How supergenes beat the odds and fuel evolution
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, North American birds called white-throated sparrows have two “morphs” with dissimilar coloration and behavior. Not only do males have to find females, they have to find a mate from the opposite morph. Otherwise, the offspring will die either from the inheritance of supergenes from both parents, or from the lack of inheritance. Only chickens that have received a “balanced lethal” inheritance of one supergene and one regular segment of the chromosome survive.
With such a high price tag, it’s surprising that supergenes have evolved at all, Berdan says. “Any set of variants will be very difficult to maintain, especially over millions of generations,” she said. “This is one of the biggest supergene mysteries.” She suggested that several types of selection may work together to keep supergenes, and that certain conditions may be most favorable for their persistence in a population.
Ironically, one of the mechanisms that can sometimes save supergenes is recombination, a phenomenon they usually resist. Amanda Larracuentean evolutionary geneticist at the University of Rochester, and her co-authors described such a case. last April V electronic life.
The Larracuente were not initially interested in supergenes or their evolutionary costs. Her focus was on selfish genes, segments of DNA that multiply in populations without benefiting their hosts. She was fascinated by a selfish gene named Segregation deformer (SD) that originated in some fruit flies in Zambia. “It’s a sperm killer,” she explained, “but it only kills sperm that don’t carry chromosomes with SD.
Somewhere in the last 3000 years, one of the versions SD trapped a large chunk of chromosomal DNA, creating a supergene known as SD-Mal that have spread to fruit fly populations throughout Africa. “It’s really the most selfish gene,” Larracuente said.
Sequencing and DNA analysis of Larracuente, Daven Presgravesand their colleagues showed that chromosomes with SD-Mal accumulate deleterious mutations, as predicted by the near absence of recombination between SD-Mal and its sister chromosome. But the researchers didn’t find as many mutations as they expected.
They found that the reason is that sometimes a fly inherits two chromosomes with SD-Mal— and these two supergenes are similar enough to allow for some recombination between them. This recombination, in turn, allows the fly’s supergenes to be cleared of several harmful mutations over time.
“As it turns out, just a little recombination is enough,” said Larracuente. She and Presgraves are now looking for others. SD supergenes in wild fruit fly populations to understand the evolution and influence of supergenes in general.
Their results show that the cleansing effects of recombination on genomes never cease to be important. The complex traits that stable and predictable supergene inheritance makes possible can be invaluable for species adaptation, but even supergenes can benefit from the random mixing of things.
original story reprinted with permission Kvanta Journal, editorially independent publication Simons Foundation whose mission is to improve the public understanding of science by highlighting research developments and trends in the mathematics, physical and life sciences.