EVOLUTION OF A TRIHYBRID UNISEXUAL FISH (POECILIOPSIS, POECILIIDAE).

Abstract

A diploid all-female fish of the genus Poeciliopsis (Poeciliidae) inhabits the Rio Fuerte of northwestern Mexico. From both morphological and biochemical studies, it is clear that this form, designated P. monachalucida (Schultz, 1969), arose by hybridization between P. monacha and P. lucida. (Schultz, 1969; Vrijenhoek, 1972). Unisexual populations persist in nature by mating with males of P. lucida; such matings result in all-female progeny with both maternal and paternal traits. All-femaleness is maintained from one generation to the next by a unique reproductive isolating mechanism, called hybridogenesis (Schultz, 1969), that prevents reassortment of monacha and lucida genomes. Oogonia, dividing prior to maturation divisions, form unipolar spindles that transmit only the monacla chromosomles to the ova; the paternal (lucida) chromosomes are lost in the process and do, not contribute to the genome of the ova (Cimino, 1972). Fertilization of the resulting haploid monacha ova by sperm from P. lucida reestablishes all-female F1 monacha-lucida hybrids. Another diploid unisexual form, P. Cx (Schultz, 1961, 1966), occurs in the Rio Mocorito, 200 km south of the Rio Fuerte. Although P. Cx resembles P. monacha-lucida and also reproduces hybridogenetically, subtle differences exist between this and the Rio Fuerte fish that might be attributable to P. viriosa, a species closely related to P. monacka. Since P. monacha is not known to occur in the Rio Mocorito, the presence of a population resembling P. monachalucida needs to be explained. Genetic ex-