Figure 4. The multiple fission cell cycle and sexual cycles of Chlamydomonas and Gonium.

During vegetative growth (A), Chlamydomonas cells may grow many fold size with the extent of growth somewhat indeterminate. Cells then divide multiple times to produce uniform-sized daughters. Two rounds of cell division are shown in this panel, with division numbers ranging from one to four in a typical culture. Gonium colonies (B) follow a similar growth and division pattern as Chlamydomonas, but the daughter cells remain attached to each other through cytoplasmic bridges and ECM (see Fig. 6). The Chlamydomonas sexual cycle (C) is induced by lack of nitrogen (-N) that causes cells to differentiate into gametes. Gametes from each mating type (plus and minus) are similar in size. Flagellar adhesion between gametes of opposite mating type precedes cell fusion to form a diploid zygote. Upon germination four meiotic progeny are produced that can reenter the vegetative cycle. In Gonium –N also triggers gametogenesis that involves colony dissolution into unicellular gametes. Post-meiotic Gonium progeny are single cells that produce a new vegetative colony after their first passage through the cell cycle.