Genomic Analyses Elucidate -locus Evolution in Response to Intra-specific Losses of Distyly in

Distyly, a floral dimorphism that promotes outcrossing, is controlled by a hemizygous genomic region known as the -locus. Disruptions of genes within the -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 and leveraging high-quality reference genomes of we tested, for the first time, predictions about the evolutionary consequences of transitions to selfing on -genes. Our results reveal a previously undetected structural rearrangement in associated with the shift to homostyly and confirm previously reported, homostyle-specific, loss-of-function mutations in the exons of the -gene . We also discovered that the promoter and intronic regions of in distylous and homostylous individuals are conserved, suggesting that down-regulation of via mutations in its promoter and intronic regions is not a cause of the shift to homostyly. Furthermore, we found that hemizygosity is associated with reduced genetic diversity in -genes compared with their paralogs outside the -locus. Additionally, the shift to homostyly lowers genetic diversity in both the -genes and their paralogs, as expected in primarily selfing plants. Finally, we tested, for the first time, long-standing theoretical models of changes in -locus genotypes during early stages of the transition to homostyly, supporting the assumption that two copies of the -locus might reduce homostyle fitness.