The team found that the proteins encoded by Slxl1/Slx and Sly, which affect sperm fitness, compete for Spindlins—proteins that regulate gene expression. The proteins vie for binding, with more X-linked gene family proteins leading to the production of more X-carrying sperm, and vice versa.
Sperm fitness determined by competition between X and Y
One of the researchers, Jacob Mueller, associate professor of human genetics and senior author of the paper, explained that these proteins are relatively recent developments in evolutionary terms, appearing only a few million years ago, well after humans diverged from chimps.
Additionally, while spermatogenesis can occur without Slxl1/Slx and Sly, these genes have persisted in mice by becoming integrated into a crucial system for the species. The researchers also found evidence that similar arms races are occurring repeatedly across different species and time periods.