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

Abstract 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. Competing Interest Statement The authors have declared no competing interest. Footnotes Figure 2: Added nucleotide diversity and combined Copia and Ty3 into LTR; Figure 3f-h: Updated the p-values from t-test to Wilcoxon Signed-rank test in Figure 3f-h; Revised the origin of CYCD6;1-S in Figure 4, taking the whole genome duplication into consideration; Figure 5: Added the expression pattern of CYCD6;1-S; Figure 6: Redrew the illustration of postulated sequence of the evolution to trimorphism from monomorphism. Paragraph on balancing selection removed, as there is not a good method to test the hypothesis with current data.