A species of yeast has weird sex. While most organisms use sex to reshuffle their genes and create offspring that are genetically different from their parents, this one goes to extreme lengths to avoid recombining its DNA.
The yeast, called Saccharomycodes ludwigii, illustrates a problem that all sexually reproducing species face: while sex has evolutionary benefits, it also has costs. In some circumstances, reshuffling genes can produce individuals that can’t survive, so it is better not to do it too much.
“This species is an extreme case,” says Michael Knop at Heidelberg University in Germany. Knop and his colleagues have spent more than a decade studying S. ludwigii. Like other yeasts, it is a single-celled fungus and can reproduce sexually.
They do this through a process called meiosis: a yeast species starts out with two copies of every chromosome, and therefore two copies of every gene. Each “adult” cell then shuffles genes between its chromosomes through a process called recombination, before producing daughter cells that carry just one copy of each chromosome.
Because of the recombination, the chromosomes that these daughter cells carry are genetically distinct from those present in the adult – and so when two such daughter cells fuse together to form offspring with two copies of every chromosome, these offspring will also be genetically distinct from the original adults.
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But this isn’t the case in S. ludwigii. Knop and his team found that this yeast hardly ever performs recombination. They sequenced parent and offspring yeast and couldn’t find any pieces of chromosome that had been swapped. This result held true when the researchers examined 10 different strains of the species, suggesting that it has behaved this way for a long time.
It isn’t that S. ludwigii can’t perform recombination – the researchers found that it still has the majority of the genes required to do so.
“There’s a pressure to keep [recombination rates] so low that it’s very rarely occurring, but it is not trapped in a situation where it can’t do it anymore,” says Knop.
“It’s great to have learned more about the unusual biology of a fascinating system,” says Maurine Neiman at the University of Iowa in Iowa City.
Although it isn’t yet clear why the yeast doesn’t shuffle its DNA, the cause may have been the deletion of a gene called MER1, which is involved in recombination but isn’t entirely essential. Knop suspects this was an accident: it may have solved a problem that the species was experiencing, but left it with reduced recombination.
For Neiman, S. ludwigii fits into a larger picture. “All sorts of organisms have evolved ways to get at least some of the benefits of sex without all the costs,” she says. Organisms aren’t simply sexual or asexual, she says: there is a spectrum.
There are many reasons why organisms might limit recombination, says Neiman. “You’re breaking up genes that work well together, without any predictions regarding how they’re going to do in their new combination.” So while recombination can produce beneficial combinations of genes, it can also produce harmful ones that mean some offspring don’t survive.
This risk may be worthwhile in highly changeable environments, but not in stable ones. However, Neiman says it is hard to pin down which aspects of the environment are most important.
She points to a 1995 paper titled “Is a little bit of sex as good as a lot?” that argued many species would do best if they only rarely recombined their genes. But not all organisms have the molecular machinery to exert that kind of precise control – hence species like humans where the mixing of sperm and egg is a lottery. “These yeast,” says Neiman, “might have gotten it exactly right.”
Reference: bioRxiv, doi.org/f9jj