Mother species-father species: unidirectional hybridization in animals with female choice

As mitochondria are inherited in a matrilinear way, an animal hybrid contai ns the mitochondrial DNA of its 'mother species'. Of 80 studies that analys ed the mitochondrial DNA of at least five hybrid individuals, 50 showed tha t all hybrids contained the mitochondrial DNA of only one of the two parent al species, indicating either mating of females of species A with males of species B but not vice versa (unidirectional hybridization) or the disappea rance of one of the two parental mtDNA types after reciprocal hybridization . I review and discuss factors promoting unidirectional or reciprocal hybri dization and present a sexual selection hypothesis for unidirectional hybri dization. The inequality of the sexes in parental investment leads to the s ex that invests more being the more discriminating one. In the presence of conspecific males, females reject allospecific males and, consequently, a m ale in an environment of both allospecific sexes is unlikely to mate, while in the absence of conspecific males, females sometimes accept fertilizatio ns by males of other species. Thus, hybrid matings are usually between the females of a rare species and the males of a common species, but not vice v ersa. (C) 1999 The Association for the Study of Animal Behaviour.