Clash of the swordtails
Hundreds of conflicts within the genomes of hybrid swordtail fish keep two species distinct—even though they often interbreed.
In nature, closely related species often interbreed to produce hybrids. However, hybrids are often less fertile or unable to compete with parent species, making them less likely to thrive in the wild.
When the genomes of two different species are mixed, versions of genes that are meant to work together can become separated, which means that these genes do not work as well as they should. This reduces the hybrids’ chances of survival, and a poor survival rate of hybrids is one barrier that keeps different species distinct, even though the species can interbreed.
Two species of swordtail fish live in the rivers in Mexico, and although they mostly live in different stretches of these rivers, the two species interbreed to produce hybrids in the regions where they overlap. These hybrids can outnumber the parental species in these ‘hybrid zones’, but the two species have remained separate in other parts of the rivers. Though some genetic incompatibilities that might keep the species distinct have previously been suggested, it is not known how many incompatibilities there are in these fish’s genomes.
Molly Schumer and co-workers have searched the genomes of wild hybrids between these species and found hundreds of genetic incompatibilities. These were identified by looking for species-specific pairs of genes that are found together more often than would be expected if there were no selection against hybrids. It is likely that many of these incompatibilities reduce the evolutionary fitness of the hybrid fish and Schumer and co-workers suggest that many could be the result of environmental pressures and the fish’s mating preferences. Furthermore, Schumer and co-workers demonstrate that genes close to identified incompatibilities are more different between species, on average, than genes that are further away. When there is on-going interbreeding between two the species (as is the case with the swordtails), this finding is expected only if these incompatibilities reduce the hybrids’ chances of finding mates or surviving.
The findings of Schumer and co-workers demonstrate how conflicts in the genomes of two species allow these species to remain distinct even when they live in overlapping environments and frequently interbreed. Future work will investigate how these genetic incompatibilities shape the hybrid populations that are found in the wild; and which incompatibilities are caused by poor survival of the hybrids or by the fish’s mating preferences selecting against the hybrids.
To find out more
Listen to Molly Schumer discuss hybridizing fish species in episode 13 of the eLife podcast.
Read the eLife research paper on which this story is based: “High-resolution mapping reveals hundreds of genetic incompatibilities in hybridizing fish species” (June 4, 2014).
Want to read more about
evolution and speciation?
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
