Supergenes can also complicate the mating process. In some species, supergenes create a breeding system that actually has four sexes. For example, due to a supergene in the North American birds called white-throated sparrows, there are two “morphs” with different coloring and different behaviors. Males must not only find females, but also find a mate from the opposing morph. Otherwise, the offspring will die from either inheriting supergenes from both parents or from inheriting neither. Only chicks that receive a “balanced lethal” inheritance of a supergene and a common chromosome segment survive.
At such a high price point, it’s a wonder supergenes evolved at all, says Berdan. “Any set of variants is going to be really difficult to maintain, especially over millions of generations,” she said. “That’s one of the great mysteries of supergenes.” She suggested that multiple modes of selection might work together to conserve supergenes and that certain environments might be most conducive to their persistence in the population.
Ironically, one of the mechanisms that supergenes can sometimes preserve seems to be recombination – the phenomenon they normally resist. Amanda Larracuente, an evolutionary geneticist at the University of Rochester, and her co-authors described such a case last April in eLife.
Larracuente was not initially interested in supergenes or their evolutionary costs. Her focus was on selfish genes, segments of DNA that replicate in populations without benefiting their hosts. She was fascinated by a selfish gene called Separation Distortion (SD) found in certain fruit flies in Zambia. “It’s a sperm killer,” she explained, but it only kills sperm that don’t carry a chromosome SD.
Sometime in the last 3,000 years, a version of SD ensnared a large piece of chromosomal DNA and created a supergene known as SD times which spread to fruit fly populations across Africa. “It really is the ultimate selfish gene,” Larracuente said.
DNA sequencing and analysis by Larracuente, Daven Presgraves and their colleagues showed that chromosomes with SD times accumulate deleterious mutations as predicted by the near complete lack of recombination between SD times and its sister chromosome. But the researchers didn’t find as many mutations as expected.
They found that this is because a fly occasionally inherits two chromosomes SD times— and these two supergenes are just similar enough to allow some recombination between them. This recombination, in turn, allows for some deleterious mutations to be removed from the flies’ supergenes over time.
“It turns out that a little bit of recombination is enough,” Larracuente said. She and Presgraves are now looking for others SD Supergenes in wild fruit fly populations for clues to evolution and the impact of supergenes in general.
Their results show that the cleansing effect of recombination on genomes never ceases to matter. The complex traits that enable the stable, predictable inheritance of supergenes can be invaluable in helping species adapt, but even the supergenes can benefit from mixing things up from time to time.
Original story Reprinted with permission from quanta magazine, an editorially independent publication Simons Foundation whose mission is to improve public understanding of science by covering research developments and trends in mathematics and the natural and life sciences.