EGG-PRODUCTION, POLYPLOIDIZATION AND EVOLUTION IN A DIPLOID ALL-FEMALE FISH OF THE GENUS POECILIOPSIS.

Abstract

The frequency of natural populations of unisexual vertebrates raises the question of the evolutionary potential of unisexual forms, both in isolation and in relation to their sympatric congeners. In order to determine this potential, their reproductive mechanisms must be known, for herein lies the control of their genetic variability. A unisexual-bisexual complex of viviparous fishes of the genus Poeciliopsis (Poeciliidae: Cyprinodontiformes) provides some interesting unisexual situations, among which are (1) present sympatry of the original progenitors of the complex; (2) two degrees of ploidy; and (3) two distinct mechanisms of unisexual reproduction, one of which is unique in vertebrates. This complex, which inhabits the Rio del Fuerte of northwestern Mexico, contains five forms. The morphology of the three all-female forms indicates a hybrid origin, with the two gonochoristic (bisexual) species of the complex, P. lucida Miller and P. monacha Miller, as parental forms. Of the three unisexual forms, one is diploid (2n = 48) like the gonochorists, and is intermediate in morphology between them (Schultz, 1961, 1969). It apparently possesses one genome of each; for this reason, it has been named P. monacha-lucida (formerly Cx; Schultz, 1969). The other two unisexuals are triploid (3n 72). The one which more closely resembles P. lucida is called P. monacha-2 lucida (formerly Cy). The other resembles P. monacha and is called P. 2 monacha-lucida. In each case, the nomenclature reflects the proposed genome dosage received from the parental species (Schultz, 1969).