Special Adaptive Problems Associated with Unisexual Fishes

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Abstract
SYNOPSIS. Two genera of fishes, Poecilia and Poeciliopsis, both in the family Poeciliidae, have established unisexual populations. One “species,” Poecilia formosa, is a diploid form that relies upon sperm from Poecilia latipinna or P. mexicana to initiate parthenogenetic development (gynogenesis). Triploid individuals of P. formosa have also been discovered but the mode of reproduction and the significance of such fish to natural populations have yet to be established. In the genus Poeciliopsis three gynogenetic triploids are known. Each is superimposed on a different bisexual host: Poeciliopsis lucida, P. monacha or P. viriosa. Five diploid hybridogeneticforms also have been identified. These are fertilized by either P. lucida, P. latidens, or P. occidentalis. In hybridogenetic forms, sperm actually feitilizes the egg. A hybrid with maternal and paternal characters is produced, but only maternal genes are transmitted through the egg to the next generation. Because of their sperm dependency, all unisexual fishes must reside with at least one of the parental species. They areforced to compete with them for sperm, space, and other limited resources. The ability to produce two females for every one that bisexuals produce suggests that the unisexuals have the potential to win this competition; but in so doing, they would lose their sexual support and eliminate themselves. A density-dependent mate selection system plays a role in establishing a workable balance between the competing elements. Unisexual forms of both Poecilia and Poeciliopsis are of hybrid origin. For most of them, the parental precursors and the proportion of genetic material that each contributed to the unisexual is known. The various genomes that make up the unisexuals are adapted to a variety of habitats. Once these genomes are combined in a unisexual hybrid, potential for adaptation to multiple niches exists; this coupled with heterosis contributes greatly to the success of gynogenetic and hybridogenetic fishes.

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