Genomic analyses of the breakdown of distyly in Linum trigynum

Distyly is a complex floral polymorphism that promotes efficient pollen transfer and outcrossing. In distylous plants two floral morphs that differ by the position of their anthers and stigmas coexist within the same population, and this specific arrangement of traits is governed by a supergene known as the S-locus. Importantly, distyly has often evolved in association with a heteromorphic incompatibility system that prevents self- and intra-morph fertilization and reinforces disassortative mating. Distyly can break down into homostyly, i.e., populations exhibiting a single floral type with stigmas and anthers located at the same height, and such flowers can usually self-fertilize. The loss of distyly could thus be expected to be associated with a shift to higher levels of self-fertilization, with potentially genome-wide consequences. Here, we investigate the genetic causes behind loss of distyly in Linum trigynum, and study the genome-wide consequences of this transition. We assembled genome sequences of the homostylous L. trigynum and the closely related distylous L. tenue to determine if S-linked genes exhibit candidate mutations for disrupting distyly. Our results suggest that L. trigynum is highly self-fertilizing and has significantly lower genetic diversity genome-wide as well as more marked population structure compared to its distylous relative L. tenue. Our findings shed light on the genetic basis and genomic effects of a prevalent evolutionary transition in flowering plants.