Determining the Genes for Distyly in Turnera

Heterostyly promotes outbreeding and reduces pollen wastage. Distylous species have two mating types: L-morph and S-morph plants, which are genetically determined by the S-locus. It was hypothesized that the S-locus carries at least three tightly linked genes constituting a supergene. The S-locus genes responsible for distyly in Turnera had not been determined. Using deletion mapping and chromosome walking, the S- and s-haplotypes of the S-locus were constructed. Three hemizygous genes, TsSPH1, TsYUC6, and TsBAHD occur only on the S-haplotype and appear to determine S-morph characteristics. TsSPH1 is expressed in filaments, TsYUC6 in anthers, and TsBAHD in styles. A long-homostyle mutant (Drh) did not possess TsBAHD while another long-homostyle mutant (Mhomo-H) possessed a substitution mutation in TsBAHD. Short-homostyle mutants did not express TsSPH1. The incompatibility system of Turnera joelii possessed a mutation that appears to be linked to S-locus. A possible mechanism of incompatibility is discussed which involves brassinosteroids and auxins.