Maintenance of the Adh polymorphism in Ambystoma tigrinum nebulosum (tiger salamanders). I. 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 evolutionary processes might be affecting genetic variation at the alcohol dehydrogenase (Adh) locus. Ponds frequently dried in late summer; salamanders that did not metamorphose and leave drying ponds died of desiccation. Low levels of water oxygen significantly slowed metamorphosis of tiger salamanders in both natural and laboratory populations. Adh- SS frequency was significantly positively correlated with daily oxygen maxima in ponds, Adh-FF frequency demonstrated a nonsignificant trend to be negatively correlated with this measure, and Adh-SF frequency was unrelated to pond oxygen. Genotype frequency changes across the season in hypoxic and supersaturated ponds suggested that differential mortality of genotypes might explain the relationships with pond oxygen, although other mechanisms cannot be ruled out. Individuals of the Adh-SS genotype metamorphosed more slowly in hypoxic water than in normoxic 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 metamorphosed at the same rate in all oxygen environments. These data suggest that the Adh polymorphism in tiger salamanders is being maintained by selection for Adh or for a linked locus through differential metamorphosis in extreme oxygen environments.
Carter
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Department of Environmental, Population and Organismic Biology, Campus Box 334, University of Colorado, Boulder, CO 80309, USA. pacarter@wsu.edu