Abstract
The house mouse X and Y chromosomes have recently acquired multicopy, rapidly evolving gene families representing an evolutionary arms race. This arms race between proteins encoded by X-linked Slxl1/Slx and Y-linked Sly gene families can distort offspring sex ratio, but how these proteins compete remains unknown. Here, we report how Slxl1/Slx and Sly encoded proteins compete in a protein family-specific and dose-dependent manner using yeast. Specifically, SLXL1 competes with SLY1 and SLY2 for binding to the Spindlin SPIN1. Similarly, SLX competes with SLY2 for binding the Spindlin SSTY2. These competitions are driven by the N termini of SLXL1, SLX, SLY1, and SLY2 binding to the third Tudor domains of SPIN1 and SSTY2. SLY1 and SLY2 form homo- and heterodimers, suggesting that the competition is between complex multimers. Residues under positive selection mapping to the interaction domains and rapid exon gain/loss are consistent with competition between the X- and Y-linked gene families. Our findings support a model in which dose-dependent competition of these X- and Y-linked encoded proteins to bind Spindlins occurs in haploid X- and Y-spermatids to influence X- versus Y-sperm fitness and thus sex ratio.