MAINTENANCE OF THE ADH POLYMORPHISM IN AMBYSTOMA-TIGRINUM-NEBULOSUM (TIGER SALAMANDERS) .1. GENOTYPIC DIFFERENCES IN TIME TO METAMORPHOSIS IN EXTREME OXYGEN ENVIRONMENTS

Populations of gilled tiger salamanders (Ambystoma tigrinum nebulosum) living in ephemeral ponds were studied in order to determine what evo lutionary processes might be affecting genetic variation at the alcoho l dehydrogenase (Adh) locus. Ponds frequently dried in late summer; sa lamanders that did not metamorphose and leave drying ponds died of des iccation. Low levels of water oxygen significantly slowed metamorphosi s of tiger salamanders in both natural and laboratory populations. Adh -SS frequency was significantly positively correlated with daily oxyge n maxima in ponds, Adh-FF frequency demonstrated a nonsignificant tren d to be negatively correlated with this measure, and Adh-SF frequency was unrelated to pond oxygen. Genotype frequency changes across the se ason in hypoxic and supersaturated ponds suggested that differential m ortality of genotypes might explain the relationships with pond oxygen , although other mechanisms cannot be ruled out. Individuals of the Ad h-SS genotype metamorphosed more slowly in hypoxic water than in normo xic water in the laboratory, and Adh-FF individuals metamorphosed more slowly in supersaturated water in the field, and in hypoxic water in the laboratory, than they did in normoxic water. Adh-SF individuals me tamorphosed at the same rate in all oxygen environments. These data su ggest that the Adh polymorphism in tiger salamanders is being maintain ed by selection for Adh or for a linked locus through differential met amorphosis in extreme oxygen environments.