Journal
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES
Volume 377, Issue 1856, Pages -Publisher
ROYAL SOC
DOI: 10.1098/rstb.2021.0207
Keywords
recombination suppression; structural variation; genome assembly; Danaus chrysippus; chromosomal rearrangement; repeats
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This study investigates the origin of the BC supergene in the African monarch butterfly and reveals multiple structural differences in this supergene. It shows the role of duplications and inversions in the evolution of the supergene, supporting a stepwise model of supergene evolution.
Supergenes maintain adaptive clusters of alleles in the face of genetic mixing. Although usually attributed to inversions, supergenes can be complex, and reconstructing the precise processes that led to recombination suppression and their timing is challenging. We investigated the origin of the BC supergene, which controls variation in warning coloration in the African monarch butterfly, Danaus chrysippus. By generating chromosome-scale assemblies for all three alleles, we identified multiple structural differences. Most strikingly, we find that a region of more than 1 million bp underwent several segmental duplications at least 7.5 Ma. The resulting duplicated fragments appear to have triggered four inversions in surrounding parts of the chromosome, resulting in stepwise growth of the region of suppressed recombination. Phylogenies for the inversions are incongruent with the species tree and suggest that structural polymorphisms have persisted for at least 4.1 Myr. In addition to the role of duplications in triggering inversions, our results suggest a previously undescribed mechanism of recombination suppression through independent losses of divergent duplicated tracts. Overall, our findings add support for a stepwise model of supergene evolution involving a variety of structural changes.This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.
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