Hybrid incompatibility "snowballs" between Solanum species.

Among the reproductive barriers that can isolate species, hybrid sterility is frequently due to dysfunctional interactions between loci that accumulate between differentiating lineages. Theory describing the evolution of these incompatibilities has generated the prediction, still empirically untested, that loci underlying hybrid incompatibility should accumulate faster than linearly with time--the "snowball effect." We evaluated the accumulation of quantitative trait loci (QTL) between species in the plant group Solanum and found evidence for a faster-than-linear accumulation of hybrid seed sterility QTL, thus empirically evaluating and confirming this theoretical prediction. In comparison, loci underlying traits unrelated to hybrid sterility show no evidence for an accelerating rate of accumulation between species.