New study on South American grass frogs:Battle between molecular forces leads to toxin resistance

31 May 2021, by MIN-Dekanat

Photo: flickr/Brian Gratwicke (https://www.flickr.com/photos/briangratwicke/5414973624)

Leptodactylus insularum.

A gene duplication leads to frogs of the genus Leptodactylus being resistant to certain poisons. This was discovered by an international research team with the participation of the Department of Biology at Universität Hamburg. The results show that the study of gene duplicates can help identify important genetic interactions.

Gene duplication is a powerful tool in evolution because it creates a copy of the gene without harmful effects for the organism. The additional copy serves as a safety backup of the genetic material, allowing the other copy to more freely mutate and possibly gain a new function. In a study, scientists from the Department of Biology at Universität Hamburg have now investigated the evolution of a gene duplication in frogs of the genus Leptodactylus and thus deciphered the mechanism that led to a specialised adaptation in the animals that makes them resistant to cardiotonic steroids. The frogs feed on toads, which use these poisons as a defence against predators.

For the study, the research team analysed genetic material from different species of Leptodactylus and found that a duplication of the gene ATP1A1 in the ancestor of these frogs led to their resistance. ATP1A1 codes an enzyme (ATPase) that is vital for all animal cells: the sodium-potassium pump (or Na+K+-ATPase). "What was striking was that all species shared twelve amino acid changes in the duplicated gene that were not found in the original gene," says Prof. Dr. Susanne Dobler from the Department of Biology and co-author of the study.

The DNA patterns further revealed that outside of these twelve changes, everything else had remained very similar over tens of millions of years between the original and duplicated gene. The team found that this similarity was caused by a molecular mechanism that restricts the evolution of gene duplicates by working to keep duplicated genes identical (known as non-allelic gene conversion). Further analyses by the researchers revealed that the twelve changes managed to escape this homogenizing force through the opposing power of natural selection – the twelve changes are important for the survival of the frogs.

Two of these so-called amino acid substitutions are known to produce resistance to cardiotonic steroids. The toxins affect the sodium-potassium pumps by deactivating them. In the worst case, this can be fatal for the hunters. "Because the frogs are known to feed on toads, it makes perfect sense that they have a copy of this gene that makes them resistant to the toxins," says Dr. Shabnam Mohammadi, co-author of the study, who is currently doing research in the Department of Biology with Prof. Dr. Dobler. "This is a textbook example of what is known as neofunctionalisation, where a gene acquires a new function after a duplication event." In this case, a new function evolved between a resistant gene copy, called the R copy, and a copy that retained the original susceptibility, the S copy.

However, protein engineering experiments and functional assays also revealed that the two amino acid substitutions come at a high price: When added both individually and together to the S protein, they drastically reduce the activity of the enzyme (Na+K+-ATPase). The resistance-promoting substitutions thus offer an advantage at the expense of enzyme function. "This is where the functional importance of the other ten substitutions becomes apparent," says Mohammadi. "Because when we added the ten amino acid substitutions together with the two resistance-producing substitutions, the enzyme activity was rescued."

The two substitutions increase the frogs' toxin resistance, while the additional ten substitutions support ATPase activity. The scientists' results show how studying the genetic signals of evolution can help trace important functional changes underlying adaptations.