In heterothallic species, sexual reproduction occurs when opposite mating types (designated + and -) come into close proximity, inducing the formation of specialized hyphae called "gametangia". The gametangia grow toward each other, then fuse, forming a diploid zygote at the point of fusion. The zygote develops a resistant cell wall, forming a single-celled "zygospore", the characteristic that gives its name to this group of fungi. Meiosis occurs within the "zygospore" (see article Phycomyces). Upon germination, a new haploid mycelium or sporangium is formed. Some species are homothallic.