Abstract
Phenotypic differences among species may evolve through genetic accommodation, but mechanisms accounting for this process are poorly understood. Here we compare hormonal variation underlying differences in the timing of metamorphosis among three spadefoot toads with different larval periods and responsiveness to pond drying. We find that, in response to pond drying, Pelobates cultripes and Spea multiplicata accelerate metamorphosis, increase standard metabolic rate (SMR), and elevate whole-body content of thyroid hormone (the primary morphogen controlling metamorphosis) and corticosterone (a stress hormone acting synergistically with thyroid hormone to accelerate metamorphosis). In contrast, Scaphiopus couchii has the shortest larval period, highest whole-body thyroid hormone and corticosterone content, and highest SMR, and these trait values are least affected by pond drying among the three species. Our findings support that the atypically rapid and canalized development of S. couchii evolved by genetic accommodation of endocrine pathways controlling metamorphosis, showing how phenotypic plasticity within species may evolve into trait variation among species.
Highlights
Phenotypic differences among species may evolve through genetic accommodation, but mechanisms accounting for this process are poorly understood
The divergence of reaction norms among spadefoot toad species is consistent with evolution by genetic accommodation of the timing of metamorphosis driven by selection in ponds with different hydroperiods[13, 16, 17]
Because there is evidence that standard metabolic rate (SMR) can be directly increased by exogenous CORT in other amphibians like red-legged salamanders (Plethodon shermani)[24], we investigated whether experimentally manipulating CORT would influence SMR in spadefoot toads
Summary
Phenotypic differences among species may evolve through genetic accommodation, but mechanisms accounting for this process are poorly understood. Individuals undergoing stress-induced metamorphosis exhibit morphological consequences of the increased hormone contents, including smaller body size, shorter relative limb length, and reduced levels of abdominal fat[16, 17] These morphological changes can be induced by exogenous treatment with TH and CORT18, 19. In comparison to the other spadefoot species, S. couchii has reduced size at metamorphosis, shorter hind limbs, altered head shape, altered timing of gonad differentiation, and smaller abdominal fat bodies[10, 13, 16, 17] These morphological traits are constitutively expressed in S. couchii but observed in P. cultripes and S. multiplicata only when they are exposed to low water levels. We hypothesize that species differences in larval period and associated traits evolved through genetic accommodation via altered endocrine regulation
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