Evolution of developmental plasticity in ambystomatid salamanders

Developmental plasticity was studied in three salamander species in the family Ambystomatidae. Two species (Ambystoma maculatum and Ambystoma texanum) have the ancestral reproductive mode of breeding in ponds while Ambystoma barbouri breeds in highly ephemeral yet highly oxygenated streams. The stream species has evolved larger egg size, more rapid development and earlier metamorphosis than its sister species, A. texanum. Oxygen and yolk reserve levels were experimentally manipulated to examine their roles in the evolution of the derived stream-breeding strategy. The response to oxygen in the pond species changed direction developmentally. Embryos grew larger in high oxygen while larvae grew larger in low oxygen. The stream species grew much larger in high oxygen throughout development. These results show oxygen's effects on body size not only change developmentally, but evolve in magnitude and direction. In the embryonic manipulation experiments, surgical yolk removal in all three species showed suites of larval traits could cascade across multiple life stages. But the response to yolk removal varied between species. In A. maculatum and A. barbouri, embryonic yolk removal had persistent effects at hatching and well into the larval period. However in A. texanum, yolk removal had minimal effects. This dissertation shows the developmental responses to oxygen levels and yolk removal have evolved in three ambystomatid salamander species that experience very different developmental conditions in natural vernal pools and streams.