Genome sequencing of a highly heterozygous African queen butterfly, Danaus chrysippus. Danaus chrysippus

Danaus butterflies make an ideal system to study introgression and species barriers using genomics. They have conspicuous wing pattern in different subspecies. In addition to their distinct wing patterns, which should be under selection to maintain consistent warning signals in each region, there is evidence that selection acts against hybrids. Moreover, the hybrid zone, a region where different subspecies interbreed, exhibits high levels of infection by Spiroplasma, which kills, based on recent evidence, male offspring before they hatch, leading to skewed sex ratios in the Rift Valley region. To date, no genomic study has investigated the full extent of male-killing and species barriers across the genus. Cytogenetics based evidence shows that female determining sex chromosome and an autosome, responsible for color pattern, is fused and segregates together. For a meaningful comparison and detection of chromosomal fusion, it’s important to have a good quality reference genome. This group (all subspecies of Danaus chrysippus) therefore offers a rare opportunity to address a range of questions about hybridization between both close and distant relatives, providing broad insights into this important biological process.