Female (above) and male of the Malawi cichlid Nimbochromis venustus. Photo: Valentina Burskaia, University of Antwerp.
In Lake Malawi, hundreds of cichlid species arose rapidly through multiple large chromosomal inversions that govern sex determination and ecological adaptation.
Dhe iconic species flock of cichlids in the East African Lake Malawi owes its extraordinary diversity to chromosomal inversions that drive sex-specific selection and adaptations for survival at different depths. This is the conclusion of an international group of evolutionary biologists, including Hannes Svardal of Naturalis and the University of Antwerp, following analyses of 1,375 genomes from 240 cichlid species (Science, 13 June).
They identified, among other things, large inversions in five chromosomes (2, 9, 10, 11 and 13) that act as ‘supergenes’ controlling traits essential for survival at different lake depths and for reproduction – including vision, hearing and behaviour. Fish living deep in the lake require different visual capabilities and eat different food from fish living close to the surface. This rapid diversification into some 800 cichlid species – encompassing large predators, algae-eaters and planktivores – all took place within the same lake.
Hybridisation plays a major role in this radiation, but because several inversions function as a kind of sex chromosome, they help to keep species reproductively isolated. ‘Now that we understand how these supergenes evolve and spread, we are getting closer to answering a fundamental scientific question: how does life on Earth become so rich and varied?’ says Svardal in an accompanying press release. A preprint by American biologists (bioRxiv, 18 March) also confirms that inversions, together with both sexual and natural selection, were essential to the adaptive radiation of Malawi cichlids.