DISTRIBUTION, ADAPTATION AND PROBABLE ORIGIN OF AN ALL‐FEMALE FORM OF POECILIOPSIS (PISCES: POECILIIDAE) IN NORTHWESTERN MEXICO

At least five, morphologically distinct, all-female forms occur in the viviparous fish genus Poeciliopsis (Schultz, 1966; Schultz, 1969). These appear to have had a similar origin, even though in different parts of their range they are not associated with the same gonochoristic (bisexual) species of Poeciliopsis. Morphological and experimental evidence indicate that the common ancestor to all of the unisexual forms was a diploid unisexual that originated through hybridization, at which time it was endowed with its obligatory unisexuality and a disruptive meiotic mechanism. This form, termed P. monacka-lucida (Schultz, 1969) has given rise to two gynogenetic triploid populations by incorporating an additional genome from one or the other of the ancestral bisexual species involved in its origin. Two other derived forms are diploid and like P. monacka-lucida sustain themselves by a unique reproductive mechanism termed hybridogenesis (Schultz, 1969). In gynogenesis, spermatozoa are required to initiate cleavage, but paternal chromatin is not incorporated into the zygotic nucleus. Offspring of hybridogenetic organisms, on the other hand, exhibit characteristics of both parents but unisexuality is retained by excluding the entire paternal genome from functional ova in the process of oogenesis. The latter mode of reproduction, theoretically, has high adaptive value. Females that reproduce by such a mechanism have a genome which is isolated from the gene pool of the bisexual species