Spadefoot toads are known to be shape-shifters; they can rapidly change their behavior and physiology to adapt to environmental changes. This makes them excellent models for the study of plasticity and adaptive evolution. Learn more from the video below.
These toads are unusual creatures. They dwell in the desert, unlike typical amphibians, and have acquired many adaptations. The genome of one type of Spadefoot Toad (Spea multiplicata) has been recently sequenced and reported in the journal G3 by a team of researchers from the University of Southern California and the University of North Carolina, Chapel Hill.
In their study, they were able to identify genes that had undergone positive selection; when a gene confers some benefit to an organism and because the gene is advantageous, it spreads through that population. Now that scientists have obtained the sequence of S. multiplicata, they will be able to learn more about the unique changes in this animal.
A 2017 study reported in Nature Communications used the spadefoot toad to learn more about how animals may change in response to a drier and warmer climate. The research team from the University of Cincinnati (UC) found that the toads were able to alter the course of their development during larval stages. By sensing cues from their environment, they could change the production of hormones, in a demonstration of 'phenotypic plasticity.'
"We found that when certain species combined stress hormones produced by the adrenal gland with hormones produced in the thyroid gland, this was key in speeding their development and shortening the larval periods. This process helps assure that they metamorphose or hatch before the pond dries up," explained the senior author of that study Daniel Buchholz, a UC associate professor.
In this work, the investigators examined American and Spanish species of Spadefoots that dwell in semipermanent or permanent ponds. When these animals perceived that their pond was drying out, they were able to regulate their development to accommodate these changes.
A species of toad that lives in the dry conditions of Arizona, however, had already changed the way they developed. This might be because their environment may have once been variable but was now primarily dry all the time. These toads were not good at adapting to changing water levels in ponds, unlike their counterparts from different areas.
"We think the ancestors enabled desert survival because their physical characteristics were flexible and they were able to respond to a drying pool by freaking out and having a stress-induced increase in metamorphic rate," said Buchholz. "This enabled them to shorten their larval periods from months and weeks to just days by switching on corticosterone stress hormones that act synergistically with thyroid hormones to accelerate the metamorphosis to speed the development.
"Eventually this ability becomes fixed as the desert species consistently encountered short duration ponds. This allowed the tadpoles to use newly developed lungs to escape the drying pond and move safely onto land."
Sources: G3, University of Cincinnati, Nature Communications