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Isogamy is a form of sexual reproduction that involves gametes of similar morphology (generally similar in shape and size), found in most unicellular organisms. Because both gametes look alike, they generally cannot be classified as male or female. Instead, organisms undergoing isogamy are said to have different mating types, most commonly noted as "+" and "−" strains. Some isogamous species have more than two mating types, but the number is usually lower than ten (which are designated by numbers or letters), though in some extremely rare cases a species can have thousands of mating types. In all cases, 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.
With motile cells
With non-motile cells
In another type, neither of the gametes is flagellated.
Another, more complex form, is conjugation (similar to the exchange of genetic material through a bridge in bacterial conjugation, but involving reproduction). This occurs in some the green algae, the Zygnematophyceae, e.g., 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.
In zygomycetes fungi, 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 the cytoplasm is fused (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) then eventually 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
- Lehtonen, Jussi; Kokko, Hanna; Parker, Geoff A. (2016-10-19). "What do isogamous organisms teach us about sex and the two sexes?". Philosophical Transactions of the Royal Society B: Biological Sciences. 371 (1706). doi:10.1098/rstb.2015.0532. ISSN 0962-8436. PMC 5031617. PMID 27619696.
- Kumar, Rahul; Meena, Mukesh; Swapnil, Prashant (2019), "Anisogamy", in Vonk, Jennifer; Shackelford, Todd (eds.), Encyclopedia of Animal Cognition and Behavior, Cham: Springer International Publishing, pp. 1–5, doi:10.1007/978-3-319-47829-6_340-1, ISBN 978-3-319-47829-6, retrieved 2020-11-02
- Krumbeck, Yvonne; Constable, George W. A.; Rogers, Tim (2020-02-26). "Fitness differences suppress the number of mating types in evolving isogamous species". Royal Society Open Science. 7 (2): 192126. arXiv:1906.07117. Bibcode:2020RSOS....792126K. doi:10.1098/rsos.192126. ISSN 2054-5703. PMC 7062084. PMID 32257356.
- Dusenbery, David B. (2009). Living at Micro Scale, Chapter 20. Harvard University Press, Cambridge, Massachusetts ISBN 978-0-674-03116-6.
- Beukeboom, L. and Perrin, N. (2014). The Evolution of Sex Determination. Oxford University Press, p. 10 . Online resources, .