In some butterfly species one sex — usually the female — mimics the wing pattern of a toxic species. In the 1960s the phenomenon was proposed to be under the control of a 'supergene'. More recently a consensus has emerged that supergenes are likely to be clusters of tightly linked genes, each influencing a different aspect of mimetic wing patterning. Now Marcus Kronforst and colleagues show, surprisingly, that in a classic supergene mimic, the swallowtail butterfly Papilio polytes, the supergene is truly a single gene. And another surprise: that gene is a well-known component in the sex determination pathway called doublesex. Gene expression and DNA sequence variation data suggest that isoform expression differences and protein sequence evolution also contribute to the differences between doublesex mimicry alleles. Thus the P. polytes mimicry supergene can be summed up as a fusion of previous hypotheses: single-gene control but with help from multiple functional mutations.
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