Hybridization, Unisexuality, and Polyploidy in the Teleost Poeciliopsis (Poeciliidae) and Other Vertebrates

The discovery of a second gynogenetic, triploid "species" in the genus Poeciliopsis is reported. This all-female form is part of a five-fish hybrid complex from the Rio Fuerte of northwestern Mexico. The complex consists of two bisexual species, P. monacha and P. lucida, and three unisexual derivatives, two of which are triploid and one of which is diploid. The 2n unisexual approximates 1n of P. monacha and 1n of P. lucida. During oogenesis the paternally derived P. lucida chromosomes are lost; thus it is primarily the P. monacha set that is transmitted to the egg. Since the allfemale form mates mainly with males of P. lucida, the hybrid nature of this fish is maintained. The term "hybridogenesis" is applied to this mode of reproduction. The two triploid forms apparently stem from the occasional failure of this mechanism and the subsequent production of 2n ova. Such ova, when combined with P. lucida sperm, result in a triploid form that morphologically appears to be 2n of P. lucida and 1n of P. monacha; but when combined with P. monacha sperm, the ova result in a triploid that is 2n of P. monacha and 1n of P. lucida. All three of the unisexual forms are abundant in nature. The discovery of this hybrid complex along with recent discoveries of similar complexes in other vertebrates suggests that hybridization, unisexuality, polyploidy, hybridogenesis, gynogenesis, and parthenogenesis are related. This relationship makes it possible to explain how large changes in DNA levels took place during the evolution of vertebrates. A reconsideration of the importance of polyploidy to animal evolution is presented.