Transcriptome and Network Analyses of Heterostyly in Provide Mechanistic Insights: Are -Loci a Red-Light for Pistil Elongation?

Heterostyly employs distinct hermaphroditic floral morphs to enforce outbreeding. Morphs differ structurally in stigma/anther positioning, promoting cross-pollination, and physiologically blocking self-fertilization. Heterostyly is controlled by a self-incompatibility -locus of a small number of linked -genes specific to short-styled morph genomes. possesses three -genes, namely (controlling pistil characters), and (controlling stamen characters). Here, we compare pistil and stamen transcriptomes of floral morphs of to investigate hypothesized -gene function(s) and whether hormonal differences might contribute to physiological incompatibility. We then use network analyses to identify genetic networks underpinning heterostyly. We found a depletion of brassinosteroid-regulated genes in short styled (S)-morph pistils, consistent with hypothesized brassinosteroid-inactivating activity of . In S-morph anthers, auxin-regulated genes were enriched, consistent with hypothesized auxin biosynthesis activity of . Evidence was found for auxin elevation and brassinosteroid reduction in both pistils and stamens of S- relative to long styled (L)-morph flowers, consistent with reciprocal hormonal differences contributing to physiological incompatibility. Additional hormone pathways were also affected, however, suggesting -gene activities intersect with a signaling hub. Interestingly, distinct -genes controlling pistil length, from three species with independently evolved heterostyly, potentially intersect with phytochrome interacting factor () network hubs which mediate red/far-red light signaling. We propose that modification of the activities of hubs by the -locus could be a common theme in the evolution of heterostyly.