Genetic differences in male development time among populations of the endangered Gila topminnow

Timing of male sexual maturity was compared in a common laboratory environment for populations from the four watersheds in which the Gila topminnow (Poeciliopsis o. occidentalis) still remains in Arizona. One population, Monkey Spring, was found to have an approximately 50% later development of male sexual maturity. Monkey Spring is the only population of the four whose natural habitat has both a constant and warm, year-round temperature and year-round reproduction. Year-round reproduction is a common strategy in tropical fish species in warm constant environments. Our findings are consistent with the hypothesis that the other three populations-Bylas Springs, Cienega Creek, and Sharp Spring-have adapted to temporally variable environments and seasonally limited reproduction with earlier male maturation. This genetic difference in a fitness-related trait lends support to the recommendation that Gila topminnows from different watersheds be managed and conserved separately.