Amazing stuff!
"... Cichlid fish from Lake Malawi [fourth largest freshwater lake in the world] in East Africa offer a clue. In this single lake, over 800 different species have evolved from a common ancestor in a fraction of the time it took for humans and chimpanzees to evolve from their common ancestor.
What’s even more remarkable is that the diversification of cichlids happened all in the same body of water. Some of these fish became large predators, others adapted to eat algae, sift through sand, or feed on plankton. Each species found its own ecological niche. ...
The researchers looked at the DNA of over 1,300 cichlids to see if there’s something special about their genes that might explain this rapid evolution. “We discovered that, in some species, large chunks of DNA on five chromosomes are flipped – a type of mutation called a chromosomal inversion,” ..."
From the editor's summary and abstract:
"Editor’s summary
Chromosomal inversions have been found to be important for adaptation. These structural variants suppress recombination, allowing groups of variants to be transmitted together. Blumer et al. sequenced 1375 genomes from 240 species of Lake Malawi cichlids, a clade of fish hallmarked by their extremely species-rich adaptive radiations in several African lakes. The authors found multiple segregating inversions in these populations, as well as genomic signatures showing that these regions had been used repeatedly in adaptation throughout these species. Some of these inversions seem to primarily include genes related to sensory tissues, and two were linked to sex determination. ...
Structured Abstract
INTRODUCTION
Ecological speciation is responsible for much of the biodiversity on our planet. Despite its fundamental importance, this process, in which new species emerge through evolutionary adaptation to distinct ecological niches, is still not fully understood. Intriguing case studies are adaptive radiations, bursts of ecological speciation that give rise to large numbers of diverse species over timescales that are short compared with the fixation time for new genetic variants. Genome sequencing studies increasingly point towards the importance of hybridization and cross-species gene flow in producing the diversity needed for ecological speciation and adaptive radiation. However, a major conundrum is the role of meiotic recombination in this process. On the one hand, recombination can create new, beneficial combinations of genetic alleles. On the other, it breaks down co-adapted allelic combinations, impeding speciation.
RATIONALE
Chromosomal inversions, stretches of DNA that are flipped in their orientation, provide a potential solution to the conflicting roles of recombination in ecological speciation. This is because inversions show suppressed recombination with the original chromosomal orientation, enabling them to lock together adaptive combinations of alleles in so-called “supergenes.” Inversions have been found to be important in ecological adaptation and speciation in many groups of organisms, but so far, there has been little evidence for their significance in adaptive radiations. To address this gap, we systematically investigated the presence and role of inversions across the lake Malawi cichlid fish adaptive radiation, the largest recent vertebrate radiation.
RESULTS
The genomes of 1375 Malawi cichlids from 240 species revealed the presence of multiple chromosomal inversions.
The five largest of these segregate within the diverse and species-rich benthic subradiation, with a strong association between inversion states and habitat depth. Phylogenetic tracking of inversion states revealed a hybrid origin of the benthic clade, along with several introgression events transporting inversions and other genetic material between lineages within and outside of the radiation.
Inversion haplotypes showed strong signals of adaptive evolution, including being enriched for sensory functions, behavior, and reproduction.
For three chromosomes, the re-introgression of haplotypes of the ancestral orientation into benthic lineages coincides with an apparent Y chromosome–like role of this haplotype in the sex determination of some benthic species but not others.
CONCLUSION
The spread of chromosome-scale inversions in Malawi cichlids coincided with the phenotypic and ecological diversification of benthic species across habitats, with evidence for a role of inversion haplotypes in ecological adaptation. The additional transient sex linkage of introgressed inversion-region haplotypes points to an interplay of sex-linked and natural selection in shaping the evolution of inversion haplotypes and the diversification of cichlids."
Five large chromosomal inversions contribute to the diversification of Malawi cichlids.
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