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HETEROZYGOSITY AND FITNESS DIFFERENCES AMONG POPULATIONS OF THE ENDANGERED SONORAN TOPMINNOW (Poeciliopsis occidentalis J.M. Quattro and R.C. Vrijenhoek Center for Theoretical and Applied Genetics Rutgers University, CN 231 New Brunswick, New Jersey 08903 Abstract Remnant populations of the Sonoran topminnow (Poeciliopsis occidentalis) from Arizona contain significantly lower levels of genetic variability than their counterparts in Sonora, Mexico. Three of five Arizona populations surveyed were genetically invariant at 25 protein- determining loci. The purpose of this study was to test the prediction that low genetic variability in remnant populations of an endangered species is associated with a reduction in fitness. Gravid P. occidentalis females were collected from three Arizona populations differing in mean levels of genic heterozygosity (H): Monkey Spring (H = 0.0%), Tule Spring (H = 1.5%), and Sharp Spring (H = 3.7%). First generation laboratory progeny were isolated in a recirculating aquatic incubator and raised to sexual maturity (12 weeks) under conditions of constant temperature, diet, and photoperiod. Four components of fitness were measured at the end of the 12 week experimental period: survivorship, growth, fecundity, and developmental stability. All four fitness components increased with an increase in mean heterozygosity across populations. Monkey Spring topminnows invariably ranked lowest in estimates of survivorship, growth, fecundity, and developmental stability; whereas, the most heterozygous population studied, Sharp Spring, consistently ranked highest. Tule Spring topminnows, intermediate in mean heterozygosity, ranked intermediate to Monkey Spring and Sharp Spring for two characters (growth and fecundity), but performed similarly to Sharp Spring in survivorship and to Monkey Spring in developmental stability. The more variable Sharp Springs fish would serve as the best native population for restocking and reclamation efforts in Arizona. The results emphasize the necessity of genetic as well as demographic information in the formulation of sound conservation and recovery practices. 71