#700-40 Genetic Mechanisms Underlying Sex Determination in the Homosporous Fern Ceratopteris richardii

Homosporous ferns, unlike seed plants, produce sexually undifferentiated spores. Instead, sex in homosporous ferns is environmentally regulated where single spores will develop into female or hermaphroditic fern gametophytes.  These gametophytes secrete the sex pheromone antheridiogen causing nearby undetermined gametophytes to develop into males. In the fern Ceratopteris richardii (Ceratopteris), several hermaphroditic (her) mutants have been characterized that develop as hermaphrodites even in the presence of antheridiogen, but the immense size of the Ceratopteris genome makes efficient identification of the underlying genes a challenge. We performed bulked segregant RNA-seq to map two her mutants (her14 and her19) to a 16Mbp interval on chromosome 29 of the Ceratopteris genome. A brassinosteroid receptor-like kinase gene located within this mapped interval contained a deletion mutation in her14 and a missense mutation in her19. Three additional co-segregating her mutants (her7, her11, and her15) had independent missense mutations in the same gene, which we name HER7, providing strong support for HER7 being required for antheridiogen perception and downstream sex determination in Ceratopteris. Interestingly, antheridiogens in other ferns have been characterized as gibberellic acids and not brassinosteroid derived hormones. This work is the first to implicate a BRL, and by extension brassinosteroids, in sex determination in a homosporous fern.  Other her mutants mapped to different loci are currently being prepared for another bulk segregant analysis by RNA-seq to identify more genes involved in the Ceratopteris sex determination pathway and learn how it may differ from other antheridiogen systems.