Genetic differences in thermal tolerance of eastern mosqyitofish (Gambusia holbrooki; Poeciliidae) from ambient and thermal ponds

Two populations of eastern mosquitofish (Gambusia holbrooki; Poeciliidae) in South Carolina, one in an ambient temperature pond and the other in a pond heated to near-lethal temperatures by nuclear reactor effluents for 60–90 mosquitofish generations, offered an excellent opportunity to observe selection for increased thermal tolerance. We performed three experiments. First, we determined the critical thermal maximum of each population and, as predicted, found the thermal population to have a higher one. We then exposed fish from both populations to an acute thermal LD50 stress and compared genetic diversity of fish that died and fish that survived. Survivors had higher heterozygosities, indicating that genetic diversity may contribute to thermal tolerance. Finally, we used a half-sib – full-sib experimental design to estimate heritabilities for temperature tolerance in fish from the heated pond. We calculated a narrow-sense heritability for temperature at death of over 32%, indicating that selection has not depleted the population of genetic variation associated with thermal tolerance. Our results have implications for climate change because adaptations to higher thermal regimes must, in part, come from selection on genetic variation for temperature tolerance within populations.