Y chromosomes of fish species diverged even without recombination
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BA Sandkam et al., “The extreme Y chromosome polymorphism corresponds to five male reproductive morphs of a freshwater fish”, Nat Eco Evol, 5: 939–48, 2021.
One of the key stages in the evolution of different sex chromosomes is a loss of recombination – the exchange of DNA during meiosis – which is both an important mechanism for maintaining genetic diversity in populations and for purging the damaging mutations. This loss has led to the progressive depletion of the functional gene content of the Y chromosome in many species, says Judith Mank, an evolutionary geneticist at the University of British Columbia who studies sexual dimorphism.
Nonetheless, some species have high levels of male phenotypic variability which appear to be related to Y. Freshwater fish and the parent guppy Poecilia parae, for example, has five different male forms that vary in size, coloration and behavior. The so-called parae males are large, silvery, and aggressive towards other fish, for example, while blue melanzona males are smaller, wear stripes, and rely on their appearance to attract females.
These morphs coexist next to each other, Mank says, and their traits are reliably passed from fathers to male offspring, indicating that the variation is related to gender. Interested in knowing if the genetic source of this variation might be lurking in P. parae‘s Y, Mank and colleagues used read-linked DNA sequencing on tail samples from 40 fish collected in Guyana, and the results confirmed the researchers’ intuition: “There were very large differences in [Y chromosome] size and genetic content ”between morphs, says Mank. The Ys of the morphs were more similar to each other than they were to the Y chromosome of a sister fish species, supporting the idea that the variation occurred after, not before, speciation.
The fact that such a variation has occurred despite low levels of genetic recombination on this chromosome suggests that other diversity-generating mechanisms, such as the movement of transposable elements, may be at play, adds Mank.
University of Edinburgh evolutionary biologist Deborah Charlesworth, who studies guppies and was not involved in the work, says she is delighted to see wild animal research P. parae populations that appear to confirm the role of the Y chromosome in controlling staining and other traits. It would be interesting to know more about the function of these sequences – do they code for proteins or do they perhaps have a regulatory role? – and on their location on the chromosome, she adds. “What are these genes and where are they? “