Isogamy refers to a form of sexual reproduction involving gametes of the same size. Since 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. Fertilization occurs when "+" and "-" gametes fuse to form a zygote.
There are several types of isogamy. Both gametes may be flagellated and thus motile. This type occurs for example in algae such as 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 "?" (alpha) instead of "+" and "-".
Another, more complex form, is conjugation. This occurs in some green algae, for example in Spirogyra. These algae grow as filaments of cells. When two filaments of opposing mating types come close together, the cells form conjugation tubes between the filaments. Once the tubes are formed, one cell balls up and crawls through the tube into the other cell to fuse with it, forming a zygote.
Fungi also use conjugation. In zygomycetes, two hyphae of opposing mating types form special structures called gametangia where the hyphae touch. The gametangia then fuse into a zygosporangium. In other fungi, cells from two hyphae with opposing mating types fuse, but only cytoplasmic (plasmogamy). The two nuclei do not fuse, leading to the formation of a dikaryon cell that gives rise to a mycelium consisting of dikaryons. Karyogamy (fusion of nuclei) occurs in sporangia and leads to the formation of diploid cells (zygotes) that immediately undergo meiosis to form spores.
In many cases, isogamous fertilization is used by organisms that can also reproduce asexually through binary fission, budding, or asexual spore formation. The switch to sexual reproduction mode is often triggered by a change from favorable to unfavorable growing conditions. Fertilization frequently leads to the formation of a thick-walled zygotic resting spore that can withstand harsh environments and will germinate once growing conditions turn favorable again.
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