The elements of our genome don't necessarily get along. Many genes are 'selfish' and only interested in their own spread and survival. New research has now revealed more about how an evolutionary 'arms race' develops between genes to keep these bad actors at bay.
While we've known about these "parasites of the genome" for many years, this latest study gives further evidence of how exactly they works. Specifically, the study looks at the genomes of three closely related species of Drosophila (fruit flies).
Fruit flies share about 70 percent of the same genes that cause human diseases and are similar to humans on the molecular level. This makes them ideal candidates for genetic research. What's more, their short reproductive cycles (under two weeks) mean that several generations of flies can be studied together.
The researchers found that each fruit fly species had 5-12 meiotic drive genes on the X chromosomes. These newly discovered genes are of the selfish type, and try to sneakily spread into more than the standard 50 percent of the parent's offspring.
"We have found that an evolutionary arms race has led to a proliferation of meiotic drive genes on the X chromosome and suppressor genes elsewhere in the genome," says evolutionary biologist and theoretical population geneticist Christina Muirhead from the University of Rochester in New York.
The genes found by the researchers are related to a meiotic drive gene called Dox, or "distorter on the X". Found on the X chromosome, Dox kills Y chromosome-bearing sperm. With that in mind, the new genes have been called Dox-like or Dxl for short, because they use a similar type of attack.
These Dxl genes were shown to produce a histone protein that interferes with immature male sex cells bearing the Y chromosome. These cells then die off, which means that future generations will have many more daughters than sons.
All the Dxl genes are concerned with is propagating themselves, the researchers report, apparently oblivious to the logical conclusion of this kind of behavior: The species and the Dxl genes being driven to extinction as fewer and fewer males are produced.
"The drive genes get an evolutionary advantage by killing Y-bearing sperm," says evolutionary geneticist Daven Presgraves, also from the University of Rochester. "But the individuals carrying the drive genes suffer reduced fertility, and the population becomes increasingly female-biased, risking eventual extinction."
To balance the scales, the team also found Dxl gene duplicates playing counter-attack against the original Dxls by pretending to be a selfish Dxl gene. These duplicates silence real Dxl genes via RNA interference, rather than expressing Dxl proteins as normal – a crucial modification. In other words, the rest of the fruit fly genome seems to have evolved to suppress the selfish parts and ensure survival.
Further work is needed to verify if something similar happens in the human genome, but given how genetically similar fruit flies are to us, what's happening here in Drosophila is likely to occur in other mammals as well: Microscopic battles between genes to gain superiority and to keep the genome in balance.
"Similar repetitive gene copies like the Dxl genes that selfishly bias sex ratios are common to the X and Y chromosomes of great apes and humans," says Presgraves.
"These are just one line of evidence that evolutionary arms races have important consequences for genome evolution."
The research has been published in Nature Ecology & Evolution.