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population genetics of speciation in two closely related wild tomatoes (solanum section lycopersicon)


作者單位:Department Biologie II, Abteilung Evolutionsbiologie, University of Munich (LMU), 82152 Planegg-Martinsried, Germany1 Corresponding author: Plant Ecological Genetics, Institute of Integrative Biology, ETH Zurich, Universitätsstrasse 1 8092 Zurich, Switzerland.

【摘要】 we present a multilocus sequencing study to assess patterns of polymorphism and divergence in the closely related wild tomato species, solanum peruvianum and s. chilense (solanum section lycopersicon, solanaceae). the data set comprises seven mapped nuclear loci ( 9.3 kb of analyzed sequence across loci) and four local population samples per species that cover much of the species' range (between 80 and 88 sequenced alleles across both species). we employ the analytical framework of divergence population genetics (dpg) in evaluating the utility of the "isolation" model of speciation to explain observed patterns of polymorphism and divergence. whereas the isolation model is not rejected by goodness-of-fit criteria established via coalescent simulations, patterns of intragenic linkage disequilibrium provide evidence for postdivergence gene flow at two of the seven loci. these results suggest that speciation occurred under residual gene flow, implying that natural selection is one of the evolutionary forces driving the divergence of these tomato species. this inference is fully consistent with their recent divergence, conservatively estimated to be 0.55 million years. we discuss possible biases in the demographic parameter estimates due to the current restriction of dpg algorithms to panmictic species.

【關鍵詞】 population genetics speciation tomatoes (solanum lycopersicon)


the biological and geographic determinants of species divergence have long been contentious, and it is now increasingly appreciated that patterns of genetic variation and differentiation may provide valuable insights into the evolutionary processes shaping this divergence. the importance of geographic isolation in facilitating evolutionary divergence as a consequence of mutation and genetic drift (or additionally, adaptive differentiation) was recognized early, and the process of allopatric speciation is uncontroversial on theoretical grounds ( m ayr 1963; l osos and g lor 2003; c oyne and o rr 2004 ). if residual gene flow characterized the divergence of incipient species, however, modes other than strict allopatric speciation must be invoked, and these invariably require natural selection as one of the factors underlying species divergence. in addition to the putatively rare cases of sympatric speciation ( e.g., s avolainen et al. 2006 ), divergence under residual gene flow may proceed in parapatry; i.e., geographically adjacent populations may be subject to directional selection that incidentally confers reproductive isolation ( e ndler 1977; t urelli et al. 2001; g avrilets 2003 ). another scenario is an initial period of divergence in allopatry followed by secondary contact allowing gene flow and thus direct selection for stronger interspecific barriers (reinforcement of reproductive isolation; r ice and h ostert 1993; c oyne and o rr 2004; h oskin et al. 2005 ). some researchers posit that interspecific hybridization and postdivergence gene flow following secondary contact may promote novel advantageous gene combinations in populations of mixed ancestry, perhaps contributing to adaptive divergence and speciation ( e.g., a rnold 1997; r ieseberg et al. 2004; s eehausen 2004; m allet 2005 ).

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