Genomic evidence for supergene control of Darwin's 'complex marriage arrangement' - the tristylous floral polymorphism

Tristyly is a polymorphism characterized by three flower morphs with reciprocal stigma and anther heights controlled by two epistatically interacting diallelic loci (S and M), hypothesized to be supergenes. Chromosome-level genome assemblies of Eichhornia paniculata identified the S- and M-loci. The S-locus is a supergene consisting of two divergent alleles: The S-allele (2.51Mb) with three S-allele specific genes hemizygous in most S-morph plants and the s-allele (596kb) with five s-allele specific genes. Two of the S-allele specific genes, LAZY1-S and HRGP-S, were specifically expressed in styles and stamens, respectively, making them tristyly candidate genes. The M-locus contained one gene (LAZY1-M), homologous to LAZY1-S, that was present in the M-allele but absent from the m-allele. Estimates of allele ages are consistent with the prediction that the S-locus evolved before the M-locus. Re-use of the same gene family highlights the potential role of gene duplication in the evolution of epistatic multilocus polymorphisms.