GENETIC-DIFFERENCES IN MALE DEVELOPMENT TIME AMONG POPULATIONS OF THEENDANGERED GILA TOPMINNOW

Timing of male sexual maturity was compared in a common laboratory env ironment for populations from the four watersheds in which the Gila to pminnow (Poeciliopsis o. occidentalis) still remains in Arizona. One p opulation, Monkey Spring, was found to have an approximately 50% later development of male sexual maturity. Monkey Spring is the only popula tion of the four whose natural habitat has both a constant and warm, y ear-round temperature and year-round reproduction. Year-round reproduc tion is a common strategy in tropical fish species in warm constant en vironments. Our findings are consistent with the hypothesis that the o ther three populations-Bylas Springs, Cienega Creek, and Sharp Spring- have adapted to temporally variable environments and seasonally limite d reproduction with earlier male maturation. This genetic difference i n a fitness-related trait lends support to the recommendation that Gil a topminnows from different watersheds be managed and conserved separa tely.