Laboratory culturing and selection for increased resistance to cadmium reduce genetic variation in the least killifish, Heterandria formosa.

Populations exposed to environmental contaminants can undergo intense selection pressures, which in turn can lead to a loss of genetic variation. We assessed this loss of genetic variation in the least killifish Heterandria formosa for laboratory populations that had undergone eight generations of selection for an increased resistance to cadmium. Using microsatellite markers, we compared genetic variation between three selection and three control laboratory populations and between these laboratory populations and the source population. Heterozygosity was lower in each selection population than it was in its paired control population, with this difference being statistically significant in two of the three comparisons. This is evidence that adaptation to environmental contaminants can result in an overall loss of genetic variation. Furthermore, the laboratory populations had much lower heterozygosity than did the source population. The latter loss of genetic variation, probably a result of random drift, did not prevent the laboratory populations from showing a strong response to the selection for cadmium resistance. The loss of genetic variation resulting from maintaining populations in the laboratory demonstrates that it is important to maintain a large population size for such populations and that the potential for loss of genetic variation in laboratory populations is taken into consideration in ecotoxicology when extrapolating from laboratory to natural populations.