Supergenes can also complicate the process of mating. In some species, supergenes create a breeding system that in effect has four sexes. Because of a supergene in the North American birds called white-throated sparrows, for example, there are two “morphs” with dissimilar coloration and behaviors. Not only do males have to find females, but they must find a partner from the opposing morph. Otherwise, offspring will die either from inheriting supergenes from both parents or from inheriting none. Only chicks that receive a “balanced lethal” inheritance of one supergene and one ordinary segment of chromosome survive.
With such a steep price, it’s a wonder that supergenes evolved at all, Berdan says. “Any set of variants is going to be really hard to maintain, especially over millions of generations,” she said. “That’s one of the big mysteries of supergenes.” She suggested that multiple types of selection might be working together to preserve supergenes, and that certain environments might be most conducive to their persistence in the population.
Ironically, one of the mechanisms that can sometimes preserve supergenes seems to be recombination—the phenomenon that they normally resist. Amanda Larracuente, an evolutionary geneticist at the University of Rochester, and her coauthors described such a case last April in eLife.
Larracuente wasn’t initially interested in supergenes or their evolutionary costs. Her focus was on selfish genes, segments of DNA that proliferate in populations without benefiting their hosts. She was fascinated by a selfish gene called Segregation Distorter (SD) that arose in certain fruit flies in Zambia. “It’s a sperm killer,” she explained, but it only kills sperm that doesn’t carry a chromosome with SD.
Sometime within the last 3,000 years, one version of SD ensnared a large piece of chromosomal DNA, creating a supergene known as SD-Mal that spread to fruit fly populations throughout Africa. “It’s really the ultimate selfish gene,” Larracuente said.
DNA sequencing and analysis by Larracuente, Daven Presgraves, and their colleagues showed that chromosomes with SD-Mal accumulate harmful mutations, as predicted by the near-complete lack of recombination between SD-Mal and its sister chromosome. But the researchers didn’t find as many mutations as they expected.
The reason, they discovered, is that occasionally a fly will inherit two chromosomes with SD-Mal—and those two supergenes are just similar enough to allow some recombination between them. That recombination in turn makes it possible for a few harmful mutations to be purged from the flies’ supergenes over time.
“As it turns out, just a little bit of recombination is enough,” Larracuente said. She and Presgraves are now looking for other SD supergenes in wild fruit fly populations for clues to the evolution and impacts of supergenes more generally.
Their results show that the purifying effects of recombination on genomes never cease to be important. The complex traits that the stable, predictable inheritance of supergenes makes possible may be invaluable in helping species adapt, but even the supergenes can benefit from mixing things up once in a while.
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.