Toads get the jump on evolution

 

UC research links hormones and changing climate to altered evolution for spadefoot toad species.

 

 

By Melanie Schefft
513-556-5213

Photos provided

Oct. 25, 2017

Deep within aquatic ecosystems all over the world, certain toad species have managed to survive warmer and drier climate change. But the method behind their metamorphic adaptation to environmental change over time has long puzzled scientists –– until now.

NSF-funded research at the University of Cincinnati reveals how certain species of the pudgy spadefoot toads manage to ramp up their larval periods. They instinctively regulate their hormone production altering their developmental timetables –– all in an adaptive response to changes in their environment.  

Daniel Buchholz, UC associate professor and biologist on the study, credits this survival process to a theory called “genetic accommodation” –– hereditary changes that occur in response to a changing environment.

“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,” says Buchholz.

The study, recently published in Nature Communications, sheds light on how metamorphosis is controlled by hormone changes in certain species of spadefoot toads, supporting the theory of genetic modification by way of “phenotypic plasticity” or a change in physical characteristics of any plant, animal or microbe as a reaction to changes in their environment over time.

Three spadefoot toad species at the same stage in the beginning of metamorphosis, but different ages. Top is 20-day-old Pelobates cultripes, middle is 13-day-old Spea multiplicata and bottom is 7-day-old Scaphiopus couchii (desert).

Life finds a way

To illustrate this phenomenon, the researchers spanned the globe and collected three species within the spadefoot family. They compared the Pelobates cultripes from Doñana, Spain, to two different species from Arizona.

In controlled experiments in the laboratory at UC, Buchholz and Saurabh Kulkarni, UC biology graduate student at the time and first author on the publication, compared larval periods directly to each other without outside environmental impact.

They found that Spanish and American species indigenous to permanent and semipermanent pond conditions had more flexible control over their development and could activate their own metamorphosis when they perceived their pond drying up too soon.

On the other hand, the desert species Scaphiopus couchii from the dry American southwest had already developed shorter and shorter larval periods –– primarily in response to environment conditions that gradually became more ephemeral or short-lived over time. At this stage of evolution, the desert toads were unable to react to pond water level and their short larval periods were now fixed.

While many of a given species will die if introduced to a profound change in their environment, Buchholz argues, some of the more hearty and healthier of the species will eventually evolve to survive in a new environment in an evolution by natural selection. But Buchholz says the key issue with these desert species is, “How did that process occur exactly?”

“One of the effects from rapid larval development in the desert toads is they don’t have certain physical characteristics that their cousins in wetter environments have,” says Buchholz. “”For example, the desert spadefoot is smaller and doesn’t have the metatarsal tubercles to dig, and they don’t have their gonads differentiated at that point.

“So there are consequences for having this rapid metamorphosis.”

But genetic modifications over time enabled desert species not only to survive but also to thrive in this new previously inhospitable environment.

Spadefoot species studied are from diverse environments. Top Pelobates cultripes from a permanent pond in Doñana National Park, Spain; middle Spea multiplicata resides in semipermanent ponds in the American southwest and bottom Scaphiopus couchii, the smallest with the shortest larval period lives in short-lived ponds in Arizona. Photo courtesy of Daniel Buchholz, Nature Communications and Creative Commons

Which came first?

In the complex cycle of survival, the researchers pondered whether the genes changed first or did a phenotypic plasticity allow for the survival that subsequently enabled their genes to change to ultimately survive in desert pools?

The researchers say their evidence is the best so far for showing how species’ differences may come about directly as a consequence of evolution of phenotypic plasticity in a changing environment.

For example, other scientists have shown similar effects in experimental populations of insects or within species with various populations living in different environments. They can see changes in the phenotype because they have been living in different environments. But the researchers say this study is the clearest example where differences between species can be understood based on evolution from phenotypic plasticity.

“One of my basic questions has been, ‘Why do these animal as well as human species look different from each other?’” asks Buchholz.

“What we have found about a major unappreciated piece of the explanation is that species differences in physical characteristics may be initially caused by environmental sensitive hormone levels controlling development followed by genetic changes that set the physical shape or morphological difference.”