Phenotypic Variation in Metamorphosis and Paedomorphosis in the Salamander Ambystoma talpoideum

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Phenotypic variation in metamorphosis and paedomorphosis in the salamander Ambystoma talpoideum was examined to determine its environmental or genetic basis. Eight artificial ponds were maintained, four at each of two environmental treatments: constant water level, to simulate fish free permanent breeding ponds, and gradual drying out, to simulate temporary breeding ponds. Two populations of salamanders were used, derived from two breeding ponds having different frequencies of paedomorphosis. The water level in the drying treatment was lowered during the last 10 wk of the experimental period with no apparent differences in water chemistry parameters between treatments and only a slight change in water temperatures during the last 2 wk. The quality of the growth environment for larvae, as measured by body size and survivorship, was the same in the four experimental treatments. The frequency of metamorphosis was significantly higher in the drying treatment that under constant water level, and also differed between populations. The frequency of paedomorphosis was significantly different between populations but not between water level treatments. The effects of water level were potentially confounded by those of water temperature, density of larvae, and amount of food. Population differences in the frequency of metamorphosis and paedomorphosis could potentially represent genetic differences resulting from the different selective regimes that individuals encounter in breeding ponds varying in drying frequency. However, phenotypic plasticity and nongenetic maternal effects cannot be dismissed without further experimentation.