Mating type

Mating types are the microorganism equivalent to sex in higher organisms{{Cite web |title=mating type |url=https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100140225 |access-date=2021-08-26 |website=Oxford Reference |language=en }} and occur in isogamous species.[http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001899#pbio-1001899-box001 From Mating Types to Sexes]. Bachtrog D, Mank JE, Peichel CL, Kirkpatrick M, Otto SP, et al. (2014) Sex Determination: Why So Many Ways of Doing It? PLoS Biol 12(7): e1001899. doi:10.1371/journal.pbio.1001899 Depending on the group, different mating types are often referred to by numbers, letters, or simply "+" and "−" instead of "male" and "female", which refer to "sexes" or differences in size between gametes. Syngamy can only take place between gametes carrying different mating types.

Mating types are extensively studied in fungi. Among fungi, mating type is determined by chromosomal regions called mating-type loci. Furthermore, it is not as simple as "two different mating types can mate", but rather, a matter of combinatorics. As a simple example, most basidiomycete have a "tetrapolar heterothallism" mating system: there are two loci, and mating between two individuals is possible if the alleles on both loci are different. For example, if there are 3 alleles per locus, then there would be 9 mating types, each of which can mate with 4 other mating types.{{Cite journal |last=Idnurm |first=Alexander |last2=Hood |first2=Michael E. |last3=Johannesson |first3=Hanna |last4=Giraud |first4=Tatiana |date=2015-12-01 |title=Contrasted patterns in mating-type chromosomes in fungi: Hotspots versus coldspots of recombination |url=https://www.sciencedirect.com/science/article/pii/S1749461315000287 |journal=Fungal Biology Reviews |series=Special Issue: Fungal sex and mushrooms – A credit to Lorna Casselton |volume=29 |issue=3 |pages=220–229 |doi=10.1016/j.fbr.2015.06.001 |issn=1749-4613|pmc=4680991 }} By multiplicative combination, it generates a vast number of mating types.

As an illustration, the model organism Coprinus cinereus has two mating-type loci called A and B. Both loci have 3 groups of genes. At the A locus are 6 homeodomain proteins arranged in 3 groups of 2 (HD1 and HD2), which arose by gene duplication. At the B locus, each of the 3 groups contain one pheromone G-protein-coupled receptor and usually two genes for pheromones.

The A locus ensures heterothallism through a specific interaction between HD1 and HD2 proteins. Within each group, a HD1 protein can only form a functional heterodimer with a HD2 protein from a different group, not with the HD2 protein from its own group. Functional heterodimers are necessary for a dikaryon-specific transcription factor, and its lack arrests the development process. They function redundantly, so it is only necessary for one of the three groups to be heterozygotic for the A locus to work.{{Cite journal |last=Kamada |first=Takashi |date=May 2002 |title=Molecular genetics of sexual development in the mushroom Coprinus cinereus |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.10083 |journal=BioEssays |language=en |volume=24 |issue=5 |pages=449–459 |doi=10.1002/bies.10083 |issn=0265-9247}}

Similarly, the B locus ensures heterothallism through a specific interaction between pheromone receptors and pheromones. Each pheromone receptor is activated by pheromones from other groups, but not by the pheromone encoded by the same group. This means that a pheromone receptor can only trigger a signaling cascade when it binds to a pheromone from a different group, not when it binds to the pheromone from its own group. They also function redundantly.

In both cases, the mechanism is based on a "self-incompatibility" principle, where the proteins or pheromones from the same group are incompatible with each other, but compatible with those from different groups.{{Cite journal |last=Riquelme |first=Meritxell |last2=Challen |first2=Michael P |last3=Casselton |first3=Lorna A |last4=Brown |first4=Andrew J |date=2005-07-01 |title=The Origin of Multiple B Mating Specificities in Coprinus cinereus |url=https://academic.oup.com/genetics/article/170/3/1105/6060408 |journal=Genetics |language=en |volume=170 |issue=3 |pages=1105–1119 |doi=10.1534/genetics.105.040774 |issn=1943-2631 |pmc=1451185 |pmid=15879506}}{{Cite journal |last=Brown |first=Andrew J. |last2=Casselton |first2=Lorna A. |date=2001-07-01 |title=Mating in mushrooms: increasing the chances but prolonging the affair |url=https://www.sciencedirect.com/science/article/pii/S0168952501023435 |journal=Trends in Genetics |volume=17 |issue=7 |pages=393–400 |doi=10.1016/S0168-9525(01)02343-5 |issn=0168-9525}}

Similarly, the Schizophyllum commune has 2 gene groups (Aα, Aβ) for homeodomain proteins on the A locus, and 2 gene groups (Bα, Bβ) for pheromones and receptors on the B locus. Aα has 9 alleles, Aβ has 32, Bα has 9, and Bβ has 9. The two gene groups at the A locus function independently but redundantly, so only one group out of the two needs to be heterozygotic for it to work. Similarly for the two gene groups at the B locus. Thus, mating between two individuals succeeds if

$[(A\backslash alpha\; 1\; \backslash neq\; A\backslash alpha\; 2)\; \backslash mathrm\{OR\}(A\backslash beta\; 1\; \backslash neq\; A\backslash beta\; 2)]\; \backslash mathrm\{AND\}\; [(B\backslash alpha\; 1\; \backslash neq\; B\backslash alpha\; 2)\; \backslash mathrm\{OR\}(B\backslash beta\; 1\; \backslash neq\; B\backslash beta\; 2)]$

Reproduction by mating types is especially prevalent in fungi. Filamentous ascomycetes usually have two mating types referred to as "MAT1-1" and "MAT1-2", following the yeast mating-type locus (MAT).{{Cite journal |last1=Yoder |first1=O.C. |last2=Valent |first2=Barbara |last3=Chumley |first3=Forrest |date=1986 |title=Genetic Nomenclature and Practice for Plant Pathogenic Fungi |url=http://www.apsnet.org/publications/phytopathology/backissues/Documents/1986Articles/Phyto76n04_383.pdf |journal=Phytopathology |volume=76 |issue=4 |pages=383–385 |doi=10.1094/phyto-76-383 |access-date=11 November 2015}} Under standard nomenclature, MAT1-1 (which may informally be called MAT1) encodes for a regulatory protein with an alpha box motif, while MAT1-2 (informally called MAT2) encodes for a protein with a high motility-group (HMG) DNA-binding motif, as in the yeast mating type MATα1.{{cite journal|last1=Turgeon|first1=B.G.|last2=Yoder|first2=O.C.|title=Proposed Nomenclature for Mating Type Genes of Filamentous Ascomycetes|journal=Fungal Genetics and Biology|date=2000|volume=31|issue=1|pages=1–5|doi=10.1006/fgbi.2000.1227|pmid=11118130}} The corresponding mating types in yeast, a non-filamentous ascomycete, are referred to as MATa and MATα.{{Cite journal |last1=Hanson |first1=Sara J |last2=Wolfe |first2=Kenneth H |date=2017-05-01 |title=An Evolutionary Perspective on Yeast Mating-Type Switching |journal=Genetics |language=en |volume=206 |issue=1 |pages=9–32 |doi=10.1534/genetics.117.202036 |issn=1943-2631 |pmc=5419495 |pmid=28476860}}

Mating type genes in ascomycetes are called idiomorphs rather than alleles due to the uncertainty of the origin by common descent. The proteins they encode are transcription factors which regulate both the early and late stages of the sexual cycle. Heterothallic ascomycetes produce gametes, which present a single Mat idiomorph, and syngamy will only be possible between gametes carrying complementary mating types. On the other hand, homothallic ascomycetes produce gametes that can fuse with every other gamete in the population (including its own mitotic descendants) most often because each haploid contains the two alternate forms of the Mat locus in its genome.{{cite journal|last=Giraud|first=T. | title = Mating system of the anther smut fungus Microbotryum violaceum: Selfing under heterothallism | journal = Eukaryotic Cell | year = 2008 | volume = 7 |issue=5 | pages = 765–775 | doi=10.1128/ec.00440-07|display-authors=etal|pmc=2394975 | pmid=18281603}}

Basidiomycetes can have thousands of different mating types.{{cite journal|author= Casselton LA | title = Mate recognition in fungi | journal = Heredity | year = 2002 | volume = 88 | pages = 142–147 | pmid = 11932772 | doi = 10.1038/sj.hdy.6800035|issue= 2| doi-access = free }}

In the ascomycete Neurospora crassa matings are restricted to interaction of strains of opposite mating type. This promotes some degree of outcrossing. Outcrossing, through complementation, could provide the benefit of masking recessive deleterious mutations in genes which function in the dikaryon and/or diploid stage of the life cycle.Bernstein H, Byerly HC, Hopf FA, Michod RE. Genetic damage, mutation, and the evolution of sex. Science. 1985 Sep 20;229(4719):1277-81. doi: 10.1126/science.3898363. PMID 3898363

{{Main|Evolution of sexual reproduction}}

Mating types likely predate anisogamy,{{Cite book |last=Andersson |first=Malte |url=https://books.google.com/books?id=lNnHdvzBlTYC&dq=anisogamy&pg=PA4 |title=Sexual Selection |date=1994-06-16 |publisher=Princeton University Press |isbn=978-0-691-00057-2 |pages=4 |language=en}} and sexes evolved directly from mating types or independently in some lineages.{{Cite journal |last=Perrin |first=Nicolas |title=What Uses Are Mating Types? The "Developmental Switch" Model |date=2012-04-06 |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.2011.01562.x |journal=Evolution |language=en |volume=66 |issue=4 |pages=947–956 |doi=10.1111/j.1558-5646.2011.01562.x|pmid=22486681 |s2cid=5798638 }}

Studies on green algae have provided evidence for the evolutionary link between sexes and mating types.{{Cite book |last1=Sawada |first1=Hitoshi |url=https://directory.doabooks.org/handle/20.500.12854/29412 |title=Sexual Reproduction in Animals and Plants |last2=Inoue |first2=Naokazu |last3=Iwano |first3=Megumi |date=2014 |publisher=Springer |isbn=978-4-431-54589-7 |pages=215–227 |language=en}} In 2006 Japanese researchers found a gene in males of Pleodorina starrii that is an [https://en.m.wiktionary.org/wiki/orthologue#:~:text=orthologue%20(plural%20orthologues),sequences%20found%20in%20different%20species. orthologue] to a gene for a mating type in the Chlamydomonas reinhardtii.{{Cite journal |last1=Nozaki |first1=Hisayoshi |last2=Mori |first2=Toshiyuki |last3=Misumi |first3=Osami |last4=Matsunaga |first4=Sachihiro |last5=Kuroiwa |first5=Tsuneyoshi |date=2006-12-19 |title=Males evolved from the dominant isogametic mating type |journal=Current Biology |volume=16 |issue=24 |pages=R1018–1020 |doi=10.1016/j.cub.2006.11.019 |issn=0960-9822 |pmid=17174904 |s2cid=15748275 |doi-access=free |bibcode=2006CBio...16R1018N }} In Volvocales, the plus mating type is the ancestor to female.{{Cite book |last1=Togashi |first1=Tatsuya |url=https://books.google.com/books?id=5eOvRTIuLXMC&dq=Title%3A+The+Evolution+of+Anisogamy+%3A+A+Fundamental+Phenomenon+Underlying+Sexual+Selection&pg=PR5 |title=The Evolution of Anisogamy: A Fundamental Phenomenon Underlying Sexual Selection |last2=Cox |first2=Paul Alan |date=2011-04-14 |publisher=Cambridge University Press |isbn=978-1-139-50082-1 |pages=1–15 |language=en}}

In ciliates, multiple mating types evolved from binary mating types in several lineages.{{Rp|75}} As of 2019, genomic conflict has been considered the leading explanation for the evolution of two mating types.{{Cite book |last=Hill |first=Geoffrey E. |url=https://books.google.com/books?id=Zt-RDwAAQBAJ&q=mating+types+evolved |title=Mitonuclear Ecology |date=2019-04-30 |publisher=Oxford University Press |isbn=978-0-19-881825-0 |pages=115 |language=en}}

Secondary mating types evolved alongside simultaneous hermaphrodites in several lineages.{{Cite book |last1=Beukeboom |first1=Leo W. |url=https://books.google.com/books?id=7yrnAwAAQBAJ&dq=secondary%2520mating%2520types%2520evolved&pg=PP1 |title=The Evolution of Sex Determination |last2=Perrin |first2=Nicolas |date=2014 |publisher=Oxford University Press |isbn=978-0-19-965714-8 |language=en}}{{Rp|71}}{{Clarify|reason=Which species exactly? What are secondary mating types?|date=August 2021}}

• Mating in fungi
• Mating of yeast
• Mating-type locus
• Saccharomyces cerevisiae (a and α mating types)
• Schizophyllum commune (23,328 mating types)
• Tetrahymena (7 mating types)

{{reflist|colwidth=30em}}

Category:Mycology

Category:Microbiology