DEVELOPMENT OF THE HYBRID SWARM BETWEEN PECOS PUPFISH (CYPRINODONTIDAE, CYPRINODON PECOSENSIS) AND SHEEPSHEAD MINNOW (CYPRINODON-VARIEGATUS) - A PERSPECTIVE FROM ALLOZYMES AND MTDNA

A comparison of allozyme and mtDNA frequencies was used for insight in to a situation in the Pecos River, Texas where contact between the end emic pupfish (Cyprinodon pecosensis) and an introduced congener (C. va riegatus) has resulted in rapid, geographically extensive genetic intr ogression. Temporal changes in mean frequencies of diagnostic allozyme markers indicate that the clinal pattern of introduced genetic materi al (Echelle and Connor 1989) is slowly decreasing in amplitude. Signif icant rank concordance in diagnostic allele frequencies among sites an d across sampling years indicates directional influences upon temporal allele frequency change. These observations are consistent with the t heory of gene flow in neutral dines. Levels of introgression indicated by each of four allozyme loci and mtDNA were roughly equivalent. The early history of the hybrid swarm is explained by genetic swamping, po ssibly mediated by selection for C. variegatus or C. variegatus X C. p ecosensis, at a time when the normally abundant endemic species had be en catastrophically depleted. High frequencies of an introduced GPI-A allele in all samples of intergrades suggests that the introduced geno me originated with a single founding event.