Transgressive segregation, adaptation and speciation

The production of extreme or 'transgressive' phenotypes in segregating hybr id populations has been speculated to contribute to niche divergence of hyb rid lineages. Here, we assess the frequency of transgressive segregation in hybrid populations,,describe its genetic basis and discuss the factors tha t best predict its occurrence. From a survey of 171 studies that report phe notypic variation in segregating hybrid populations, we show that transgres sion is the rule rather than the exception. In fact, 155 of the 171 studies (91%) report at least one transgressive trait, and 44% of 1229 traits exam ined were transgressive. Transgression occurred most frequently in intraspe cific crosses involving inbred, domesticated plant populations, and least f requently in interspecific crosses between outbred, wild animal species. Qu antitative genetic studies of plant hybrids consistently point to the actio n of complementary genes as the primary cause of transgression, although ov erdominance and epistasis also contribute. Complementary genes appear to be common for most traits, with the possible exception of those with a histor y of disruptive selection. These results lend credence to the view that hyb ridization may provide the raw material for rapid adaptation and provide a simple explanation for niche divergence and phenotypic novelty often associ ated with hybrid lineages.