THE PROGRAMMING OF SEXUAL PHENOTYPE IN THE HOMOSPOROUS FERN CERATOPTERIS-RICHARDII

Genetically identical Ceratopteris gametophytes are either hermaphrodi tic or male. The determinant of sex type is antheridiogen (A(CE)), a p heromone that is secreted by the hermaphrodite and promotes male devel opment of sexually undetermined gametophytes. The hormone abscisic aci d (ABA) blocks the A(CE) response. The morphological and physiological changes in developing wildtype and mutant hermaphroditic1 (her1) game tophytes have been characterized and used to define six discrete stage s of gametophyte development. Stage 1 begins when the spore is inocula ted on culture medium. During this stage, the spore imbibes water but the spore wall remains intact and exposure to A(CE) will not promote m ale development, probably because of levels of ABA in the spore that a re sufficiently high to block the A(CE) response. During stage 2, 3-4 d after spore inoculation, the spore wall cracks. Exposure of the two- to three-celled gametophyte to A(CE) during stage 2 is necessary for initiating a male program of expression. If not exposed to A(CE) at st age 2, the gametophyte initiates a hermaphroditic program of expressio n that cannot be reversed by exposure to A(CE) at later stages. During stage 3, 4-5 d after inoculation, the gametophyte consists of a unise rate protonema of three to five cells with one to three rhizoids. Game tophytes must be continuously exposed to exogenous A(CE) from stage 2 through stage 3 to develop as males. At stage 4, 5-6 d after inoculati on, bidimensional growth of the prothallus begins. Although the male a nd hermaphroditic prothalli are morphologically indistinguishable at t his stage, gametophytes begin to secrete A(CE). At stage 5, beginning 6-7 d after spore inoculation, the male and hermaphroditic morphologie s become distinct and mature sexually by stage 6. While the hermaphrod itic program of expression is stable once initiated, the male program of expression, if initiated at stage 2, is reversible by the withdrawa l of A(CE) or by exposure to ABA, indicating that A(CE) is required fo r both the initiation and maintenance of the male program of expressio n in Ceratopteris.