GAMETANGIA, GAMETES, FERTILIZATION AND ZYGOTE DEVELOPMENT IN ULOTHRIX-FLACCA VAR ROSCOFFENSIS (CHLOROPHYTA)

The mature gametangia, gametes, gamete fusion, young motile zygotes and development of the settled zygotes into unicellular sporophytes (Codiolum stages) are described in Ulothrix flacca (Dillwyn) Thuret var. roscoffensis Berger-Perrot (Ulvophyceae, Chlorophyta) using light and electron microscopy. Flagella of gametes narrow to a hair-like point. Gamete fusion takes place near two lateral thickenings of the plasma membrane, which are considered to be poorly differentiated mating structures. The dilated perinuclear space contains an unidentified electron-dense substance. Karyogamy generally takes place 8-24 h after gamete fusion and chloroplasts have fused 12 h after gamete fusion. Flagella are shed approximately 8 h after fertilization. Twelve hours after, abundant osmiophilic droplets are secreted on the plasma membrane of the zygotes prior to formation of the cell wall. Forty-eight hours later, the quiescent zygotes become stalked sporophytes after striking cytological changes. These features are discussed in relation to other members of the Ulvophyceae and other green algae.