It's complicated: Researchers discover unexpected complexity of sexual differentiation in African clawed frog

A genomic analysis has uncovered a total of eight different sex chromosomes in just 11 species of the frog, many or all of which may contain unique and newly evolved genes that trigger male or female sexual differentiation.

Previously, researchers knew of only three different sex chromosomes in these frogs.

"In these frogs, we've discovered extraordinary variation even among closely related species, which allows us to explore how important things like sex determination evolve rapidly," says Ben Evans, a professor in the department of Biology, and lead author of a new study in the journal Molecular Biology and Evolution. Evans conducted the work with colleagues from the Czech Republic, France, the U.S., and South Africa.

The African clawed frog is used as a model organism for biological research because of its close evolutionary relationship to humans, and because early development occurs externally, allowing fundamental processes to be readily observed and manipulated.

The frogs are found in sub-Saharan Africa and live in slow-moving or stagnant water. They are known for their flat bodies, vocal organs that can produce sound underwater, and claws on the first three toes of the hind feet, which they use to tear food apart.

In this study, the researchers pinpointed the locations of the newly identified sex chromosomes, which added to their surprise.

Prevailing theory had suggested that sex-determining genes might typically arise in regions of the genome with a low rate of recombination -- the exchange of genetic material within each parent that creates new mixtures of traits in their offspring.

But in this study, they found these newly evolved genes were almost universally located in regions where genetic recombination is high, raising questions about how and why the genetic basis of very important biological traits -- such as sexual differentiation -- may evolve so quickly, and how new genes and genetic function arise.

"If you conducted these same tests within some even older groups such as most mammals or all birds, you would find that their sex chromosomes are all the same," explains Evans. "But this group of frogs -- in sharp contrast -- has incredible variation."

"It is very likely that new genes arose many times in these frogs to orchestrate sexual differentiation, by acting as an 'on-off switch' or a 'male-female switch' at the top of the developmental cascade," he says.

In 2015, Evans -- who has studied the African clawed frog for over two decades -- led a team that discovered six new species and added another back to the list of known species, providing the foundational information for subsequent work, including the new study.