Gene for promiscuity found in yeast
Sexual origin Researchers have discovered 'selfish' genes that increase promiscuity in yeast -- a finding that supports one theory about the evolution of sex.
"We compared the sexuality of yeast that had the selfish gene compared to those that didn't and found that those with this selfish gene were more sexy," says Goddard. "They engaged in meiosis more readily than those that didn't."
While most people think of genes as working together for the good of the organism, for some decades now scientists have known there are some genes that are more "selfish" than this.
"There are some genes that are essentially parasitic. They are present in the organism but they don't actively contribute to the organism's benefit in any way and in fact they may even be detrimental," says Goddard.
He says these genes spread by cheating Mendel's rules of inheritance in which approximately 50 per cent of genes come from each parent.
"These selfish genes have evolved mechanisms to mean that there's not a 50:50 split in the offspring but there's a massive bias towards the presence of the selfish gene," says Goddard.
In a study of the yeast, Saccharomyces cerevisiae, Goddard looked closely at the impact of a small selfish gene, called a Homing Endonuclease Gene (HEG), which produces an enzyme that cuts DNA.
By engineering identical yeasts to contain the HEG or not, the researchers found the gene was indeed parasitic, and caused infected hosts to grow more slowly.
More interestingly, they found that infection with HEG affected the yeast's sexuality.
Selfish genes can only spread and increase their numbers by passing from an infected individual to an uninfected individual during sex. "Think of them as a sexually transmitted disease," says Goddard.
So it's reasonable to predict that HEGs increase promiscuity of their host would have an evolutionary advantage.
And this is exactly what Goddard and colleagues found.
"The individuals that carry the gene are more likely to engage in sex than those that don't," he says.
The researchers used a mathematical model to confirm the increase in promiscuity massively increased the speed at which HEGs invade populations.
Origins of sex
Sex first evolved in eukaryotes and this is the first example of a selfish gene increasing promiscuity in a eukaryote, says Goddard.
The findings could help explain the puzzle of why sexual reproduction, which is more complicated and costly than asexual reproduction, came to be the dominant mode of reproduction on Earth in eukaryote.
While it provides genetic variance that allows faster adaptation to environmental circumstances, this does not explain how sex could have evolved in the first place.
"Right back before sex evolved, how would one rare individual that was sexual begin to increase in frequency?" asks Goddard.
The findings support the idea sex evolved as a result of selfish genes forcing asexual individuals to become sexual.
Goddard says HEGs are mostly found in microbes and other selfish genes have been found in mice, although they have not been tested for effects on promiscuity.
"It opens up the possibility that such selfish genes are present in other species including humans," he says.
And, he says it's not "inconceivable" that viruses may also manipulate host promiscuity.