On the Instability of Polygenic Sex Determination: the Effect of Sex-specific Selection.

A combination of analytical and simulation models is used to explore the initial evolution of genic sex determination from polygenic sex determination. Prior studies have indicated that polygenic sex determination is rare or absent in extant species but that it has likely played an important intermediate role in the evolution of other genetic sex-determination systems. This study explores why polygenic sex determination does not persist. Two possibilities are considered. First it is assumed that a major sex-determining gene also pleiotropically increases fitness. Second it is assumed that the sex-determining gene is neutral but linked to another locus segregating for a rare selectively favored allele. The major conclusion from the models is that sex-specific natural selection will cause polygenic sex determination to be a transient state in most populations. Polygenic sex determination may be an important intermediate step in the evolution of genetically controlled sexual differentiation, but it is unlikely to persist unless there is some selective advantage compared to genic sex determination. This may in part explain the relatively small number of extant species that have polygenic sex determination.