Phenotypic and genomic changes associated with the evolutionary loss of heterostyly in primroses

Abstract

Trait loss has been documented across broad phylogenetic scales and dissected genetically within species, but how the genetic basis of loss varies among lineages and over evolutionary time remains poorly understood. Heterostyly, a floral polymorphism, represents an ideal case study, as it has been lost repeatedly across angiosperms. In Primula, heterostyly is controlled by the S-locus supergene, and loss-of-function mutations in CYP cause transitions to homostyly. We investigated the critically endangered Primula boveana, whose floral morphology is debated, in the diploid, self-compatible P. sect. Sphondylia. We quantified herkogamy dynamics during anthesis, performed controlled pollination experiments, assembled a chromosome-scale genome, compared S-locus structure and gene expression with heterostylous relatives, and reconstructed phylogenetic relationships within the section. Our results revealed that P. boveana is homostylous, with a disrupted CYP. Herkogamy declines during anthesis, resulting in early anther-stigma contact, autonomous selfing, and reproductive assurance. Thus, unlike other homostylous Primula species, these developmental changes do not represent a delayed selfing strategy. These findings link developmental changes in herkogamy with autonomous selfing and support CYP inactivation as the genetic basis of long homostyly in P. boveana. The extensive degeneration of CYP suggests that prolonged relaxed selection promotes progressive degeneration of non-functional S-genes following heterostyly loss.