Sexual Morphogenesis in Achlya: Ultrastri Induction of Antheridial Hyphae* (water molds/steroid hormones/vesicles/freeze-etching)

Male strains of the water mold Achlya ambisexualis produce antheridial hyphae in response to the steroid hormone antheridiol. The antheridial hypha is postulated to be initiated through a localized wall softening with the enzyme cellulase. Freeze-etch studies of hormone-treated hyphae were conducted to determine if aggregates of vesicles are induced at the location of new antheridial hyphae. Localized aggregates of vesicles were found in conjunction with areas of wall thinning. These data suggest that the processes of vesiculation and secre- tion provide a mechanism for concentration of cellulase at the site of initiation of antheridial hyphae. A distinctive pattern of sexuality characterizes heterothallic members of the biflagellate water molds, such as Achlya ambisexualis Raper. In this species, sexual morphogenesis is initiated and sequentially controlled by a series of diffusible