q. cerris x Q. suber ima pojavu luksuriranja. Kod roda Eucalyptus postoji pojava luksuriranja u dva slučaja međuvrsnog križanja (60). Kod roda Betula izgleda da heterozis postoji kod hibrida B. papyrifera x B lenta. Johnsso n (25) navodi da hibridi B. verrucosa x B. japonic;», B. verrucosa x B. papyrifera i B. pubescens x B. papyrifera nadmašuju prosječne visine roditeljskih vrsta kod starosti od 7 godina. Pored navedenih primjera međuvrsnih hibrida s pojavom heterozisa smatramo da je ovdje potrebno spomenuti i takve hibride koji u rastu nadmašuju jednog roditelja ili su pak otporni na bolesti. ,Za hibrid između Pinus nigra i P. đensiflora 116 |
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utvrđeno je da raste sporije nego P. đensiflora, ali mnogo brže od P. nigra. Mi smo također proveli križanje P. nigra x P. đensiflora i ustanovili da hibridne biljke, stare dvije godine ne rastu uvijek brže od majčinskog roditelja Pinus nigra. To je ovisno o pojedinom stablu koje se uzme za majku. Hibrid između Pinus nigra i P. silvestris izgleda da raste brže od crnog bora, a sporije od običnog bora (52). Hibridi: P. thunbergii x P. đensiflora, P. thunbergii x P. taiwanensis, P. thunbergii x P. Yunnanensis, P. thunbergii x P. massoniana, P. thunbergii x P. nigra austriaca, P. đensiflora x P. thunbergii P. đensiflora x P. silvestris, P. nigra austriaca x P. tabulaeformis, i P. ponderosa x P. latifolia rastu brže od ženskog roditelja koji je prvi naznačen. Hibrid P. echinata x P. taeđa raste u kalifornijl brže od P.echinata. Hibrid P. rigida x P. taeđa na nekim lokalitetima nadmašuje u rastu jednog roditelja, a na drugom drugog roditelja. Hibrid između P. monticola. i P. strobus je značajan ne samo po tome što bolje prirašćuje na nekim lokalitetima od jednog roditelja nego da se odabiranjem roditeljskih partnera za tu kombinaciju križanja može dobiti potomstvo otporno na rđu (5, 6). Otpornost na rđu obećava i hibrid između P. strobus i P. peuce (16). Spontani hibrid između Picea abies i P. sitchensis pokazuje kod starosti od 17 godina brži rast za 25"/o od sitkanske smrče. Pored toga sitka je mnogo više stradala od hladnoće nego hibrid (47). Isto tako hibrid između Fraxinus excelsior i F. americana kod starosti od 12 godina pokazuje bolji rast od F. americana. Osim toga taj hibrid pokazuje veliku vitalnost (47). Albenskij (2, 4) je križao Acer negundo s A. platanoiđes, te Acer negundo s A. sacharinum. U oba slučaja dobio je potomstvo koje je imalo bujniji rast od majčinskog roditelja. Isto tako spomenuti je autor kod međuvrsne hibridizacije brijestova dobio potomstvo koje je imalo bolji prirast od majčinskog roditelja. Te kombinacije križanja su: Ulmus pinnatoramosa x U. foliacea, U. pinnatoramosa x ti. laevis. OSTALE MOGUĆNOSTI ZA DOBIVANJE HETEROZISA KOD HIBRIDA: a) Križanje u srodstvu. Kod oplemenjivanja stranooplodnoga poljoprivrednog bilja vrlo je dobro poznata metoda križanja u srodstvu s ciljem da se kod konačnog križanja dvije »inbreed « linije dobije potomstvo s pojavom heterozisa. Budući da je većina vrsti šumskog drveća stranooplodna, ta se metoda može koristiti i kod šumskog drveća, pogotovo kada se zna da je dovoljno proizvesti li do Is generaciju. Međutim, postoji poteškoća za one stranooplodne vrste šumskog drveća koje ispoljavaju potpuni sterilitet. U tom pogledu nemamo još dovoljno rezultata, što se očituje i u tome da se podaci u literaturi za jednu te istu vrstu ne slažu. Uzrok tome je i taj što samofertilnost varira od stabla do stabla (43, 44). Potpuni sterilitet utvrđen je kod Castanea mollissima, Liriodendron tulipifera, i više vrsta roda Eucalyptus. Skoro potpuno sterilni su: Alnus glutinosa, Larix decidua, Larix leptolepis, Picea sitchensis, Pinus strobus, Pinus silvestris, Pinus nigra i dr. Vrste kao npr. Pinus monticola i Picea abies pokazuju različiti stupanj steriliteta i »inbreeding« depresije. Od samofertilnih vrsta spomenut ćemo Alnus incana, a tu spada i više vrsta roda Eucalyptus. Depresija inbreedinga kod navedenih vrsta varira od vrlo malo do gotovo 50´%>. Važno je spomenuti i to da je sjeme dobiveno samooplodnjcm slabe klijavosti, njegovo potomstvo ima slabi prirast, a pojedine jedinke često su još i nenormalnog izgleda. Dosadašnji radovi na oplemenjivanju tom metodom uglavnom su u prvoj fazi, tj. proizvodnja li i Is generacije. Ima više primjera koji potvrđuju opće postavke. Tako je L a n g 1 e t (29) kod obične smrče utvrdio da se kod križanja u bliskom srodstvu smanjuje procent klijavosti. Biljke pokazuju depresiju već u prvoj godini rasta, a u kasnijim godinama ona ide nešto preko 500/o Depresija je utvrđena i kod drugih vrsta. npr. Pinus silvestris, Pinus nigra, Pinus Mugo i Larix decidua. Jovanović i Tu čo v i ć (28) dobili su od jednoga monoecijskog stabla crne topole li biljke koje u prve dvije godine života ne pokazuju veći stepen depresije. ,S. C Larsenjes a svojim suradnicima već dobio kod evropskog ariša i I2 generaciju. Daljnja faza rada tom metodom je međusobno križanje »inbred« linija s ciljem da se dobiju potomstva s pojavom heterozisa. i 117 |
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Ako vršimo oplemenjivanje putem »inbreedinga« i koristimo dvije različite rase ili provenijencije, mogućnost dobivanja većeg efekta heterozisa je po našem mišljenju veća nego kad taj rad provodimo unutar jedne rase ili provenijencije. Langne r (31) i preporučuje kod te metode oplemenjivanja korišćenje različitih provenijencija. Tako je npr. Nils son (38) dobio jaku pojavu heterozisa križajući jednu švedsku provenijenciju smrče s jednom kontinentalnom. Autor objašnjava da je do pojavt heterozisa došlo uslijed »inbreeding« depresije unutar svake provenijencije, te se kao posljedica toga kod potomstva pojavio efekt luksuriranja. b) Viševrsni hibridi. Takva vrsta hibrida može imati isto tako pojavu heterozisa. U Placervillueu proizvedeni su hibridi (P. peuce x P. strobus) x P. monticola, koji kod starosti od 11 godina visinom jako nadmašuju tamo autohtoni P. monticola. U istom Institutu proizveden je i hibrid (P. ponderosa x P. apacheca) x P. montezumae koji raste brže čak i od hibrida P. ponderosa x P. montezumae. Mi smo dobili hibrid od triju vrsta P. silvestris x (P. densiflora x P. nigra) koji raste brže od crnog bora. Prirodni viševrsni (»multispecies«) hibrid je P. nigra austracia x (P. thunbergii x P. densiflora) koji raste brže od muškog roditelja. Kod roda Castanea poznat je hibrid od triju vrsta C. mollissima x (C. crenata x C. dentata) koji prema Wright u mnogo obećaje s obzirom na kestenov rak, od čega strada američki kesten. Kao što se vidi iz primjera, i ta metoda rada može biti od velike koristi pa zbog toga treba kod planiranja dugoročne međuvrsne hibridizacije voditi računa i o ovim mogućnostima, pogotovo sada kad već posjedujemo velik broj hibrida, nastalih križanjem dviju različitih vrsta. c) Primjena zračenja. Smatramo za potrebno da spomenemo još jednu mogućnost proizvodnje biljaka s pojavom heterozisa. Genetičari koji se bave drozofilom utvrdili su da je heterozis često povezan s heterozigotnosti karaktera delecije, inverzije i translokacije hromosoma. U takvom slučaju jedan hromosom u paru je potpuno normalan dok se drugi, koji potječe od drugog roditelja, razlikuje strukturalno od prvoga. Na osnovi te spoznaje Osborn e (39) postavlja hipotezu da bi se prije križanja ozračio samo jedan roditelj te bi se na taj način mogao inducirati heterozis. Smatra da bi se ta metoda mogla koristiti baš kod šumskog drveća koje je dugog života. Zračenjem polena i oprašivanjem nezračenog (normalnog) ženskog gametofita dobile bi se heterozigotne a i heterotične biljke između kojih bi se vršila selekcija tokom uzgoja. Rudolp h (49) je na osnovi navedeno hipoteze postavio pokus s Picea glauca. Zasada je ustanovio da je zračenje polena imalo stimulativni efekt na zametanje sjemena te na valjanost sjemena. Polen, zračen s 600 r dao je i teže sjeme. Istraživanja koja u tom pravcu vršimo s crnim i običnim borom pokazala su da se mogu dobiti veće i deblje biljke kada je polen prije oprašivanja zračen gama zrakama. Radi ilustracije donosimo slijedeće podatke: trogodišnje biljke običnog bora dobivene kontroliranim oprašivanjem s nezrsčenim polenom imale su prosječnu visinu 21,11 cm, a debljinu 8,22 mm; biljke kod kojih je polen zračen s 400 r i 5000 r imale su prosječne visine 23,88 odnosno 26,78 cm dok su im promjeri iznosili 9,38 odnosno 11,14 mm. HIBRIDIZACIJA IZMEĐU RASA Wrigh t (61) je u svom referatu već u naslovu poglavlja o međurasnoj hibridizaciji dobro podvukao da je taj predmet zanemaren. Zasada imamo malo radova o križanju rasa unutar jedne vrste premda se od takvih radova mogu u mnogo slučajeva očekivati dobri rezultati. Poznato je da se kod poljoprivrednog bilja međusobnim križanjem različitih rasa mogu dobiti hibridi s pojavom luksuriranja. Isto to može se postići i kod šumskog drveća. Razumljivo je da je prije toga potrebno, ukoliko još nije ispitano, ustanoviti rase, varijetete i podvrste koje dolaze kod jedne vrste. Što su rase i taksonomske jedinice bolje izražene, očekuje se da će se kod njihova međusobnog križanja dobiti potomstvo koje će biti u mnogo slučajeva vrlo vitalno pa i s pojavom heterosisa. Tako jeJohnsso n (27) u Švedskoj, kada je križao trepetljiku južne provenijencije sa sjevernom, dobio potomstvo koje je kod starosti 10 godina bilo snažnije od oba roditelja. Tu mogućnost potvrđuju i rezultati autora van V1 o t e n (57) i Schönbac h (51) kod križanja različnih provenijencija trepetljike. 118 |
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Poučan primjer o vrijednosti međusobnog križanja nižih sistematskih jedinica unutar jedne vrste postoji i kod nas u Istri. U jednoj kulturi Pinus nigra zapazili smo da postoji velika razlika u morfološkim karakteristikama pojedinih biljaka. Neke od njih odgovaraju korzičkom, druge austrijskom crnom boru dok je jedan dio intermedijaran. O historijatu te kulture nemamo pisanih podataka, ali je najvjerojatnije da je sjeme od kojih su uzgojene te biljke sabrano iz mješovite kulture korzičkoga i austrijskog crnog bora. Okularnim promatranjem može se vidjeti da intermeđijarne biljke, za koje pređpostavljamo da su hibridi austrijskoga i korzičkog crnog bora, imaju veće visine i promjere od austrijskoga crnog bora. Langne r (31) objašnjava pojavu luksuriranja kod međurasnog križanja primjerom smrče. Rasa sa sjevera raste pod uvjetima dugog dana Ona je s obzirom na duljinu dana koju zahtijeva za svoj rast homozigotna i ima genetsku konstituciju LLkk. Južna rasa raste pod uvjetima kraćeg dana te je s obzirom na to svojstvo njezina genetska konstitucija IIKK. Ako se križaju te dvije rase, nastaje potomstvo1 s najvećim mogućim heterozigotetom LIKk. Ovakovo objašnjenje može se prihvatiti premda znamo da je u mnogim slučajevima uvjetovano s više faktora, pa je i objašnjenje mnogo kompliciranije. Mislimo da je zasada najvažnije da se uoči ta pojava te da se hibridizacija po mogućnosti što više usmjerava u tom pravcu. Međurasna hibridizacija može imati svoju vrijednost i u slučaju ako se ne dobije hcterotično potomstvo. Križanjem dviju rasa te kombinacijom njihovih svojstava može se dobiti potomstvo koje će imati željena svojstva. Takav slučaj je sa smrčom u Švedskoj gdje križanac između švedske i njemačke rase raste bolje od domaće rase, a otporan je na hladnoću kao i domaća švedska rasa. POTEŠKOĆE KOD MEĐUVRSNE HIBRIDIZACIJE I NJIHOVO SVLADAVANJE Kod međuvrsne hibridizacije često se nailazi na poteškoće. Mnoge kombinacije križanja dale su potpuno negativne rezultate. Smatramo da je ovdje potrebno nešto reći i o tom problemu. Uzroke negativnim rezultatima treba tražiti u srodstvenoj vezi vrsta koje križamo, morfologiji cvjetova, dobi cvatnje kao i inkompatibilnosti vrsta. i Razumljivo je da se obično ne mogu međusobno križati vrste koje su srodstveno jako udaljene. U takvom slučaju poteškoće su vrlo velike i trebat će još mnogo truda uložiti da se prebrode, ukoliko će se uopće i moći prebroditi. Ako postoji bitna razlika u gradnji cvjetova, križanje se uglavnom ne može sprovesti s uspjehom. Isto tako, ako postoji razlika u vremenu cvatnje između dvije vrste, postoji poteškoća kod njihove hibridizacije, ali svladavanje tih poteškoća je mnogo lakše. U takvom slučaju potrebno je ubrzati ili usporiti vrijeme cvatnje muških odnosno ženskih cvjetova kako bi se oprašivanje moglo izvršiti u pravi čas. Najprikladnija metoda je u tome da se polen spremi i koristi kada su ženski cvjetovi spremni za oprašivanje. Kod inkompatibilnih vrsta poteškoće su isto velike, ali smatramo da se u nekim slučajevima mogu svladati.* Da se ustanovi u kojoj se fazi razvoja nalazi nakon oprašivanja polen, jajna stanica, zigota ili proembrio, potrebno je vršiti embriološka istraživanja kao što su prikazana u radovima Buchholz (7), M c W i 11 i a m (36), Hagman i Mi k kola (21) i Hvun i Yim (23). Takva istraživanja ukazat će nam dokle je prodrla pole- nova mješinica, da li je došlo do oplodnje i šta se desilo nakon oplodnje sa zigotom. Kada dobijemo jasnu sliku o tim procesima kod križanja dviju inkompatibilnih vrsta, onda ćemo možda lakše moći svladati poteškoće uslijed opiranja križanju. Osim toga kod križanja inkompatibilnih vrsta potrebno je razrađivati metode hibridizacije. Smatramo da su interesantne metode prethodnoga vegetativnog zbližavanja i nanošenja dijelova njuški tučkova od očinskog stabla na njuške tučkova materinskog stabla, koje metode preporučuje za međuvrsnu hibridizaciju J a b 1 ciko v (24). Prema našim nepotpunim istraživanjima (55) kod hibridizacije inkompatibilnih vrsta oprašivanje ženskih cvjetova jedne vrste sa zračenim polenom druge * Pod inkompatibilnošću razumijevamo nesposobnost polena da oplodi jajnu stanicu, a ukoliko je oplodi, nesposobnost zigote da preživi ili pak kasnije nastaje degeneracija proembrija. |
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vrste ukazuje na novu mogućnost rješavanja toga problema. Interesantno je u vezi s tim istaći, premda su rezultati nepotpuni, da kod oprašivanja crnoga bora sa zračenim polenom običnog bora kod doza od 800 r i 1000 r sjemeni zametak u manje slučajeva pokazuje degeneraciju, nego kad polen običnog bora nije zračan. Na kraju ovog referata slobodan sam iznijeti jedan prijedlog. Dosadašnji radovi na unutarvrsnoj, a naročito na međuvrsnoj hibridizaciji dali su vidne rezultate. Dobiveni su mnogi hibridi s pojavom heterozisa. Danas već imamo mnogo podataka o različnim kombinacijama križanja. Naša iskustva kao i teoretska znanja o mogućnostima križanja dvjiu vrsta stalno se upotpunjuju, pa i pored toga pred nama stoje još mnogi problemi koje treba rješavati. Tako npr. punu pažnju zaslužuje problem masovne proizvodnje hibrida, na što dobro ukazuje Right er (44). Nije manje važan problem i hibridizacije između rasa s ciljem da se dobije heterotično potomstvo. Zbog toga smatram da bi bilo korisno uspostaviti organiziranu međunarodnu suradnju u vezi s pitanjima međuvrsne i međurasne hibridizacije. LITERATURE CITED 1. Albensk i j , V. A. (1940): Projavlenie priznakov u mezvidovyh gibridov listvennic pervogo pokolenija. Dokl. vsesojuz. Akad. sel.-hoz. Nauk Lenina, 23/24, pp. 20—23. 2. Albenskij , V. A. (1951): Itogi gibridizacii listvennic, kljonov, il´movih i jasenej. Akad. Nauk SSSR — Trudy Inst. Lesa, 8, pp. 88—93. 3. Albenskij , V. A. ( ): Vtoroe pokolenie gibridov drevesnyh porod, ih hozjajstvennye svojstva i priznaki. Vsesojuz. nauč.-issled. Inst. Agrolesomel., pp. 521—525. 4. Albenskij, V. A. (1959): Selekcija drevesnyh porod i semenovodstvo. Goslesbumizdat, Moskva-Leningrad, 306 pp. 5. Barnes, B. V. and Bingham, R. T. (1962): Juvenile performance of hybrids between Western and Eastern White Pine. Intermt. For. Range Exp. Sta., Res. Note, No. 104, 6 pp. 6. Bingham, R. T., S qui 11 ace, A. E. and P a 11 o n, R. F. (1956): Vigor, disease resistance, and field performance in juvenile progenies of the hybrid Pi- nus monticola Dougl. x Pinus strobus L. Z. Forstgenet, 5, pp. 104—112. 7. B u ch h o 1 z, J. T. (1944): The cause of sterility in crosspollinations between species of Pines. Amer. J. Bot., 31 (8): Suppl. p. 2 s. 8. C h i n g, K. K. (1959): Hybridization between Douglas-Fir and Bigcone Douglas- Fir. For. Sci., 5 (3), pp. 246—254. 9. Cri tchf ield, W. B. (1962): Hybridization of the Southern Pines in California. Proc. of a forest genetics workshop, Macon, Georgia, pp. 40—48. 10. Cr i tchf ield, W. B. (1963): The Austrian x Red Pine hybrid. Silvae Genet., 12 (6), pp. 187—192. 11. D i m p f 1 m e i e r, R. (1959): Die Bastardierung in der Gattung Larix. Forstwiss. Forsch., Beih. Forstwiss., 12, 75 pp. 12. Duffield , J. W. (1952): Relationships and species hybridization in the genus Pinus. Z. Forstgenet., 1 (4), pp. 93—100. 13. Duffield , J. W. (1954): The importance of species hybridization and polyploidy in forest tree improvement. J. For., 52 (9), pp. 645—646. 14. Duffield, J. W. and R i g h t e r, F. I. (1953): Annotated list of Pine hybrids made at the Institute of Forest Genetics. Calif. For. Range Expt. Sta., Res. Note, No. 86,9 pp. 15. Duffield, J. W. and Snyder, E. B. (1958): Benefits from hybridizing American forest tree species, J. For. 56 (11), pp. 809—815. 16. Fowler, D. P. and Heimburger, C. (1958): The hybrid Pinus peuce Griseb. x Pinus strobus L., Silvae Genet., 7 (3), pp. 81—86. 17. F u k a r e k, P. (1963): Nova hibridna ili prelazna svojstva javora iz Crne Gore (x Acer pseuodoobtusatum Fuk.) (A. new hybrid or transitional race of Maple from Montenegro). Šumarstvo, 10/12, pp. 357—362. 18. Fukarek , P. — C e 1 j o, A. (1959): Hibridi između gorskog i planinskog javora. (Hybrids between Acer pseudoplatanus and Acer Heldreichii). Šumarstvo, 11/12, pp. 543—548. |
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19. Ga thy, P. (1957): A propos de 1´ hybride nature! Abies concolor (Gord.) Engelm. x Abies grandis Link. Silvae Genet., 6 (6), pp. 186—190. 20. Go the, H. (1951/52, 1953, 1956): Ein Kreuzungsversuch mit Larix europaea D. C, Herkunft Schlitz und Larix leptolepis Gord. Z. Forstgenet, 1 (1951/52), pp. 108— 110; 2 (1953), pp. 122—125; 5 (1956), pp. 116—125. 21. H a g m a n, M. and Mikkola , L. (1963) observations on cross-, self-, and interspecific pollinations in Pinus peuce Griseb. Silvae Genet., 12 (3), pp. 73—79. 22. H y u n, S. K. (1960): Mass production of control-pollinated seed of conifers. Prot. Fifth World For. Congress, pp 787—792. 23. Hyun , S. K. and Yim, K. B. (1863): On the fertilization in the crossings of Pinus rigida. with P. taeda, P. radiata and P. elliottii. World Consultation on Forest Genetics and Tree Improvement, Stockholm, 2b/3. 24. Ja b 1 o kov, A. S. (1962): Selekcija drevesnyh porod. Moskva, 487 pp. 25. John s son, H. (1949): Studies on birch species hybrids. I. Betula verrucosa x B. japonica, B. verrucosa x B. papyrifera, and B. pubescens x B. papyrifera. Hercditas, 35, pp. 115—135. 26. J o h n s s o n, H. (1953): Hybridaspens ungdomsutveckling och ett försök till framtidsprognos. Svenska Skogsv Foren. Tidskr., 51, pp. 73—96. 27. Johnsson , H. (1956): Heterosiserscheinungen bei Hybriden zwischen Breitengradrassen von Populus tremula. Z. Forstgenet., 5, (5/6), pp. 156—160. 28. Jovanović, B. — Tucović, A. (1964): Prva inbriding generacija monoecijskog stabla crne topole (Populus nigra L.) u okolini Kosovske Mitrovice. (The first inbreeding generation of a monoecious Black Poplar tree in the environs of Kosovska Mitrovica) Topola, 42/43, pp. 13—19. 29. L a n g 1 e t, O. (1940): Om utvecklingen ov granar ur frö efter självbefruktning och efter fri vindpollinering. Med. Stat SkogsFörsAnst, 32 (1), pp. 1—22. 30. L a n g n e r, W. (1951—52): Kreuzungsversuche mit Larix europaea D. C. und Larix leptolepis Gord. Z. Forstgenet., 1, (1,2), pp. 2—18, 40—65. 31. L a n g n e r, W. (1957, 1958): Einführung in die Forstpflanzenzüchtung. Allg. Forstzeitschr., 48/1957, 16/1958, 32 pp. 32. Langner , W. (1958): Vorsicht mit Hybridlärchen. Allg. Forstzeitschr. 13, pp. 654—655. 33. Langner , W. (1959): Ergebnisse einiger Hybridisierungsversuche zwischen Picea sitchensis (Bong.) Carr. und Picea omorika (Pančić) Purkyne. Sylvae Genet.. 8 (5), pp. 138—143. 34. L a r s e n, C. S. (1937): The employment of species, types, and individuals in forestry. Roy. Vet. Agric. Coll. Yearb., Copenhagen pp. 74—154. 35. L a r s e n, C. S. (1956): Genetics in silviculture. Oliver and Boyd, Edinburgh- London, 224 pp. 36. M c W i 11 i a m, J. R. (1959): Interspecific incompatibility in Pinus. Amer. J Bot., 46 (6), pp. 425—433. 37. Moulopoulos, C. and Bassiotis, C. (1961): Artificial hybrids of Pinus halepensis and Pinus brutia. Lab. Silv. mountainous Hydron., Aristot. Univ. Thessaloniki, pp. 160—179. 38. Nils son, B. (1963): Intraspecific hybridization and heterozis within Picea abies. World Consultation on Forest Genetics and Tree Improvement, Stockholm, 2b/6. 39. Osborne , T. S. (1957): Past, present and potential uses of radiation in southern plant breeding. Proc. 9th Oak Ridge Reg. Symp. on Radiation in Plant Breeding, pp. 5—10. 40. Pauley , S. S. (1963): Performance of some aspen seed sources and hybrids in eastern Massachusetts. World Consultation on Forest Genetics and Tree Improvement, Stockholm, 2b/2. 41. P i a t n i t s k y, S. S. (1960): Evolving new forms of oak by hybridization. Proc. Fifth World For. Congress, pp. 815—818. 42. Righ t er, F. I. (1946): New perspectives in forest tree breeding. Science, 104, pp. 1—3. 43. Righ t er, F. I. (1960): Evidence of hybrid vigor in forest trees. Pap. Int. Conf. For. Tree Growth, Tucson, Ariz., pp. 345-—355. 44. Righ t er, F. I. (1960): Forest tree improvement through inbreeding and intraspecific and interspecific hybridization. Proc. Fifth World For. Congress, pp. 783—787. 121 |
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45. Right er, F. I. and Duff i eld, J. W. (1951): Hybrids between ponderosa and Apache Pine. J. For., 49 (5), pp. 345—349. 46. Righ t er, F. I. and Du f field , J. W. (1951): Interspecies hybrids in Pines. J. Hered., 42 (2), pp. 75—80. 47. Rohmeder , E. (1963): Experiments on forest tree hybrids in Bavaria from 1936 to 1962. World Consultation on Forest Genetics and Tree Improvement, Stockholm, 2b/l. 48. Rohmeder, E. — Schönbach, H. (1959): Genetik und Züchtung der Waldbäume. Paul Parey, Hamburg u. Berlin, 338 pp. 49. Rudolnh , T. D. (1964): The effect of gamma irradiation of pollen on seed characteristics in white spruce. Technical Meeting on the Use of Induced Mutations in Plant Breeding, FAO, Rome, 8 pp. 50. S a y 1 o r, L. C. (1964): Karyotype analysis of Pinus — Group Lariciones. Silvae Genet., 13 (6), pp. 165—169. 51. Schönbach , H. (1957): Die bisherigen Ergebnisse der Züchtungsarboiten mit verschiedenen Pappelarten der Sektion Leuce. Beitr. Pappelforsch. II, Wiss. Abhandl. Dtsch., Akad. Landwirtschftswiss., pp. 149—178. 52. V i d a k o v i ć, M. (1958): Investigations on the intermediate type between the Austrian and the Scots Pine. Silvae Genet., 7 (1), pp. 12—18. 53. V i d a k o v i ć, M. (1959): Oplemenjivanje ariša. (Breding of Larch). Šum. List, 10/11, pp. 347—354. 54. Vidaković , M. (1962): Novi prilog oplemenjivanju ariša. (A new contribution to the breeding of Larch). Šum. List, 1/2, pp. 47—62. 55. V i d a k o v i ć, M. (1963): Interspecific hybridization of several pine species from the subgenus Diploxylon Koehne. World Consultation on Forest Genetics and Tree Improvement, Stockholm, 2b/5. 56. Vidaković , M. (1963): Međuvrsno križanje Pančićeve omorike (Picea omorica) Paučić (Purkyne) sa sitkanskom smrčom (Picea sitchensis (Bong.) Carr. Interspecific crossing between Picea omorica (Pančić) Purkyne and Sitka Spruce (Picea sitchensis (Bong.) Carr. Šumarstvo, 10´12, pp. 337—342. 57. Vio ten, H. van (1954): T. N. O. Körte medeling nr. 22. (Kurze Mitteilungen über Kreuzungen mit Pappelarten der Sektion Leuce). 58. Wright , J. W. (1955): Species crossability in spruce in relation to distribution and taxonomy. For. Sci., 1, pp. 319—349. 59. Wright , J. W. (1959): Species hybridization in the White Pines. For. Sci., 5, pp. 210—222. 60. Wright , J. W. (1962): Genetics of forest tree improvement. FAO, Rome, 399 pp. 61. Wright , J. W. (1963): Hybridization between species and races. World Consultation on Forest Genetics and Tree Improvement, Stockholm, 2b´0. 62. Wright, J. W. and Gabriel, W. J. (1958): Species hybridization in the Hard Pines, series sylvestres, Silvae Genet., 7 (4) pp. 109—115. |