A fixation-weighted null model for the proportion of sex chromosome-autosome fusions

Recent null models for sex-autosome fusions have often relied on occurrence probability alone. However, evolutionary expectations must account for fixation probability. Under neutrality, fixation probability is inversely proportional to chromosomal population copy numbers, which vary across autosomes, X chromosomes, and Y chromosomes. We extend the original fusion model by weighting each fusion class by its relative neutral fixation probability. This updated model predicts substantially higher proportions of fixed SA-fusions, establishing a more biologically realistic null hypothesis. We provide an updated R function in the R package evobiR and demonstrate its impact by re-evaluating empirical fusion patterns in Drosophila and Habronattus .