Biologists explain evolution and the consequences of selfish genetic elements

The human genome is littered with “selfish genetic elements”, which do not appear to benefit the host, but only seek to reproduce.

Selfish genetic elements can wreak havoc by, for example, distorting sex ratios, impairing fertility, causing dangerous mutations, and potentially even causing population extinction.

Biologists at the University of Rochester, including Amanda Larracuente, professor of biology, and Daven Presgraves, University Dean’s Professor of Biology, are using population genomics for the first time to explain the evolution and consequences of known selfish genetic elements. as Segregation Distortion (SD).

In a paper published in the journal eLifethe researchers reported that SD has led to dramatic changes in chromosomal organization and genetic diversity.

Sequencing the genome first

Researchers use fruit flies as model organisms to study SD, a selfish genetic element that deviates from the rules of just genetic transmission. Fruit flies share about 70 percent of the same genes that cause disease in humans, and because they have a short reproductive cycle-; less than two weeks-; scientists were able to create generations of flies in a relatively short time.

Female flies transmit SDinfected chromosomes to about 50 percent of their offspring, as would be expected under Mendelian inheritance laws. Men, however, send SD chromosomes to nearly 100 percent of their offspring, because SD kill sperm that do not carry a selfish genetic element.

How SD do this?

Because it has evolved into what researchers call a “supergene”-; a group of selfish genes on the same chromosome that are inherited together.

Researchers have known for decades that SD evolved to form supergenes. But this is the first time they’ve used what’s known as population-genomics; examine genomic patterns of DNA sequence variation among individuals in a population-; to study the dynamics, evolution, and long-term effects of SD on genome evolution.

This is the first time that anyone has sequenced the entire genome of SD chromosomes and are therefore able to make inferences about the history and consequences of the genome being a supergene.”

Daven Presgraves, Professor of Biology

Evolutionary fallout on the horizon

The advantage of being a supergene is that many genes can act together to cause SDalmost complete transmission to their offspring. However, as the researchers discovered, there is a major drawback to being a supergene.

In sexual reproduction, the chromosomes from the mother and father exchange genetic material to produce new genetic combinations that are unique to each offspring. In most cases, the chromosomes line up correctly and cross over each other. Scientists have long recognized that genetic material exchanges by cross-; known as-recombination; is very important because it empowers natural selection to eliminate deleterious mutations and allows the spread of beneficial mutations.

However, as the researchers point out, one of the main costs of SDAn almost perfect transmission is not undergoing recombination.

The selfish genetic element benefits from short-term transmission by turning off recombination to ensure it is passed on to all offspring. But SD not looking forward: preventing recombination has led to SD accumulates more deleterious mutations than normal chromosomes.

“Without recombination, natural selection cannot effectively clean up deleterious mutations, so they can accumulate in SD chromosomes,” Larracuente said. “These mutations may interfere with gene function or regulation.”

Lack of recombination can also cause SD’evolutionary downfall, says Presgraves.

“Due to their lack of recombination, SD the chromosomes are already starting to show signs of evolutionary degeneration.”

Source:

Journal reference:

Navarro-Dominguez, B., et al. (2022) Epistatic selection in the Distorter supergene Selfish segregation – drive, recombination, and genetic burden. eLife. doi.org/10.7554/eLife.78981.

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