Genetics of a Sexually Reproducing Fish in a Highly Fluctuating Environment

Abstract

The sexually reproducing fish Poeciliopsis monacha inhabits the highly fluctuating desert streams of northwestern Mexico. Severe annual fluctuations in population size along with periodic local extinctions are typical for this species. Genetic differentiation among local populations apparently results from random genetic drift. In this species as a whole, levels of protein polymorphism and heterozygosity are not significantly lower than in other fish inhabiting less variable environments. P. monacha played a central role in the hybrid origins of several all-female forms of Poeciliopsis that perpetuate their hybrid genotypes by clonal reproduction. The reproductive mechanisms of the unisexual forms apparently arose as a consequence of meiotic failure in the hybrids, since they are not found in the ancestral P. monacha populations. Occasional breakdown of clonal reproduction in these hybrids might permit a significant amount of gene exchange with P. monacha and could explain the higher levels of variation in this sexual species than in other desert dwelling fish in this genus and related families.