A masculinizing supergene underlies an exaggerated male reproductive morph in a spider.

In many species, individuals can develop into strikingly different morphs, which are determined by a simple Mendelian locus. How selection shapes loci that control complex phenotypic differences remains poorly understood. In the spider Oedothorax gibbosus, males either develop into a hunched morph with conspicuous head structures or as a fast developing flat morph with a female-like appearance. We show that the hunched-differs from the flat-determining allele by a hunch-specific genomic fragment of approximately 3 megabases. This fragment comprises dozens of genes that duplicated from genes found at different chromosomes. All functional duplicates, including doublesex - a key sexual differentiation regulatory gene, show male-specific expression, which illustrates their combined role as a masculinizing supergene. Our findings demonstrate how extensive indel polymorphisms and duplications of regulatory genes may contribute to the evolution of co-adapted gene clusters, sex-limited reproductive morphs, and the enigmatic evolution of exaggerated sexual traits in general.