Gardeners and botanists have known since the 1500s that some plant species have two forms of flowers that differ reciprocally in the length of their male and female sexual organs. Darwin first proposed that such distylous flowers promoted efficient cross-pollination through insect pollinators. Early geneticists showed that the two forms of flowers were controlled by a single chromosomal region likely harboring a cluster of genes, a supergene. But until recently this supergene had never been sequenced.

Now, researchers at Stockholm University, together with partners at Uppsala University, Durham University, University of Granada, and University of Seville, have solved the mystery of the supergene. They studied a system where already Darwin described distyly, wild flaxseed species, Linum, and used modern DNA sequencing methods to identify the supergene.

Surprisingly, they found that the supergene responsible for differing lengths of male and female sexual organs itself varied in length. Specifically, the dominant form of the supergene contained about 260,000 base pairs of DNA that were missing from the recessive form. The 260,000 base pair stretch of DNA harbored several genes likely to cause length variation in sexual organs.