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INTERSPECIFIC HYBRIDIZATION AND THE EVOLUTIONARY ORIGIN OF A GYNOGENETIC FISH, POECILIA FORMOSA

In recent years, largely through the application of allozyme techniques, several unisexual (thelytokous) vertebrates have been shown to be genetically equivalent to F1 hybrids of certain related bisexual species. The causal relationship, however, between interspecific hybridization and the origin of unisexuality has not been elucidated, and the role of hybridization per se is controversial (Cuellar, 1974, 1977, 1978; Cole, 1978; Wright, 1978). The amazon molly, Poeciliaformosa (Girard), an ovoviviparous gynogentic teleost (family Poeciliidae) was the first known unisexual vertebrate (Hubbs and Hubbs, 1932). The discoverers of its unisexuality recognized early that it was probably of hybrid origin, as it was an almost exact morphological intermediate between the sailfin molly (Poecilia latipinna) and the shortfin mollies (which were then regarded as a single species, P. sphenops). Laboratory hybridization of the presumptive parental species, however, produced only bisexual progeny, including fertile males (Hubbs, 1933, 1955, 1961; Meyer, 1938; Hubbs and Hubbs, 1946a, 1946b). Subsequent to those early hybridization experiments, P. "sphenops," as it was then recognized, has been shown to be an assemblage of morphologically similar but genetically quite distinct species (Hubbs, 1961; Schultz and Miller, 1971; Miller, 1975; Brett et al., unpubl.). A variety of zoogeographic, morphological (Darnell and Abramoff, 1968), chromosomal (Prehn and Rasch, 1969), and biochemical genetic (Abramoff et al., 1968; Turner et al., 1980) data suggest that the shortfin species involved in the ancestry of P. formosa was P. mexicana, a species restricted to the