isogametes

Isogametes are gametes that are morphologically indistinguishable from one another. In organisms exhibiting isogamy, both types of gametes are similar in size and appearance; they may be motile or non-motile. Despite their visual similarity, they are compatible only with gametes of the opposite mating type, which is typically determined by genetic factors and may be mediated by pheromonal or chemical signals.

Isogamy contrasts with anisogamy, where gametes differ in size, and oogamy, where a large non-motile egg is fertilized by a small motile sperm. In isogamous species, fertilization involves the fusion of two compatible gametes, followed by cytoplasmic (plasmogamy) and nuclear (karyogamy) fusion to form a diploid zygote. The zygote often may enter a period of dormancy or immediately engage in meiosis to produce haploid offspring, depending on the life cycle.

Occurrence and examples: Isogamy is observed in a range of unicellular and colonial organisms. Notable examples include some green algae, such as certain strains of Chlamydomonas and related taxa, where two mating types fuse to produce a zygote. It also occurs in some early-diverging fungi, including certain chytrids and zygomycetes, where gametes are morphologically similar and compatibility is governed by mating-type genes rather than visible differences in gamete size or form.

In summary, isogamy describes a form of sexual reproduction with indistinguishable gametes that require opposite mating types for fusion, representing an ancestral or alternative strategy in the evolution of sexual dimorphism.