Converging on long and short: The genetics, molecular biology and evolution of heterostyly

Heterostyly is a fascinating floral polymorphism that enhances outcrossing. In heterostylous species the flowers of the two or three morphs differ in multiple traits, including reciprocal reproductive-organ placement and self-incompatibility. These traits are controlled by individual genes within an S-locus supergene, whose suppressed recombination ensures the coordinated inheritance of the morph phenotypes. Recent breakthroughs about the genetic and molecular basis of heterostyly have resulted from studies on many independently evolved instances and include the following: The S-locus is a hemizygous region comprising several individual genes in multiple heterostylous taxa. In many systems, a single gene within the S-locus plays dual roles in regulating both female traits of style length and self-incompatibility type, often involving brassinosteroid signalling. The S-loci have evolved through stepwise or segmental duplication in different lineages. The frequent breakdown of heterostyly generally results from individual mutations at the S-locus and leads to a genomic selfing syndrome. These discoveries suggest convergent and genetically constrained evolution of heterostyly at the molecular level.