September 2, 2021

A new publication on a long-term comparative study of amphibian hybrid zones, for which Tomasz Suchan collaborated, made the cover of the PNAS.

How can related populations evolve into distinct species unable to crossbreed anymore? This question has puzzled evolutionary biologists ever since Darwin. Some researchers imagine a rapid process caused by mutations at a few key genes that govern mate choice and survival in the wild. Hybridizing then becomes a bad idea, because the intermediate offspring are unattractive and maladapted to either of their parental environments. Others rather see the effects of the gradual yet random differentiation of their entire genomes through time. During the time two lineages remain isolated, all their genes diverge and progressively become incompatible, eventually inducing developmental and fertility issues in hybrids.

To find out where the truth lies, the team has been studying amphibian hybrid zones – the regions where genetic lineages meet and eventually interbreed, so they offer natural laboratories to study the evolution of species. For each, they sequenced parental and hybrid individuals at thousands of genetic markers to see how many genes are involved in reproductive isolation. This was only possible through a massive collaborative effort involving renowned colleagues from many countries. At the end more than forty pairs of species could be compared, representative of all genera of frogs and toads known to the old continent.

Results show that reproductive isolation in amphibians initially evolve through multiple genetic incompatibilities spread across the entire genome, not just a few key genes.

Old lineages have more problems to hybridize than younger lineages, because incompatibilities affect more genes. When lineages are young (a few glacial cycles), the entire genome mix up without trouble, sometimes across large geographic distances – more than one hundred kilometers for subspecies of parsley frogs in Catalonia. This is a sign that their similar genomes have not yet lost compatibility.

Several million years later, however, the exchange of genes becomes extremely restricted –no more than a few kilometers for species of fire-bellied toads inhabiting the Carpathian Mountains. The reason is that hundreds of genes no longer function properly. We call them “barrier genes” since they are causing reproductive barriers even if hybridization occurs.

Moreover, the data indicate that these barrier genes are not necessarily those located on the sex chromosomes, as in mammals or birds. Sex chromosomes (XY) are expected to pose more problems in hybrids than any other chromosome pair, partly many genes have disappeared from the Y. Consequently, incompatibilities involving X-linked genes are fully expressed in XY male hybrids. These males are often sterile, if they even develop at all.

But the Y chromosomes of frogs are in good health (see the recent MBE paper on Hyla here). They share the same genes as the X, only sometimes with slightly diverged copies. Accordingly there are more barrier genes on the sex chromosomes of frogs where the Y is slightly differentiated from the X, but not in those that carry nearly identical sex chromosomes. This is an indirect cue that X-Y differentiation might bolster genetic incompatibilities, and thus accelerates reproductive isolation.

These findings thus give new explanations as to why amphibian species are usually older than other vertebrates. Reproductive isolation might evolve more slowly in amphibians, because it takes time to accumulate the many incompatibilities requested to restrict hybridization. Even more so if sex chromosomes do not boost the process as in mammals or birds.

Citation

Dufresnes C, Brelsford A, Jeffries AL, Mazepa G, Suchan T, Canestrelli D, Nicieza A, Fumagalli L, Dubey S, Martínez-Solano I, Litvinchuk SN, Vences M, Perrin N, Crochet PA, 2021. Mass of genes rather than master genes underlie the genomic architecture of amphibian speciation. Proceedings of the National Academy of Sciences 118: e2103963118. DOI