Drivers of achiasmatic meiosis: sexual antagonism versus heteromorphy-dependent aneuploidy across sex-chromosome divergence

Crossing over during meiosis ensures proper chromosome segregation and promotes genetic diversity. In species with chromosomal sex determination, recombination between sex chromosomes is often reduced or eliminated; yet, the evolutionary forces driving this shift remain debated. One extreme outcome, achiasmatic meiosis, typically completely halts recombination in the heterogametic sex. Here, we use a population genetic model to compare 2 leading hypotheses for the evolution of achiasmy: (1) selection to reduce recombination load from sexually antagonistic alleles and (2) selection to avoid aneuploidy driven by heteromorphic sex chromosomes. We analyze how mutations promoting achiasmy can invade autosomes, X chromosomes, or Y chromosomes under each selective regime. Our results reveal that the Y chromosome provides the most permissive context for invasion due to male-limited expression and selection. Moreover, we predict a shift in the primary selective forces across the trajectory of sex-chromosome divergence: sexually antagonistic selection is more likely to drive achiasmy in young, homomorphic sex chromosomes, whereas heteromorphy-dependent aneuploidy becomes the primary force in highly diverged, heteromorphic sex chromosomes. These results provide a unified framework for understanding transitions to achiasmy across diverse taxa.