The Programming of Sexual Phenotype in the Homosporous Fern Ceratopteris richardii

Genetically identical Ceratopteris gametophytes are either hermaphroditic or male. The determinant of sex type is antheridiogen (ACE), a pheromone that is secreted by the hermaphrodite and promotes male development of sexually undetermined gametophytes. The hormone abscisic acid (ABA) blocks the ACE response. The morphological and physiological changes in developing wildtype and mutant hermaphroditic1 (her1) gametophytes have been characterized and used to define six discrete stages of gametophyte development. Stage 1 begins when the spore is inoculated on culture medium. During this stage, the spore imbibes water but the spore wall remains intact and exposure to ACE will not promote male development, probably because of levels of ABA in the spore that are sufficiently high to block the ACE response. During stage 2, 3-4 d after spore inoculation, the spore wall cracks. Exposure of the two- to three-celled gametophyte to ACE during stage 2 is necessary for initiating a male program of expression. If not exposed to ACE at stage 2, the gametophyte initiates a hermaphroditic program of expression that cannot be reversed by exposure to ACE at later stages. During stage 3, 4-5 d after inoculation, the gametophyte consists of a uniserate protonema of three to five cells with one to three rhizoids. Gametophytes must be continuously exposed to exogenous ACE from stage 2 through stage 3 to develop as males. At stage 4, 5-6 d after inoculation, bidimensional growth of the prothallus begins. Although the male and hermaphroditic prothalli are morphologically indistinguishable at this stage, gametophytes begin to secrete ACE. At stage 5, beginning 6-7 d after spore inoculation, the male and hermaphroditic morphologies become distinct and mature sexually by stage 6. While the hermaphroditic program of expression is stable once initiated, the male program of expression, if initiated at stage 2, is reversible by the withdrawal of ACE or by exposure to ABA, indicating that ACE is required for both the initiation and maintenance of the male program of expression in Ceratopteris.