Does size matter? Risk of extinction does not increase in species with larger genomes

Daniel Pincheira-Donoso, Lilly P. Harvey, Jack V. Johnson, Dave Hudson, Catherine Finn, Luke E. B. Goodyear, Jacinta Guirguis, Edel M. Hyland, Dave J. Hodgson

This is a plain language summary of a Functional Ecology research article which can be found here.

The DNA that carries the genetic code of species varies enormously across nature. While the genomes of some species are made of very few genes, the genomes of others can be several orders of magnitude larger in the numbers of genes they contain. However, these differences in DNA size (or ‘genome size’) among species are not only caused by content of functional genes that encode for traits (‘coding genes’), but importantly by large portions of non-functional ‘junk’ DNA that plays no role in coding for the expression of traits. In some organisms, such as salamanders and lungfish, the size of DNA can be exceptionally large, and these peculiar dimensions are importantly explained by recurrent accumulations of junk DNA.

Emerald glass frog (Espadarana prosoblepon) from Ecuador (credit: Roberto García-Roa)

Needless to say, letting junk pile up is likely to have some unpleasant costs. And DNA is not immune to this rule – there’s a lot of evidence that the accumulation of junk DNA can increase the probability of accumulating detrimental mutations that impact on the health of a species as a whole. It is from this observation that an intriguing theory emerged: species that carry larger genomes are burdened with genetic costs that increase their vulnerability to extinction relative to species that have smaller genomes.

In the era of human-induced environmental change as the cause of species extinctions worldwide, this theory has gained popularity. Could scientists predict a species’ risk of extinction from the size of their genomes? Amphibians (frogs, salamanders, caecilians) stand out as unparalleled models to test this theory – these animals not only have the largest range of genome sizes among vertebrates, but also their alarming rates of human-driven extinctions have made them the ‘poster child’ of the modern biodiversity crisis. Using the worldwide diversity of amphibians, this study presents the largest-scale test of the theory that larger genomes increase extinction risk. Contrary to expectations, no evidence to support this prediction was found. A number of factors, including the size of the geographic region occupied by species, their body size and UV-B levels help to explain risk of extinction. Yet, no such role seems to exist for larger genomes.


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