Neo-sex chromosomes are found in many taxa, but the forces driving their emergence and spread are poorly understood. The female-specific neo-W chromosome of the African monarch (or queen) butterfly Danaus chrysippus presents an intriguing case study because it is restricted to a single ‘contact zone’ population, involves a putative colour patterning supergene, and co-occurs with infection by the male-killing endosymbiont Spiroplasma. This study by Martin et al. investigates the origin and evolution of this system using whole genome sequencing. They identify the ‘BC supergene’, a broad region of suppressed recombination across nearly half a chromosome, which links two colour patterning loci that control the forewing colour pattern differences between D. chrysippus subspecies. They then show that the same chromosome has recently formed a neo-W that has spread through the contact zone within approximately 2,200 years. Analysis of the genome of the male-killing Spiroplasma reveals perfect genealogical congruence with the neo-W, suggesting that the neo-W has hitchhiked to high frequency as the male-killer has spread through the population. Overall. the findings show how hitchhiking can occur between the physically unlinked genomes of host and endosymbiont, with dramatic consequences. The image shows a female D. chrysippus drinking nectar from a Calotropis milkweed flower. She will also lay her eggs on this plant species, and the caterpillars who feed on it will carry its toxic cardenolide glycosides; the butterfly’s bright orange, black and white patterns warn predators that it is distasteful.

Image Credit: Chris Ward