isogamy

Isogamy is a mode of sexual reproduction in which the male and female gametes are morphologically similar or indistinguishable. In isogamous species, two compatible gametes fuse to form a zygote, but the gametes may still be functionally differentiated by mating type rather than by size or form. The term contrasts with anisogamy, where gametes differ in size or form, and with oogamy, where a large non-motile egg is fertilized by a small motile sperm. In many isogamous organisms, the two mating types are genetically determined and may be labeled as plus and minus. The gametes are often motile with flagella, though some may be nonmotile. Fertilization typically yields a zygote that can undergo meiosis to return to the haploid phase, depending on the organism's life cycle. Isogamy occurs in several groups of primitive eukaryotes, including some green algae and certain fungi (chytrids) and protists. It is thought to represent an ancestral condition from which anisogamy and oogamy evolved in many lineages.