The evolution of hybrid infertility: perpetual coevolution between gender-specific and sexually antagonistic genes.

A new hypothesis is proposed for the rapid evolution of postzygotic reproductive isolation via hybrid infertility. The hypothesis is motivated by two lines of experimental research from Drosophila melanogaster that demonstrate that sexually antagonistic fitness variation is abundant and that epistatic fitness variation on the Y chromosome is common. The hypothesis states that the expression of sexually antagonistic genes leads to a 'gender-load' in each sex. In response, gender-limited reproductive genes are selected to ameliorate, through pleiotropy, the expression of sexually antagonistic genes. Chronic coevolution between gender-limited genes and gender-unlimited sexually antagonistic genes causes rapid divergence of reproductive proteins among allopatric populations, ultimately leading to hybrid infertility.