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A key metabolic gene for recurrent freshwater colonization and radiation in fishes

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Science  31 May 2019:
Vol. 364, Issue 6443, pp. 886-889
DOI: 10.1126/science.aau5656

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Well prepared

It is well known that species radiate into new niches by adapting to novel environments. But why do some species radiate in this way, while other, related, species do not. Ishikawa et al. looked across sticklebacks to determine why some, originally marine, lineages were able to colonize postglacial freshwater environments (see the Perspective by Weber and Tong). They found that a gene involved in fatty acid desaturation was duplicated in freshwater lineages. Transgenic manipulation of this gene allowed marine lineages to synthesize fatty acids and thus survive on fatty acid–deficient freshwater diets.

Science, this issue p. 886; see also p. 831

Abstract

Colonization of new ecological niches has triggered large adaptive radiations. Although some lineages have made use of such opportunities, not all do so. The factors causing this variation among lineages are largely unknown. Here, we show that deficiency in docosahexaenoic acid (DHA), an essential ω-3 fatty acid, can constrain freshwater colonization by marine fishes. Our genomic analyses revealed multiple independent duplications of the fatty acid desaturase gene Fads2 in stickleback lineages that subsequently colonized and radiated in freshwater habitats, but not in close relatives that failed to colonize. Transgenic manipulation of Fads2 in marine stickleback increased their ability to synthesize DHA and survive on DHA-deficient diets. Multiple freshwater ray-finned fishes also show a convergent increase in Fads2 copies, indicating its key role in freshwater colonization.

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