Biologists shed light on the evolution and consequences of egoistic genetic elements

The human genome is littered with “selfish genetic elements,” which don’t seem to benefit their hosts, but instead just want to reproduce themselves.

Selfish genetic elements can wreak havoc by, for example, disrupting sex ratios, affecting fertility, causing harmful mutations and possibly even causing population extinction.

Biologists at the University of Rochester, including Amanda Larracuente, associate professor of biology, and Daven Presgraves, professor of biology at the university, have used population genomics for the first time to shed light on the evolution and consequences of a selfish genetic element known just as Segregation distorterSD

In an article published in the magazine eLifethe researchers report that SD has caused dramatic changes in chromosome organization and genetic diversity.

A first genome sequencing

The researchers used fruit flies as model organisms to study SD, a selfish genetic element that skews the rules of fair genetic transfer. Fruit flies share about 70 percent of the same genes that cause disease in humans, and because they have such a short reproductive cycle — less than two weeks — scientists are able to create generations of flies in a relatively short time.

Female flies transmit SD-infected chromosomes to about 50 percent of their offspring, as expected under Mendel’s laws of heredity. However, males transmit SD chromosomes to nearly 100 percent of their offspring, because SD kills any sperm that doesn’t carry the selfish genetic element.

How works? SD do this?

Because it’s evolved into what researchers call a “supergene”: a cluster of selfish genes on the same chromosome that are inherited together.

Researchers have known for decades that SD evolved into a supergene. But this is the first time they’ve used what’s known as population genomics — examining genome-wide patterns of DNA sequence variations between individuals in a population — to explore the dynamics, evolution, and long-term effects of SD on the evolution of a genome.

“This is the first time someone has taken the whole of SD chromosomes and was therefore able to draw conclusions about both the history and genomic implications of being a supergene,” says Presgraves.

An evolutionary demise on the horizon

The advantage of being a supergene is that multiple genes can work together to cause SD‘s almost perfect transmission to offspring. However, as the researchers found, there are major drawbacks to being a supergene.

In sexual reproduction, maternal and paternal chromosomes exchange genetic material to produce new genetic combinations unique to each offspring. In most cases, the chromosomes are properly aligned and intersect. Scientists have long recognized that the exchange of genetic material through crossing – known as recombination – is vital because it allows natural selection to eliminate harmful mutations and allow the spread of beneficial mutations.

However, as the researchers showed, one of the main costs of SDThe near-perfect transfer is that it does not undergo recombination.

The egoistic genetic element gains a short-term transfer advantage by stopping recombination to ensure it is passed on to all its offspring. But SD is not forward-looking: preventing recombination has led to: SD accumulates much more harmful mutations compared to normal chromosomes.

“Without recombination, natural selection cannot effectively remove harmful mutations so they can accumulate” SD chromosomes,” says Larracuente. “These mutations can disrupt the function or regulation of genes.”

The lack of recombination can also lead to: SD’s evolutionary demise, says Presgraves.

“Due to their lack of recombination, SD chromosomes begin to show signs of evolutionary degeneration.”

Source:

Reference magazine:

Navarro Dominguez, B., et al. (2022) Epistatic Selection on a Selfish Segregation Distorter Supergene – Drive, Recombination and Genetic Load. eLife doi.org/10.7554/eLife.78981

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