Light microscopy of meiotic zoosporogenesis and mitotic gametogenesis in Ulva mutabilis Foyn.

Among the chlorophycean algae the members of the family Ulvaceae are characterized by a multicellular structure and a haplo-diplontic life-cycle that proceeds without formation of special generative organs. The transformation in these algae of ordinary vegetative cells into generative ones producing either gametes mitotically or zoospores meiotically was a major reason for introducing Ulva mutabilis Foyn as an object for the biochemical study of meiosis (Nordby & Hoxmark, 1972). In this connexion it was felt necessary to carry out a closer examination of the cytology of the 2 kinds of generative divisions mentioned. In the literature, the generative divisions in Ulva species have been partly described by Carter (1926), Feyn (1934), Smith (1947) and Yabu & Park (1969). The most detailed description of the nuclear events is given by Feyn, who also was the first to show that genetic segregation took place during meiosis in this alga. For Ulva mutabilis specifically, short electron-microscopical descriptions have been given by Braten (1971), Braten & Levlie (1968) and Braten & Nordby (1973). Of studies on related algae, the most extensive report is probably that of Ramanathan (1939) on Enteromorpha. The present report concerns only the gametophytes and diploid sporophytes. The parthenosporophytes are dealt with elsewhere (Hoxmark & Nordby, 1974).