Geographic variation in skin structure in male Andrew's toad (Bufo andrewsi): supplementary material

Variation in organ structure likely provides important clues on local adaptation and reflects the pressure target of natural selection. As one of the important organs, the skin plays a key role in adapting to complex environments by reducing water loss or increasing water absorption. Nevertheless, variation in the skin structure across different populations in a single species of anurans remains enigmatic. Here, we studied geographical variation in the skin structure of male Andrew’s toads (Bufo andrewsi) across ten populations using histological methods. We quantified thickness of the skin, the epidermis, the loose layer, the compact layer, and of the epidermis, area of granular glands (GGs) and of ordinary mucous glands (OMGs), width of the calcified layer, and number of capillary vessels. We found that the thickness of the skin, dermis and loose layer in dorsal skin increased with latitude whereas the area of granular glands decreased with altitude. Moreover, the width of the calcified layer in ventral skin decreased with latitude among populations. Our findings suggest that geographical variation in skin structure in male B. andrewsi is likely to reduce water loss or make water absorption occur faster in complex high-latitude environments, improving local adaptation.