Sex Chromosome Fight: How Competition Is Affected X vs Y Sperm Fitness

Evolutionarily, fitness is defined as the ability of an organism to survive and replicate that gene for the next generation. Genes affect fitness and sometimes compete with each other within organisms.

This competition, or weapons competition, is usually difficult to observe when the gene in question is alive on X and Y chromosomes, which determine the gender of the mammalian offspring.

In the mouse, this weapons race can result in a chick with more men or women.

A study published in Proceedings of the National Academy of Sciences by University of Michigan researchers revealed the mechanisms behind the arm race of the sperm of mouse X and Y to fertilize eggs resembling space races to reach the moon.

“The first X-carry or Y-carry sperm that you reach is what will fertilize the egg normally,” says Dr. Martin Arlt, a quasi-research scientist in the Department of Human Genetics.

“If there is a gene that gives benefits to sperm containing X, then female offspring will increase and we begin to see the opposite. But we see a split of nearly 50-50,” said Arlt, the first author of the study. “Over evolutionary time, a 50-50 split is the optimal ratio of species with a small shift, potentially leading to species loss.”

The balance of sex assessment is maintained as genes on the X and Y chromosomes and is repressed from each other. It was a mystery how this would happen, as sperm cannot be raised in the lab.

The UM team found a unique solution, moving the X-linked SLXL1/SLX and Y-linked slice gene families out of the mouse and placing them in yeast.

“We introduced each player in the competition to yeast and got a better idea of ​​how they work. We then combined them to see how they interact and compete.”

In doing so, they discovered that proteins encoded by SLXL1/SLX and SLY, which affect sperm fitness, appear to compete for a protein called spindolin, which affects gene expression. These proteins compete with each other for binding. The more X-linked gene family proteins that bind, the more the resulting X-carry sperm.

“These proteins are relatively new innovations in the period of evolution, only millions of years ago, after humans diverge from the chimpanzees,” says Dr. Jacob Mueller, an associate professor of human genetics and senior author.

“Spermatogenesis can occur normally without SLXL1/SLX and SLY, but these genes persist in mice by integrating themselves into systems that are very important to the species. There is evidence that these arms races happen again and again at different times.”

In the future, the team will use yeast model systems to investigate the evolution of X/Y arm races and other competitive genes.