Homothallism

Homothallic refers to the possession, within a single organism, of the resources to reproduce sexually;{{cite web|url=http://www.plantpath.cornell.edu/glossary/Defs_H.htm |archive-url=https://web.archive.org/web/20060908130055/http://www.plantpath.cornell.edu/glossary/Defs_H.htm |url-status=dead |archive-date=2006-09-08 |title=On-Line Glossary: H |access-date=2008-10-24 }} i.e., having male and female reproductive structures on the same thallus. The opposite sexual functions are performed by different cells of a single mycelium.{{cite web|url=https://www.thefreedictionary.com/Homothallic|title=Homothallic|access-date=16 January 2018|via=The Free Dictionary}}

It can be contrasted to heterothallic.

It is often used to categorize fungi. In yeast, heterothallic cells have mating types a and α. An experienced mother cell (one that has divided at least once) will switch mating type every cell division cycle because of the HO allele.

Sexual reproduction commonly occurs in two fundamentally different ways in fungi. These are outcrossing (in heterothallic fungi) in which two different individuals contribute nuclei to form a zygote, and self-fertilization or selfing (in homothallic fungi) in which both nuclei are derived from the same individual. Homothallism in fungi can be defined as the capability of an individual spore to produce a sexually reproducing colony when propagated in isolation.{{cite journal | vauthors = Wilson AM, Wilken PM, van der Nest MA, Steenkamp ET, Wingfield MJ, Wingfield BD | title = Homothallism: an umbrella term for describing diverse sexual behaviours | journal = IMA Fungus | volume = 6 | issue = 1 | pages = 207–14 | date = June 2015 | pmid = 26203424 | pmc = 4500084 | doi = 10.5598/imafungus.2015.06.01.13 }} Homothallism occurs in fungi by a wide variety of genetically distinct mechanisms that all result in sexually reproducing cultures from a single cell.

Among the 250 known species of aspergilli, about 36% have an identified sexual state.{{cite journal | vauthors = Dyer PS, O'Gorman CM | title = Sexual development and cryptic sexuality in fungi: insights from Aspergillus species | journal = FEMS Microbiology Reviews | volume = 36 | issue = 1 | pages = 165–92 | date = January 2012 | pmid = 22091779 | doi = 10.1111/j.1574-6976.2011.00308.x | doi-access = free }} Among those Aspergillus species for which a sexual cycle has been observed, the majority in nature are homothallic (self-fertilizing). Selfing in the homothallic fungus Aspergillus nidulans involves activation of the same mating pathways characteristic of sex in outcrossing species, i.e. self-fertilization does not bypass required pathways for outcrossing sex but instead requires activation of these pathways within a single individual.{{cite journal | vauthors = Paoletti M, Seymour FA, Alcocer MJ, Kaur N, Calvo AM, Archer DB, Dyer PS | title = Mating type and the genetic basis of self-fertility in the model fungus Aspergillus nidulans | journal = Current Biology | volume = 17 | issue = 16 | pages = 1384–9 | date = August 2007 | pmid = 17669651 | doi = 10.1016/j.cub.2007.07.012 | doi-access = free | bibcode = 2007CBio...17.1384P }} Fusion of haploid nuclei occurs within reproductive structures termed cleistothecia, in which the diploid zygote undergoes meiotic divisions to yield haploid ascospores.

Several ascomycete fungal species of the genus Cochliobolus (C. luttrellii, C. cymbopogonis, C. kusanoi and C. homomorphus) are homothallic.{{cite journal | vauthors = Yun SH, Berbee ML, Yoder OC, Turgeon BG | title = Evolution of the fungal self-fertile reproductive life style from self-sterile ancestors | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 96 | issue = 10 | pages = 5592–7 | date = May 1999 | pmid = 10318929 | pmc = 21905 | doi = 10.1073/pnas.96.10.5592 | bibcode = 1999PNAS...96.5592Y | doi-access = free }} The ascomycete fungus Pneumocystis jirovecii is considered to be primarily homothallic.{{cite journal | vauthors = Richard S, Almeida JM, Cissé OH, Luraschi A, Nielsen O, Pagni M, Hauser PM | title = Pneumocystis MAT Genes Suggest Obligate Sexuality through Primary Homothallism within Host Lungs | journal = mBio | volume = 9 | issue = 1 | date = February 2018 | pmid = 29463658 | pmc = 5821091 | doi = 10.1128/mBio.02201-17 }} The ascomycete fungus Neosartorya fischeri is also homothallic.{{cite journal | vauthors = Rydholm C, Dyer PS, Lutzoni F | title = DNA sequence characterization and molecular evolution of MAT1 and MAT2 mating-type loci of the self-compatible ascomycete mold Neosartorya fischeri | journal = Eukaryotic Cell | volume = 6 | issue = 5 | pages = 868–74 | date = May 2007 | pmid = 17384199 | pmc = 1899244 | doi = 10.1128/EC.00319-06 }} Cryptococcus depauperatus, a homothallic basidiomycete fungus, grows as long, branching filaments (hyphae).{{cite journal |vauthors=Passer AR, Clancey SA, Shea T, David-Palma M, Averette AF, Boekhout T, Porcel BM, Nowrousian M, Cuomo CA, Sun S, Heitman J, Coelho MA |title=Obligate sexual reproduction of a homothallic fungus closely related to the Cryptococcus pathogenic species complex |journal=eLife |volume=11 |issue= |pages= |date=June 2022 |pmid=35713948 |pmc=9296135 |doi=10.7554/eLife.79114 |doi-access=free }} C. depauperatus can undergo meiosis and reproduce sexually with itself throughout its life cycle.

A lichen is a composite organism consisting of a fungus and a photosynthetic partner that are growing together in a symbiotic relationship. The photosynthetic partner is usually either a green alga or a cyanobacterium. Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps. Most lichenized fungi produce abundant sexual structures and in many species sexual spores appear to be the only means of dispersal (Murtagh et al., 2000). The lichens Graphis scripta and Ochrolechia parella do not produce symbiotic vegetative propagules. Rather the lichen-forming fungi of these species reproduce sexually by self-fertilization (i.e. they are homothallic), and it was proposed that this breeding system allows successful reproduction in harsh environments (Murtagh et al., 2000).{{cite journal | vauthors = Murtagh GJ, Dyer PS, Crittenden PD | title = Sex and the single lichen | journal = Nature | volume = 404 | issue = 6778 | pages = 564 | date = April 2000 | pmid = 10766229 | doi = 10.1038/35007142 | s2cid = 4425228 }}

Homothallism appears to be common in natural populations of fungi. Although self-fertilization employs meiosis, it produces minimal genetic variability. Homothallism is thus a form of sex that is unlikely to be adaptively maintained by a benefit related to producing variability. However, homothallic meiosis may be maintained in fungi as an adaptation for surviving stressful conditions; a proposed benefit of meiosis is the promoted homologous meiotic recombinational repair of DNA damages that are ordinarily caused by a stressful environment.{{cite book | vauthors = Bernstein H, Hopf FA, Michod RE | title = Molecular Genetics of Development | chapter = The molecular basis of the evolution of sex | series = Advances in Genetics | volume = 24 | pages = 323–70 | date = 1987 | pmid = 3324702 | doi = 10.1016/s0065-2660(08)60012-7 | isbn = 9780120176243 }}