Isogamy

Isogamy is a form of sexual reproduction that involves gametes of similar morphology (similar shape and size), differing in general only in allele expression in one or more mating-type regions. Because both gametes look alike, they cannot be classified as "male" or "female." Instead, organisms undergoing isogamy are said to have different mating types, most commonly noted as "+" and "−" strains, although in some species there are more than two mating types (designated by numbers or letters). Fertilization occurs when gametes of two different mating types fuse to form a zygote.

It appears that isogamy was the first stage of sexual reproduction. In several lineages (plants, animals), this form of reproduction independently evolved to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is very much larger than the male and has no ability to move. There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction.

In Ascomycetes, anisogamy (sexes) evolved from isogamy before mating types.

There are several types of isogamy. Both gametes may be flagellated and thus motile. This type occurs for example in algae such as some but not all species of Chlamydomonas.

In another type, neither of the gametes is flagellated. This is the case for example in the mating of yeast. Yeast mating types are commonly noted as "a" and "α" (alpha) instead of "+" and "-".

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