Ancestral variation and the potential for genetic accommodation in larval amphibians: implications for the evolution of novel feeding strategies

Few studies provide empirical evidence for phenotypic plasticity's role in the evolution of novel traits. One way to do so is to test whether latent plasticity is present in an ancestor that can be refined, enhanced, or diminished by selection in derived taxa (through genetic accommodation), thereby producing novel traits. Here, we evaluated whether gut plasticity preceded and promoted the evolution of a novel feeding strategy in spadefoot toad tadpoles. We studied Scaphiopus couchii, whose tadpoles develop an elongate gut and consume only detritus, and two derived species, Spea multiplicata and Sp. bombifrons, whose tadpoles also express a novel, short-gut phenotype in response to a novel resource (anostracan shrimp). Consistent with the expectations of plasticity-mediated trait evolution, we found that shrimp induced a range of phenotypes in Scaphiopus that were not produced with detritus. This plasticity was either suppressed or exaggerated in Spea depending on whether the induced phenotypes were adaptive. Moreover, in contrast to its effects on morphology, shrimp induced little or no functional plasticity, as assessed by gut cell proliferation, in Scaphiopus. Shrimp did, however, induce substantial proliferation in Sp. bombifrons, the species that consumes the most shrimp and that produces the short-gut phenotype the most frequently. Thus, if Spea had ancestral morphological plasticity in response to a novel diet, their shrimp-induced short-gut morphology may have undergone subsequent genetic accommodation that improved its functionality. Hence, diet-induced phenotypic plasticity may have preceded and even promoted the evolution of a novel phenotype.