Clonal Diversity and Evolutionary Dynamics in a Diploid-Triploid Breeding Complex of Unisexual Fishes (Poecilia)

A unisexual-bisexual 'breeding complex' of viviparous poeciliid fishes ('mollies,' genus Poecilia) exists in the Río Soto la Marina basin in northeastern Mexico. The complex consists of an indigenous bisexual 'host' species, P. 'mexicana' (possibly distinct on the specific or subspecific level from nominally conspecific populations elsewhere in Mexico), a diploid allfemale gynogen, P. formosa (ultimately of exogenous origin), and triploid all-female gynogens that result from fertilization of diploid P. formosa ova by host sperm. As assessed by allozyme techniques, the host population has roughly average levels of heterozygosity (H = .053), but is highly differentiated geographically. The level of differentiation, even among samples collected along a single tributary, is much higher than is encountered in shortfin mollies in other drainages, and is at the upper limit of the known variation of conspecific fish populations in a single drainage. Allozyme variability is low in the diploid unisexual. Three genotypes were detected, two at but a single location. All three diploid clones have been detected in adjacent river systems. In contrast, variability among the triploids was rather high. A total of eight distinct allozyme genotypes were detected among 48 individuals, and the likelihood is great that clonal diversity or at least the number of clonal origins has been severely underestimated. The triploid genotypes do not represent the random combination of host and P. formosa genomes. In particular, a host allele at a Pgm locus is over-represented among the triploids. There is some evidence that interclonal selection may be a significant force within the triploid population, but that evidence is not yet conclusive. Pregametic selection (so that some triploid genotypes are intrinsically more probable than others) and gametic selection may also have a role in regulating triploid clonal diversity. The possibility that interclonal selection, with its associated clonal specializations, is a significant factor within the system makes it of potential general interest. Each triploid clone represents a 'flash frozen' haploid host genotype that is likely a unique array of alleles at polymorphic loci. Thus, the triploids may provide useful insight into the role of linkage disequilibrium (or other types of allelic association) in the emergence of ecologically significant adaptations.