Testing for "snowballing" hybrid incompatibilities in Solanum: impact of ancestral polymorphism and divergence estimates.

Two recent high-profile studies offered empirical evidence for a "snowballing" accumulation of postzygotic incompatibilities in Drosophila and Solanum (tomatoes). Here we present a reanalysis of the Solanum data that is motivated by population genetic principles. Specifically, the high levels of intraspecific nucleotide polymorphism in wild tomato species and presumably large effective population size throughout the divergence history of this clade imply that ancestral polymorphism should be taken into account when evaluating sequence divergence between species. Based on our reanalyses of synonymous-site divergence between the four focal Solanum species and a wide range of ancestral polymorphism, we assessed under which conditions the reported accumulation of seed sterility factors supports the snowball effect. Our results highlight the pivotal impact of levels of ancestral polymorphism and alternate divergence values, and they illustrate that robust tests of the snowball effect in Solanum require genome-wide estimates of divergence.