9723154 Banks Technical section Gametophytes of the fern Ceratopteris richardii are sexually dimorphic, either hermaphroditic or male. The determinant of sex in this and other ferns is antheridiogen, a gibberellin-like pheromone that is secreted by the hermaphroditic gametophytes and which promotes male development of other sexually undetermined gametophytes. The plant hormone abscisic acid blocks the antheridiogen response. Several key regulatory genes involved in sex determination have been identified by generating and characterizing mutations that alter the normal sex of the gametophyte. A genetic analysis of these mutants defines a regulatory network of at least eight interacting genes that are involved in determining sex type. Among these are two major sex-determining genes, TRA-1 and FEM-1, whose activities are required to promote the differentiation of female and male traits, respectively. Each gene also negatively regulates the expression of the other such that the sex of the gametophyte ultimately depends on which gene is expressed first. Antheridiogen activates a signal transduction pathway defined by the HER genes which affects the sex of the gametophyte by regulating the activity of TRA-1.This model of sex determination will be tested by continuing the genetic and morphological analysis of new and existing sex-determining mutants. One such gene, a Ceratopteris homolog of KNOTTED-1 is likely to also be involved in meristem development in the hermaphrodite and a target of repression by antheridiogen during male development. Several genes that are up-regulated in response to exposure to exogenous antheridiogen have also been isolated and the functions of these genes will be determined. A final objective is to develop an Agrobacterium-mediated transformation system in the fern. If successful, the Ac/Ds transposable elements of maize will be introduced and their ability to transpose in the fern will be tested. If they are mobile, mutants induced by transposon tagging will be sought . Non-technical section How sex is determined in plants is an important developmental question whose answer may ultimately have practical applications in agriculture and plant breeding. The proposed studies will provide a basic understanding of the genes involved in sex determination and the signaling pathway for known pheromonal and hormonal cues. ***
