Characterization of the S locus supergene controlling heterostyly in Darwin’s Primroses

Most Primula species produce two forms of flower known as pin and thrum.   Pin flowers have a long style and low anthers that produce small pollen, thrum flowers have a short style and high anthers that produce large pollen.  Although the two forms of flower were noted in the 16th Century, it was Charles Darwin’s observations on heterostyly in Primula that described the reciprocal position of male and female reproductive structures in the two forms of flower as a mechanism to promote insect-mediated cross-pollination.  Plants of most Primula species produce only pin or only thrum flowers and maximal seed set follows cross-pollination between the two forms; a self-incompatibility (SI) system in many species also reduces self-pollination.  The development of the two forms of flower and the SI system are controlled by a cluster of genes known as the S (Style length) locus.  The thrum form is dominant over the pin form; thrums behave as heterozygotes and pins as homozygous recessives; a pin x thrum cross therefore yields equal numbers of pin and thrum progeny.  Rare homostyle individuals with the anthers and stigma at the same height in self-fertile flowers occur and these were assumed to result from recombination between dominant and recessive S alleles.

To identify the genes controlling heterostyly we identified S locus-linked loci and used these in classical genetic mapping to generate a linkage map around the S locus.  These sequences facilitated a BAC walk across the region, which together with de novo genome sequencing of P. vulgaris, enabled us to identify and characterise the complete S locus gene cluster.  This cluster contains five genes, one controls anther elevation; another reduces style length in thrum flowers.  The role of the other three genes remains to be determined. Our recent findings provide new insight into the mechanism controlling heterostyly in Primula.  Thrum plants are not, as predicted, heterozygous but are hemizygous; there is no pin allele, the S locus gene cluster is absent from the pin genotype.  Furthermore, the hemizygous nature of the S locus means that homostyles cannot, as predicted, occur by recombination.  We dated the origin of one duplicated S locus gene to 51.7M years ago, before speciation of the ~430 Primula species; we therefore predict that heterostyly arose just once in the Primulaceae.  Identification of the S locus gene cluster provides answers to some long standing questions on the control of heterostyly, and also raises new questions around the maintenance of heterostyly and control of the associated self-incompatibility system.