Distinct haplotypes and reversed dominance at a single-gene balanced polymorphism controlling heterodichogamy in two...

In the angiosperm mating system of heterodichogamy, two hermaphroditic morphs temporally alternate between male and female flowering phases, promoting disassortative mating between morphs. Heterodichogamy is widespread within Juglandaceae and is controlled by distinct ancient structural variants in Juglans and Carya. Here, we identify a novel inherited basis in two additional Juglandaceae genera, Pterocarya and Cyclocarya. In Pterocarya, we map dichogamy types to an ∼70 kb trans-species structural variant overlapping a single candidate gene, GFAFL, with a dominant haplotype for female-first flowering. Heterodichogamy in Cyclocarya is associated with a distinct pair of ancient haplotypes at the same locus but with a dominant allele for male-first flowering. Both sets of alleles are ancient (ca. 45 and 50 Ma, respectively), but their divergence postdates the divergence between the genera. GFAFL expression in staminate flowers in both systems reveals trans-regulation of the recessive allele by the dominant haplotype, which in both systems contains tandem duplicates of sequence in the 3' region of GFAFL. We suggest that reciprocal sex matching in heterodichogamy can result from a combination of cis-regulatory divergence and trans-regulation involving "fast" and "slow" alleles at a single flowering-time gene, yielding "early" vs. "late" sex-specific flowering. Our findings shed light on the molecular basis of heterodichogamy and contribute to an emerging view that diverse genetic pathways can be co-opted during its evolution and facilitate turnover in its genetic control.