A population genetic approach to speciation in wild tomatoes

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In this interdisciplinary project at the interface of population genetics and phylogenetics, speciation processes in the genus Lycopersicon (wild tomatoes) are examined measuring DNA sequence variation within and between species at a genome-wide scale (i.e., at multiple loci across the genome). Although the extant nine tomato species are morphologically distinct, molecular evidence is mounting that these species are very young. This allows us to use a population genetic approach to analyze the effect of mating system, demography and natural selection on the origin and divergence of the tomato species. Of particular interest is the role of the mating system. Despite the small number of species, the tomato genus encompasses a great diversit of mating systems. In addition to the mating system, demographic factors are important. In order to assess the role of gene flow between the tomato species following incipient species separation, a speciation model is fitted to the data. This model assumes that an ancestral species split into two descendent species at some point in the past, with no further gene flow between the incipient species. The genealogical information contained in sequence date (i.e., shared polymorphic sites, sites polymorphic in only one of the two taxa, and fixed differences between species) is extracted to yield estimates of the model parameters, including the divergence time between species.