Abstracts (first author)
Phenotypic divergence among spadefoot toad species reflects accommodation of mechanisms underlying developmental plasticity
Selection in heterogeneous environments favours plasticity as it allows organisms to adapt to rapidly changing conditions. Developmental plasticity allows populations to withstand rapid environmental changes and confers an overall faster rate of adaptation. Conversely, if plasticity costs are high and the environment stabilises, selection results in genetic assimilation, which could result in trait divergence and species diversification. Current evolutionary theory contemplates that phenotypic divergence between species may initiate as environmentally-induced expression of alternative phenotypes. Descendant lineages of a plastic ancestor evolving in stable divergent environments may lose plasticity over time, their development becoming specialised to produce fixed phenotypes matching each environment. In that case, we would expect ancestral plasticity to mirror differences among taxa and that the same mechanism allowing ancestral plasticity was also the main mechanism explainig species divergences. In that light, we are studying mechanisms of plasticity behind the evolutionary divergence of spadefoot toads. Old World species (Pelobates) breed in long lasting ponds and have long but plastic larval periods, whereas New World species (Scaphiopus) have specialised in ephemeral ponds and have evolved very short larval periods. We hypothesise that Scaphiopus has undergone genetic accommodation of ancestral plasticity, which has resulted in canalised short larval periods. To test this hypothesis we have studied the mechanisms underlying developmental acceleration in response to pond drying and compared it across species. We have found that Pelobates tadpoles, which reflect the ancestral state of the group, increase their metabolic rate, and thyroid hormone and corticosterone concentrations in response to decreased water levels. All these parameters, however, seem to have been canalised in Scaphiopus, lending support to the hypothesis of genetic accommodation.