DIGITALNA ARHIVA ŠUMARSKOG LISTA
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However, it was identical also with haplotype LZ (Accession Number JF411594) that was detected in China (Lu et al. 2011). Very close genetically were the Chinese haplotypes LBX and FS (Accession Numbers JF411598 and JF411595, respectively), whereas haplotypes CC and SY (Accession Numbers JF411597 and JF411596, respectively) are more distantly related (Lu et al. 2011) (Table 2).
Molecular techniques are being now increasingly used in studies of invasive species, with applications that range from assessing genetic variation (Roderick 1996; Sakai et al., 001), identifying population origin (Cognato et al. 2005; Grapputto et al. 2005) or even developing control strategies (Conord et al. 2006; Fu et al. 2010). The chestnut gall wasp, D. kuriphilus was introduced in north-west Italy with chestnut cultivars from China in 2002 (Quachia et al. 2008). This Chinese origin of D. kuriphilus was verified by the molecular data. All the Italian individuals exhibited a single haplotype that was identical with a Chinese one that was already deposited in NCBI GenBank under the Accession Number JF411594 (Lu 2011), confirming that the founder population had a Chinese origin. Interestingly this Chinese haplotype was the most widespread one as it was found in several provinces of China (Henan, Jiangxi, Hunan and Fujian) (Lu et al., 2011). As this haplotype was retrieved once again in Italy (Ács et al. 2007), it can be easily deduced that D. kuriphilus expanded in the three investigated countries after a single introduction event, with a founding population of limited size or at least low genetic diversity. Should chestnut gall wasp have invaded Italy with multiple introductions, then genetic diversity would have been restored at least to the levels of native populations if not higher (Kolbe et al. 2004; Zalewski et al. 2011), giving rise to an admixture of haplotypes. However, not only was a single haplotype retrieved among the Italian individuals, but this haplotype was the same with the one retrieved five years before (Ács et al. 2007). Conclusively, this population bottleneck during the initial colonization of D. kuriphilus could only be indicative of a single colonization event that was additionally performed by few individuals (small founder population) (Russell et al. 2009). In any other case, multiple introductions and/or large founder numbers should soon have increased genetic variation (Stepien et al. 2005).
However, populations founded under such unfavorable conditions (single introduction of a small number of individuals) suffer so evidently from low genetic variation that might even lose their adaptive ability (Frankham & Ralls 1998; Saccheri et al. 1998; Duglosch & Parker 2008), putting ultimately their very existence in risk (Nei et al. 1975; Courchamp et al. 1999). One of the most efficient ways to overcome the defects of low population levels coupled with low genetic diversity is parthenogenesis. Uniparental propagation is highly adaptive as it permits small populations to quickly expand and efficiently exploit ephemeral resources (Niemela & Mattson 1996; Davis 2009). The advantage of parthenogenetic reproduction has been demonstrated in other organisms (e.g. Potamopyrgus antipodarum) that have been introduced succesfully, overcoming low genetic diversity at the phase of invasion through uniparental propagation (Dybdahl & Drown 2011). It thus seems logical to assume that the parthenogenetically reproducing chestnut gall wasp took advantage of the same biological feature (parthenogenesis) to compensate for the severe bottleneck it suffered during invasion and establishing the initial population in Italy.
In general, the post-introductory expansion of an invasive species is determined either by evolutionary adaptation or plasticity across a range of novel environments. Evolutionary adaptation facilitates the range expansion in invasive species particularly when there is sufficient genetic variation within the invading population for natural selection to act on (Garcia-Ramos & Rodriguez 2002; Maron et al. 2004; Bossdorf et al. 2005). This remark however, leads to a logical paradox: how can bottlenecked invasive species be successful when sometimes bottlenecked native species are indeed susceptible to extinction (Frankham 2005; Roman & Darling 2007)? Loss of genetic diversity during colonization has been observed in several invading insects (Kourti 2002; Bonizzoni et al. 2004; Grapputo et al. 2005; Tung et al. 2009; Rubinoff et al. 2010; Bray et al. 2011); nevertheless, in none of these cases was the invasion really interrupted. Based on our findings, D. kuriphilus can well be included among the above mentioned examples of succesful expansion despite the initial low genetic diversity during introduction. In less than ten years, chestnut gallwasp has dispersed from the point of introduction in Italy to several European countries (France, Slovenia, Switzerland, Croatia, Netherlands, Austria, Hungary), covering a distance of about 1200km to the south (Sicily) and 750 km to the east (Croatia). To our surprise, this expansion was accomplished by a unique mtDNA haplotype, identical to the one that invaded Europe in the first place. It thus seems possible that the paradoxically succesful post-introductory expansion of D. kuriphilus could possibly be attributed to phenotype plasticity that maintained high fitness over a broad range of environments (Richards et al. 2006). The concept that general-purpose genotypes are positively associated with invasiveness has been expressed before (Baker 1965). Conclusively, the fact that the single haplotype that expanded in the countries investigated is the very same with the most abundant one in China (Lu et al. 2011) argues for the hypothesis of a general-purpose genotype adapted to various environmental conditions.