Chromosomal rearrangements: tempo and mode of karyotype evolution in Scarabaeoidea.

Karyotype evolution, including changes in chromosome number and structure, is a fundamental process that can influence genome organization and speciation. Here, we analyze patterns of chromosome number evolution and sex chromosome-autosome (SA) fusions across Scarabaeoidea, a diverse beetle superfamily with broad variation in karyotypes and sex chromosome systems. We compiled 478 karyotype records and constructed a phylogeny for 211 species to estimate rates of chromosome number change, modeled as fission and fusion events, in Scarabaeidae, Lucanidae, and Passalidae. Passalidae show markedly higher rates of both fission and fusion along with consistently higher chromosome numbers. This pattern is consistent with genetic drift facilitating the fixation of rearrangements in lineages with low dispersal and small effective population sizes. We also tested whether SA-fusions occur more often than expected by chance, where the null expectation assumes all chromosomes are equally likely to undergo fusion. The observed proportion of SA-fusions was more than double the null expectation, with non-overlapping credible intervals. This pattern is consistent with a prediction of models where sex chromosome fusions are indirectly favored by sexually antagonistic selection. We also found that neo-XY systems are consistently associated with lower autosome numbers, consistent with repeated SA-fusions. Together, these results show that Scarabaeoidea exhibit dynamic karyotype evolution and provide comparative support for core predictions of the canonical model of sex chromosome evolution.