Haploid (n) vegetative cells occur as two mating types, mt+ and mt-, that divide by mitosis (“Asexual reproduction”; Harris, 2001, Goodenough et al., 2007). Gametogenesis can be induced by nitrogen starvation (-N) in the presence of light, and gametes of opposite mating types can fuse to form diploid (2n) zygotes (“Sexual reproduction”). Within a few hours of fertilization, zygotes resorb their four cilia to become immotile. Over the course of several days these zygotes are remodeled into highly resistant, dormant zygospores. In this process, a strong, multilayered cell wall is formed, and chlorophyll is degraded (Harris, 2001; Goodenough et al., 2007). As a result, mature zygospores appear orange, which reflects their carotenoid content (Lohr, 2009). When environmental conditions improve, the zygote undergoes meiosis to release four haploid cells (sometimes 8 and 16 when mitosis also occurs within the zygote wall; “Germination”). The haploid cells then resume vegetative growth. In the laboratory, zygote germination is induced by the addition of nitrogen (+N) to the medium in the light (Harris, 2001); nitrogen also causes reprogramming of gametes to vegetative cells (Pozuelo et al., 2000). Image credit: Debbie Maizels.