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Biology

Ain’t no killing the killifish (for now): on the virtues of genetic diversity

Reid N.M., Proestou D.A., Clark B.W., Warren W.C., Colbourne J.K., Shaw J.R., Karchner S.I., Hahn M.E., Nacci D., Oleksiak M.F., Crawford D.L., and Whitehead A. The genomic landscape of rapid repeated evolutionary adaptation to toxic pollution in wild fish. Science. 9 December 2016. DOI: 10.1126/science.aah4993.

Ladies and gentleman, the death defying killifish!

Ladies and gentlemen, the death defying killifish!

We humans have figured out how to harness lots of chemicals to great benefit (e.g. as pesticides, herbicides, flame retardants), but not without consequence to the environment. The release of chemical pollutants into the environment over time results in a cocktail of toxins that wreak havoc on ecosystems. Resistant to degradation, these chemicals persist in the environment, making their way up the food chain, and accumulating in fatty tissues of top predators. Animals unfortunate enough to be caught in polluted environments must adapt or face extinction.

The genetic changes that enable adaptation require a luxury of time over the course of many generations, which most animals simply haven’t got. Nonetheless, some species manage to survive. One of those death defying species is the banded killifish, which inhabit the coastal marshlands of the northeastern United States. Remarkably, some populations of killifish in this region show resistance to typical environmental pollutants at levels that are lethal to individuals of the same species that occupy nearby, but geographically isolated habitats. So, how has resistance come to be among the Atlantic killifish? A recent study conducted by a team of U.S. and U.K. scientists led by Noah Reid at the University of California, Davis published in the journal Science suggests that the reason these fish survived is that they had it in them to do so from the start.

Geographic locations of populations of resistant (T) and susceptible (S) killifish sampled in this study.

Geographic locations of populations of resistant (T) and susceptible (S) killifish sampled in this study. (Image courtesy Reid et al., Science 2016)

The researchers set about their study by contrasting the genetic make-up of four proximal pairs of resistant-susceptible populations of killifish occupying the stretch of coastline from Virginia to Rhode Island. By randomly sequencing the genomes of fish from these populations, as well as examining the genes that are most “turned on” in given populations (called the “transcriptome”), the researchers were able to pull out signatures associated with resistance to environmental pollutants. In particular, they found changes to genes involved in the detection of “aryl hydrocarbons,” a class of molecules of which many pollutants are a part. These mutations appear to desensitize fish to the affects of toxins. While all resistant populations possessed mutations in the same genetic regions, mutations across separate populations bore their own signatures, suggesting a phenomenon known as “convergent evolution,” one adaptive strategy arrived via manifold genetic variations. Interestingly, the resistant traits also existed in the susceptible population, albeit at a lower rate, implying that resistance was derived from a pre-existing pool of genetic variants. In other words, susceptible variants in polluted waters died off, while the lucky few survived—a phenomenon known as a “genetic sweep.”

The story of the banded killifish reinforces the importance of genetic diversity as an insurance policy against adversity. The good news is that some animals have the capacity to withstand the environmental impacts of human activities thanks to the natural richness of their genetic pool. The not so good news is that with every such perturbation to the environment, diversity goes down, making species ever more susceptible to extinction. Simply put, if we hope to keep our planet’s ecosystem’s healthy, we must be careful where we tread.

Abrahim El Gamal
Abrahim is a PhD student at Scripps Institution of Oceanography in San Diego where he studies marine chemical biology.

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