GENETIC MECHANISMS UNDERLYING SEX DETERMINATION IN THE HOMOSPOROUS FERN CERATOPTERIS RICHARDII

Homosporous ferns, unlike seed plants, produce sexually undifferentiated spores. Instead, sex is environmentally regulated by antheridiogen, a pheromone secreted from female/hermaphrodite gametophytes. Nearby undetermined gametophytes perceive antheridiogen which induces male development over female/hermaphrodite development. The homosporous fern Ceratopteris richardii (Ceratopteris) is an ideal model for studying the genetics of antheridiogen systems, with 34 sex determination mutants mapped to at least 13 different loci characterized in this species. Modern sequencing and genomic tools make the molecular identification of these mutants in the 11 Gbp genome of Ceratopteris now possible. These mutants include hermaphroditic (her) mutants, which develop as hermaphrodites even in the presence of antheridiogen. We mapped two linked mutants, her7-14 and her7-19, to the same 16 Mbp interval on chromosome 29 of the Ceratopteris genome using bulked segregant analysis by RNA-seq. A homolog of the receptor kinase Brassinosteroid Insensitive1 within this interval encoded a deletion mutation in her7-14 and a missense mutation in her7-19. Three other linked her mutants encoded missense mutations in the same gene, which we name HER7. Antheridiogens in other ferns have been characterized as gibberellins and not brassinosteroid-derived hormones. Analysis of gene expression showed brassinosteroid biosynthesis was upregulated in hermaphrodites compared to male gametophytes, while gibberellin expression patterns were unclear. This work supports HER7 being required for sex determination in Ceratopteris and suggests that Ceratopteris antheridiogen may be a brassinosteroid. In the future, other sex determination mutants could be used to identify more genes involved in this pathway and learn how it may differ from other antheridiogen systems.