Briefly, Chlamydomonas usually exist in haploid single cells, formed by the "mating" of two strains of opposite mating type (plus and minus). An explanation of the diagrammatic representation of the Chlamydomonas life cycle shown below is taken from Zhao H et al., Genes Dev. 2001;15:2767-2777 (click on the link for the full paper). Gametes are formed when vegetative mt+ and mt− cells undergo gametogenesis induced by nitrogen deprivation (middle left). When gametes of opposite mating type (or sex) are mixed together, they adhere to each other via mating type specific adhesion molecules (agglutinins) on their flagella to form large collections of agglutinating cells (upper center). Flagellar adhesion induces gamete activation that leads to release of cell walls, erection of mating structures at the apical ends of the gamete cell bodies (upper right), and several other cellular and biochemical changes that prepare the gametes for cell fusion. All of the events associated with gamete activation can be induced in gametes of a single mating type by incubating them in dibutyryl cAMP (Pan and Snell 2000b). The tips of the mating structures fuse to establish cytoplasmic continuity, and soon thereafter the two gametes merge completely to become a zygote (lower right). Zygote formation can be extremely rapid, and within 5–10 min after gametes are mixed together, 70%–90% of the cells fuse. Immediately after cell fusion, the zygote developmental pathway is initiated and at 2–4 h after fusion, the zygotes form large aggregates of tightly adherent cells (lower center) as a consequence of synthesis of zygote-specific extracellular matrix molecules and cell body adhesion molecules. At about the same time, the mt− chloroplast DNA is degraded. After an obligatory several days in the dark and upon return to nutrient-rich medium, meiosis and germination occur, and each zygote forms four progeny, two mt+ and two mt− vegetative cells (lower right). Vegetative cells undergo mitotic divisions until the nitrogen is depleted from their medium and the cycle begins again. Diagram modified and adapted from Figure 1 of Pan and Snell 2000b.
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| Source: http://genesdev.cshlp.org/content/15/20/2767/F1.large.jpg |
