STORZ, Brian/L; Florida State University: A Reassement of the Proximate Mechanisms of Polyphenism in Spadefoot Toad Tadpoles

Understanding the proximate mechanism of ontogenetic polyphenism is the first step in gaining a mechanistic and evolutionary understanding of the factors responsible for the control and evolution of polyphenism. Tadpoles of the spadefoot toad Spea multiplicata can display either a �typical� omnivorous phenotype or a carnivorous phenotype in ephemeral ponds in the deserts of the American Southwest. Exogenous thyroxine and feeding on conspecific tadpoles have been accepted as the proximate mechanisms for developmental polyphenism in these tadpoles since they were first proposed 20 years ago. Recent and past research on the effects of exogenous thyroxine on amphibian development are at odds with the current understanding of the role of exogenous thyroxine on developmental polyphenism in spadefoot toad tadpoles. I demonstrate that neither exogenous thyroxine nor feeding on conspecific tadpoles triggers developmental polyphenism in spadefoot toad larvae and show how heterochronic processes mislead us into believing that these proximate mechanisms control developmental polyphenism in spadefoot toad tadpoles.