Toxic frogs with weak defenses persist in the gene pool alongside stronger competitors

A multi-national team of evolutionary biologists shows that diversity exists even when expecting otherwise

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Credit: University of Jyvaskyla

Diversity is a hallmark of life and it shows up in unexpected places. A multi-national team of evolutionary biologists investigated how two types of poison frog co-exist when we expect only one. Their innovative study uncovers conditions where diversity flourishes against the odds, and offers new perspectives on chemical defense. The project was a collaboration between the University of Jyvaskylä, the University of Mississippi, Centre national de la recherche scientifique (CNRS) and John Carroll University. The findings were published in the Proceedings of the National Academy of Sciences (PNAS) on September 2nd 2019.

Many creatures carry warning colors that signal toxicity, such as wasps with yellow bands. The Dyeing Poison Frog (Dendrobates tinctorius) of French Guiana also has yellow stripes to keep predators away, but some have white stripes instead. This is an anomaly of evolution because predators learn to avoid warning colors through bad experience, and rare warning colors are harder to learn.

The team headed by Doctors Bibiana Rojas from University of Jyvaskyla and J.P Lawrence from the University of California Irvine propose evolutionary explanations for how nature allows rare warnings to persist.

Firstly, rare or weak signals can persist under the protection of stronger signals. The researchers show that predators learn to avoid more distasteful yellow frogs and generalize this avoidance to white frogs. Although the stronger defenses of yellow frogs should allow them to out-compete the whites over time, genomic analysis revealed that the population of white frogs is genetically separate from the yellows. Thus, diversity can exist within a single population (due to generalized learning) and between different populations (due to genetic isolation).

New methods to measure chemical defence were developed

The initial experiments were modest in scope, but strange results inspired the researchers to develop new methods.

“We faced the difficulty of not having an established method to quantify predator reaction to the chemical defenses. However, that difficulty turned into one of the biggest assets of our study, as we eventually succeeded at developing a method”, says Doctor Bibiana Rojas from University of Jyvaskyla.

The new method improves the ecological relevance and precision of measuring predator-prey interactions. Past studies have assumed that toxicity is everything, and measure it by injecting mice with defensive chemicals. But prey survival and evolution depends on whether they are eaten or spat out, not the blood chemistry of their predators. Thus, the researchers used skin extracts mixed with oats to get a separate measure of distastefulness. They show that toxicity and distastefulness are better considered independently.

“The biggest surprise came from the fact that the frogs [with] higher amount of toxins in their skin are not necessarily the ones that birds find most distasteful. This finding challenges previous assumptions that most toxic equals most unpalatable”, says Rojas.

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The research was done with supervision from Brice Noonan (University of Missisippi) and Johanna Mappes (University of Jyvaskyla).

The research was funded by the Academy of Finland, the Agence Nationale de la Recherche, the American Society of Ichthyologists and Herpetologists, and the Society for the Study of Amphibians and Reptiles.

The research “Weak warning signals can persist in the absence of gene flow” (Lawrence, Rojas, Fouquet, Mappes, Blanchette, Saporito, Bosque, Courtois and Noonan) will be published in PNAS on 2nd September, 2019.
DOI:10.1073/pnas.1901872116

ALL INFORMATION CONCERNING THE ARTICLE IS EMBARGOED UNTIL 3 PM Eastern Time, SEPTEMBER 2, 2019!

For further information:

Doctor Bibiana Rojas, Department of Biological and Environmental Sciences, University of Jyvaskyla, Finland, [email protected], +358 40 805 4622

Doctor J.P. Lawrence, Department of Ecology and Evolutionary Biology, University of California Irvine, USA, [email protected]

Communication officer Tanja Heikkinen, University of Jyvaskyla, [email protected], +358 50 85183 51
https://www.jyu.fi/science/en

Media Contact
Bibiana Rojas
[email protected]

Related Journal Article

http://dx.doi.org/10.1073/pnas.1901872116

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