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Isogamy is a form of sexual reproduction that involves gametes of similar morphology (similar shape and size), differing 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 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 "-".
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 and ciliates 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 often 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.
Notes and references
Sa Geng, Peter De Hoff, James G. Umen (July 8, 2014). "Evolution of Sexes from an Ancestral Mating-Type Specification Parthway". PLOS biology. doi:10.137/journal.pbio.10001904.
- Dusenbery, David B. (2009). Living at Micro Scale, Chapter 20. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
- Bernstein H, Bernstein C and Michod RE (2011). Meiosis as an Evolutionary Adaptation for DNA Repair. Chapter in DNA Repair Inna Kruman (Ed.), ISBN 978-953-307-697-3, InTech (Publisher). Available online from: http://www.intechopen.com/books/dna-repair/meiosis-as-an-evolutionary-adaptation-for-dna-repair