Effects of deleterious mutations on the fixation of chromosomal inversions on autosomes and sex chromosomes

Deleterious mutations have multiple effects on the fate of chromosomal inversions. In this article, we use individual-based simulations to estimate the fixation probabilities of neutral inversions under a constant input of deleterious mutations. As shown previously, we find that lucky inversions carrying a lower-than-average mutation load are initially favored and tend to selectively spread. Our results also outline the importance of Mullers ratchet caused by the absence of recombination in Y-linked and in rare autosomal inversions, reducing their fixation probabilities. Despite the fact that Y-linked inversions are more susceptible to Mullers ratchet, they can more easily fix than autosomal inversions despite initially carrying mutations, when these mutations have sufficiently low selection or dominance coefficients (sheltering effect). Similarly, the sheltering of deleterious alleles can increase the fixation probability of inversions capturing a mating-type locus, particularly under intermediate rates of self-fertilization. Overall, our results confirm that when mating is random and for realistic parameter values, the presence of deleterious alleles reduces the average fixation probability of autosomal, sex-linked or mating type-linked inversions below that of a neutral mutation. Nevertheless, the occasional fixation of a lucky inversion may have important macroevolutionary consequences, potentially contributing to the evolution of recombination arrest on sex chromosomes.