Distyly, a floral dimorphism that promotes outcrossing, is controlled by a hemizygous genomic region known as the S-locus. Disruptions of genes within the S-locus are responsible for the loss of distyly and the emergence of homostyly, a floral monomorphism that favors selfing. Using whole genome resequencing data of distylous and homostylous individuals from populations of Primula vulgaris and leveraging high-quality reference genomes of Primula we tested, for the first time, predictions about the evolutionary consequences of transitions to selfing on S-locus genes. Our results confirm the presence of previously reported homostyle-specific, loss-of-function mutations in the exons of the S-locus gene CYPᵀ, while also revealing a previously undetected structural rearrangement in CYPᵀ associated with the shift to homostyly. Additionally, we discovered that the promoter region of CYPᵀ in distylous and homostylous individuals is identical, suggesting that down-regulation of CYPᵀ via mutations in its promoter region is not a cause of shift to homostyly. Furthermore, we found that hemizygosity leads to reduced genetic diversity and less efficient purifying selection in S-locus genes compared to genes outside the S-locus, and that the shift to homostyly further lowers genetic diversity, as expected for mating-system shifts. Finally, we tested, for the first time, long-standing theoretical models of changes in S-locus genotypes during early stages of the transition to homostyly, supporting the assumption that two (diploid) copies of the S-locus might reduce homostyle viability.

The repeated transition from outcrossing to selfing is a key topic in evolutionary biology. However, the molecular basis of such shifts has been rarely examined due to lack of knowledge of the genes controlling these transitions. A classic example of mating system transition is the repeated shift from heterostyly to homostyly. Occurring in 28 angiosperm families, heterostyly is characterized by the reciprocal position of male and female sexual organs in two (or three) distinct, usually self-incompatible floral morphs. Conversely, homostyly is characterized by a single, self-compatible floral morph with reduced separation of male and female organs, facilitating selfing. Here, we investigate the origins of homostyly in Primula vulgaris and its microevolutionary consequences by integrating surveys of the frequency of homostyles in natural populations, DNA sequence analyses of the gene controlling the position of female sexual organs (CYPᵀ), and microsatellite genotyping of both progeny arrays and natural populations characterized by varying frequencies of homostyles. As expected, we found that homostyles displace short-styled individuals, but long-style morphs are maintained at low frequencies within populations. We also demonstrated that homostyles repeatedly evolved from short-styled individuals in association with different types of loss-of-function mutations in CYPᵀ. Additionally, homostyly triggers a shift to selfing, promoting increased inbreeding within and genetic differentiation among populations. Our results elucidate the causes and consequences of repeated transitions to homostyly within species, enabling a likely explanation for the fact that homostyly has not become fixed in P. vulgaris. This study represents a benchmark for future analyses of losses of heterostyly.