A classic trait in the study of the genetics of natural variation is the heterostyly polymorphism in primroses (Primula). As once described by Darwin himself in 1861, two floral morphs segregate in natural populations. The “pin” morph has flowers with a long style and short anthers, and the “thrum” morph has flowers with a short style and long anthers. This variation among individuals helps enforce outcrossing and prevent reproductive interference because pollen from the pin morph will only be transferred by pollinators to the stigma of the thrum morph, and pollen from the thrum morph only gets places on the stigma of the pin morph.

For this breeding system to work, the genes controlling the differences in floral organ length between pin and thrum need to be very close together in the genome, an arrangement known as a supergene, preventing recombination that leads to flowers with styles and anthers that are flush. Indeed, in 1935 DeWinton and Haldane screened over 18,000 sibling plants and could not find a single recombinant plant with styles and anthers of the same length. New work shows the molecular basis of the supergene, and interestingly, the polymorphism is a deletion variant that kicks a region with 5 genes in it out of the genome. Pin morphs lack these genes, thrum morphs have them, and the studies separately implicate two of these genes in the style and anther length phenotypes.

--Ben Blackman

References:

Li J et al. Genetic Architecture and evolution of the S locus supergene in Primula vulgaris. (2016) Nature Plants 2:16188.

Huu et al. Presence versus absense of CYP734A50 underlies style-length dimorphism in primroses. (2016) Elife 5: e17956.