A -acting bidirectional transcription switch controls sexual dimorphism in the liverwort.

Plant life cycles alternate between haploid gametophytes and diploid sporophytes. While regulatory factors determining male and female sexual morphologies have been identified for sporophytic reproductive organs, such as stamens and pistils of angiosperms, those regulating sex-specific traits in the haploid gametophytes that produce male and female gametes and hence are central to plant sexual reproduction are poorly understood. Here, we identified a MYB-type transcription factor, MpFGMYB, as a key regulator of female sexual differentiation in the haploid-dominant dioicous liverwort, MpFGMYB is specifically expressed in females and its loss resulted in female-to-male sex conversion. Strikingly, Mp expression is suppressed in males by a -acting antisense gene at the same locus, and loss-of-function mutations resulted in male-to-female sex conversion. Thus, the bidirectional transcription module at the Mp locus acts as a toggle between female and male sexual differentiation in gametophytes. Mp orthologs are known to be expressed in embryo sacs and promote their development. Thus, phylogenetically related MYB transcription factors regulate female gametophyte development across land plants.