Therefore, a few randomly chosen markers may not be a good predictor of adaptive potential. Second, these markers themselves are assumed to be neutral, so they cannot provide direct information about genetic variations that are important for adaptation (Ouborg et al. First, a small number of markers represents only a tiny portion of the genome and may fail to identify genomewide diversity or phenotypic variation (Reed and Frankham 2001). Although such markers are useful for obtaining basic information about population genetics parameters, they have limitations (Harrisson et al. During the last few decades, genetic analysis of introduced populations has been conducted mainly using molecular markers, such as microsatellites and amplified fragment length polymorphisms. The history of hybridization is generally inferred with population genetic methods. Hybridization between distantly related populations not only increases standing genetic variation but also sometimes leads to the formation of hybrid populations with extreme (transgressive) phenotypes (Rieseberg et al. 2004 Roman and Darling 2007 Dlugosch and Parker 2008 Forsman 2014 Rius and Darling 2014 Bock et al. Hybridization between divergent lineages and/or multiple introductions often underlie the maintenance of relatively high genetic variance and adaptation in invasive species (Lee 2002 Kolbe et al. For example, it is reported that only 37% of introduced populations showed lower genetic diversity compared with the source populations (Roman and Darling 2007). However, recent studies have revealed that reduction in genetic variation of introduced populations is not as common as expected. Initial colonization by an introduced population often involves a population bottleneck (Allendorf and Lundquist 2003), which mostly reduces genetic diversity and may also reduce the ability to adapt to diverse environments. 2001 Pejchar and Mooney 2009 Ricciardi et al. Such introduced species or populations pose a major threat to biological diversity (Sakai et al. Whole-genome sequencing of even a small number of individuals can therefore provide higher resolution inference of history of introduced populations.Īn increasing number of organisms have been anthropogenically transferred from native habitats to novel environments. Thus, interspecies introgression might predate introduction and increase genetic variation in the source population. Further amplicon sequencing revealed linkage disequilibrium around an introgression site, which suggests the possibility of selective sweep at the introgression site. The sizes of the regions were too small to be detected with traditional marker-based analysis or even some reduced-representation sequencing methods. We found that both populations have several small genomic regions with high genetic diversity, which resulted from introgression from a closely related species ( Gasterosteus nipponicus).
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Here, we conducted whole-genome sequencing of eight threespine stickleback ( Gasterosteus aculeatus) individuals, four from a recently introduced crater lake population and four of the putative source population. Recent advances in genome sequencing enable us to trace the evolutionary history of invasive species even at whole-genome level and may help to identify the history of past hybridization that may be overlooked by traditional marker-based analysis. Although introduced populations often experience population bottlenecks, some invasive species are thought to be originated from hybridization between multiple populations or species, which can contribute to the maintenance of high genetic diversity. Invasive species pose a major threat to biological diversity.