HABITAT CHOICE BY ALLELIC VARIANTS IN XIPHOPHORUS VARIATUS (PISCES; POECILIIDAE) AND IMPLICATIONS FOR MAINTENANCE OF GENETIC POLYMORPHISM.

Stream pools in northeastern Mexico exhibit gradients in dissolved oxygen (DO) concentration: low DO at the upstream ends and higher DO at the downstream ends. These pools contain populations of the variable platyfish, Xiphophorus variatus, which are polymorphic at an autosomal locus that affects tailspot pigmentation. Previous work showed that adult males of the cut-crescent morph have a lower metabolic rate than wild type males (Borowsky, 1984). Field collections were made to determine whether the morphs are distributed at random in the pools. As predicted, among adult males, all morphs were found in equal proportions at the upstream and downstream ends of the pools, except cut-crescents, which were distributed preferentially toward the upstream ends. In contrast, the opposite distributional pattern was found among adult females. Habitat choice may contribute to the stability of the tailspot polymorphism, especially so because preferences differing between the sexes can lead to disassortative mating.