Haldane's duel: intragenomic conflict, selfish Y chromosomes and speciation.

Haldane's rule, which states that the heterogametic sex (XY or ZW females) fares more poorly in interspecific hybrids, is generally attributed to absence of one of the two species' X/Z chromosomes. However, Haldane's rule is also observed in mouse placentas despite paternal X silencing. This pattern could reflect Y chromosomes having evolved to promote growth due to maternal-paternal conflict. If so, balanced sex investment arises from a complex intra- and intergenomic duel.