Sex determination in the fern Ceratopteris

The fern Ceratopteris richardii produces only one type of spore, yet each can develop as a male or a hermaphrodite gametophyte. The determinant of sex is the pheromone antheridiogen, which is secreted by the hermaphrodite and promotes the development of males from sexually undetermined gametophytes. Recent analyses of sex determination mutants have defined a hypothetical network of at least eight interacting genes that are involved in specifying sex. Among these are the two major sex determining genes, TRA1 and FEM1, which are required for promoting the development of female and male traits, respectively. Each also negatively regulates the expression of the other, so that the sex of the gametophyte depends on which of the two genes 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 TRA1. The cloning of the sex-determining genes identified by mutation should allow many new questions in plant development and evolution to be addressed.