Skin development and metamorphic re-arrangement in Arthroleptella villiersi Hewitt, 1935 reveals distinct heterochronic patterns in anurans with different life histories

BackgroundThe evolvability of ontogenetic trajectories, driven by modular developmental processes and their evolutionary repatterning, plays a crucial role in the diversity of animal diversity. Anurans offer a model for exploring how developmental repatterning, including the loss of larval features and heterochronic shifts of metamorphic processes lead to the diversity of, often terrestrialized, life histories. Ancestrally, anurans exhibit an aquatic larva (tadpoles) that differs tremendously from their semi-terrestrial adult forms. The transition from this tadpole to the adult is a thyroid hormone-dependent metamorphosis that results in the rearrangement of almost all organ systems. One of the organ systems that experiences the most extensive rearrangements is the skin. Is the ancestral pattern of metamorphic skin rearrangement altered in anurans with a completely terrestrial tadpole phase? ResultsSkin development and metamorphic rearrangement in the terrestrial tadpole of Arthroleptella villiersi is investigated based on histological sections. It follows the major pattern observed in anurans with an aquatic tadpole phase and correlates with thyroid gland maturation. Neuromasts develop and are present during a short developmental time. Both, metamorphic skin rearrangement and neuromast degeneration correlate with histological thyroid gland maturation. A few minor heterochronic shifts are detectable compared to aquatic indirect and terrestrial direct developing anurans. ConclusionSkin development and metamorphic rearrangement in anurans with different reproductive modes is very similar and correlates with thyroid gland maturation indicating that skin development in anurans might be constrained by thyroid hormone activity. Neuromasts are the major exception being present in aquatic and terrestrial indirect developing species but not in terrestrial direct developing anurans. Heterochronic shifts detected during skin development indicate that at least the differentiation of multicellular gland complexes might represent a developmental module similar to neuromasts.