Diet and hormonal manipulation reveal cryptic genetic variation: implications for the evolution of novel feeding strategies.

When experiencing resource competition or abrupt environmental change, animals often must transition rapidly from an ancestral diet to a novel, derived diet. Yet, little is known about the proximate mechanisms that mediate such rapid evolutionary transitions. Here, we investigated the role of diet-induced, cryptic genetic variation in facilitating the evolution of novel resource-use traits that are associated with a new feeding strategy--carnivory--in tadpoles of spadefoot toads (genus Spea). We specifically asked whether such variation in trophic morphology and fitness is present in Scaphiopus couchii, a species that serves as a proxy for ancestral Spea. We also asked whether corticosterone, a vertebrate hormone produced in response to environmental signals, mediates the expression of this variation. Specifically, we compared broad-sense heritabilities of tadpoles fed different diets or treated with exogenous corticosterone, and found that novel diets can expose cryptic genetic variation to selection, and that diet-induced hormones may play a role in revealing this variation. Our results therefore suggest that cryptic genetic variation may have enabled the evolutionary transition to carnivory in Spea tadpoles, and that such variation might generally facilitate rapid evolutionary transitions to novel diets.