PT - JOURNAL ARTICLE AU - Paul Jay AU - Mathieu Chouteau AU - Annabel Whibley AU - Héloïse Bastide AU - Violaine Llaurens AU - Hugues Parrinello AU - Mathieu Joron TI - Mutation accumulation in chromosomal inversions maintains wing pattern polymorphism in a butterfly AID - 10.1101/736504 DP - 2019 Jan 01 TA - bioRxiv PG - 736504 4099 - http://biorxiv.org/content/early/2019/08/15/736504.short 4100 - http://biorxiv.org/content/early/2019/08/15/736504.full AB - While natural selection favours the fittest genotype, polymorphisms are maintained over evolutionary timescales in numerous species. Why these long-lived polymorphisms are often associated with chromosomal rearrangements remains obscure. Combining genome assemblies, population genomic analyses, and fitness assays, we studied the factors maintaining multiple mimetic morphs in the butterfly Heliconius numata. We show that the polymorphism is maintained because three chromosomal inversions controlling wing patterns express a recessive mutational load, which prevents their fixation despite their ecological advantage. Since inversions suppress recombination and hamper genetic purging, their formation fostered the capture and accumulation of deleterious variants. This suggests that many complex polymorphisms, instead of representing adaptations to the existence of alternative ecological optima, could be maintained primarily because chromosomal rearrangements are prone to carrying recessive harmful mutations.