Evolutionary consequences of repeated loss of distyly in Linum

The breakdown of distyly, a polymorphism that promotes disassortative pollination between two floral morphs, has significant evolutionary implications. Here we examine the consequences of repeated loss of distyly in Linum by testing for relaxed selective pressure in homostylous relative to distylous lineages, and by characterising the evolutionary genomic patterns of the homostylous Linum leonii in comparison to its distylous close relative Linum perenne. We generated whole-genome sequences and target-capture data from sixteen Linum species, and additionally built a high-quality genome assembly and acquired population-level whole-genome sequencing data for L. leonii (n=20). We reconstructed plastome and nuclear phylogenies, estimated selective pressure for chloroplast and nuclear genes, inferred ancestral floral morph states, and tested for signatures of selfing in L. leonii. Compared to theoretical expectations, results were mixed, with partial identification of relaxed selective pressure in homostyles. One clade exhibited signs of potentially accelerated plastome evolution. Population genomic analyses of L. leonii revealed a moderate selfing rate of 0.32, suggesting that loss of distyly was associated with mixed mating rather than selfing, contrary to previous results on loss of distyly. Reduced nucleotide diversity and evidence for relaxed selection efficacy in L. leonii was likely due to a historical bottleneck. These results highlight the complex evolutionary dynamics associated with the breakdown of distyly. The genomic consequences are more heterogeneous than previously thought, and likely depend on clade- and species-specific evolutionary and demographic history dynamics. This study emphasizes the need for comparative population genomic studies to clarify how such transitions can shape evolutionary processes. Significance statementPlant mating system variation is central to evolution as it shapes genetic diversity, adaptability and fitness. Loss of distyly, an iconic example of a complex mating system favouring cross-pollination, can drives shift from outcrossing to selfing, with potentially severe evolutionary consequences for the long-term persistence of the species in which it occurs. Using high-quality genome assembly and omic data for multiple Linum species, we tested for relaxed selective pressure following loss of distyly in homostylous species and tested for a signature of selfing in homostylous Linum leonii compared to the closely related distylous Linum perenne. Contrary to theoretical expectations, evidence for relaxation of selection was mixed in Linum homostyles and L. leonii did not exhibit a genomic signature of selfing. Our study reveals multiple evolutionary pathways following the loss of distyly, and highlights how mating system transitions, together with complex demographic processes, can differentially shape plant genetic diversity and evolution.