Isogamy

Isogamy is a form of sexual reproduction involving gametes that are morphologically similar, meaning they are the same size and shape. Unlike anisogamy or oogamy, where gametes differ in size and/or motility (e.g., a large, non-motile egg and a small, motile sperm), isogamous gametes are indistinguishable from one another under microscopic examination.

The distinction between "male" and "female" gametes is typically absent in isogamy; instead, compatibility is determined by mating types or strains that must be different for fertilization to occur. These mating types are often designated with simple symbols such as "+" and "-", or numbers. Fertilization involves the fusion of two isogametes from compatible mating types.

Isogamy is considered an ancestral form of sexual reproduction, and is found in certain algae, fungi, and protists. It represents an earlier evolutionary stage in the development of differentiated gametes. Over evolutionary time, isogamy may have given rise to anisogamy and oogamy due to selective pressures favoring different reproductive strategies. While the gametes themselves are similar in size, their genetic content is still essential for genetic recombination. The process allows for increased genetic diversity within a population, which is a key benefit of sexual reproduction.