Abstract:By finding some important culture conditions as below, we succeeded in experimentally controlling the whole life history of a dioecious marine green alga, Bryopsis plumosa. In this study, we focused on the primary and secondary sex ratios (i.e. at inception and maturity) using these culture techniques. Gametogenesis was induced by culturing haploid gametophytes with PES medium under a 14:10 (Light:Dark) condition at 14˚C. Formed zygotes grew into diploid sporophytes, which were cultured for three months with PES medium under a 14:10 (L:D) condition at 18˚C. Then they were transferred into Schreiber medium and cultured under a 10:14 (L:D) condition at 22˚C. Within one week, zoosporogenesis was observed. Zoospores were released within a couple of days. Each zoospore soon germinated and grew into an unisexual gametophyte. The primary sex ratio was examined in gametophytes that originated from a single sporophyte. The secondary sex ratio was studied in the field. Both were estimated as 1:1. Synchronized meiotic cell divisions might occur during zoosporogenesis dividing each sex determining factor evenly among zoospores. Given the equal sex ratio at maturity, there seems to be no environmental factor that differentially affects survival of male or female gametophytes in nature.