Geographic Dispersal and Clonal Diversity in Unisexual Fish Populations

Abstract

Tissue grafting was used to investigate clonal diversity in populations of the unisexual hybrid fish, Poeciliopsis monacha-occidentalis from five rivers in northwest Mexico. The ranges of the two sexual progenitor species, P. monacha and P. occidentalis, overlap only in the southernmost of these rivers. Synthesis of new clones of P. monacha-occidentalis via interspecific matings may still occur there. The other four rivers are north of the range of P. monacha, but contain populations of P. monacha-occidentalis and its sexual host species, P. occidentalis. A surprisingly large number of clones was detected in the southernmost river. Unisexual strains descended from 33 wild-collected females comprised 17 distinct clones. Clonal diversity was drastically lower in the northern rivers. In one of the rivers two clones were detected; the other three rivers had only one clone each. This pattern strongly indicates that multiple-hybrid origins are the most important source of clonal diversity in P. monacha-occidentalis. Evolution does not cease with the advent of unisexuality, however. An example of genetic change in unisexuals subsequent to their hybrid origin was detected. The single clone from the northernmost river contains unique alleles at two electrophoretic loci. This clone was found to be antigenically similar to the single clone from the adjacent river. It is likely that these two clones have diverged genetically from a common ancestral clone that originated in the south. Factors affecting the clonal composition of populations are discussed. The clonal composition of the populations is not a result of random processes. Among the selective forces influencing clonal diversity, habitat complexity probably plays a minor role in comparison to the high degree of temporal instability which is characteristic of these desert stream habitats.