Homothallism

Homothallic refers to the possession, within a single organism, of the resources to reproduce sexually;[1] 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.[2]

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.[3] Homothallism occurs in fungi by a wide variety of genetically distinct mechanisms that all result in sexually reproducing cultures from a single cell.[3]

Among the 250 known species of aspergilli, about 36% have an identified sexual state.[4] Among those Aspergillus species for which a sexual cycle has been observed, the majority in nature are homothallic (self-fertilizing).[4] 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.[5] 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.[6] The ascomycete fungus Pneumocystis jirovecii is considered to be primarily homothallic.[7] The ascomycete fungus Neosartorya fischeri is also homothallic.[8]

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).[9] 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.[10]

See also

References
  1. "On-Line Glossary: H". Archived from the original on 2006-09-08. Retrieved 2008-10-24.
  2. "Homothallic". Retrieved 16 January 2018 – via The Free Dictionary.
  3. Wilson AM, Wilken PM, van der Nest MA, Steenkamp ET, Wingfield MJ, Wingfield BD (June 2015). "Homothallism: an umbrella term for describing diverse sexual behaviours". IMA Fungus. 6 (1): 207–14. doi:10.5598/imafungus.2015.06.01.13. PMC 4500084. PMID 26203424.
  4. Dyer PS, O'Gorman CM (January 2012). "Sexual development and cryptic sexuality in fungi: insights from Aspergillus species". FEMS Microbiology Reviews. 36 (1): 165–92. doi:10.1111/j.1574-6976.2011.00308.x. PMID 22091779.
  5. Paoletti M, Seymour FA, Alcocer MJ, Kaur N, Calvo AM, Archer DB, Dyer PS (August 2007). "Mating type and the genetic basis of self-fertility in the model fungus Aspergillus nidulans". Current Biology. 17 (16): 1384–9. doi:10.1016/j.cub.2007.07.012. PMID 17669651.
  6. Yun SH, Berbee ML, Yoder OC, Turgeon BG (May 1999). "Evolution of the fungal self-fertile reproductive life style from self-sterile ancestors". Proceedings of the National Academy of Sciences of the United States of America. 96 (10): 5592–7. Bibcode:1999PNAS...96.5592Y. doi:10.1073/pnas.96.10.5592. PMC 21905. PMID 10318929.
  7. Richard S, Almeida JM, Cissé OH, Luraschi A, Nielsen O, Pagni M, Hauser PM (February 2018). "Pneumocystis MAT Genes Suggest Obligate Sexuality through Primary Homothallism within Host Lungs". mBio. 9 (1). doi:10.1128/mBio.02201-17. PMC 5821091. PMID 29463658.
  8. Rydholm C, Dyer PS, Lutzoni F (May 2007). "DNA sequence characterization and molecular evolution of MAT1 and MAT2 mating-type loci of the self-compatible ascomycete mold Neosartorya fischeri". Eukaryotic Cell. 6 (5): 868–74. doi:10.1128/EC.00319-06. PMC 1899244. PMID 17384199.
  9. Murtagh GJ, Dyer PS, Crittenden PD (April 2000). "Sex and the single lichen". Nature. 404 (6778): 564. doi:10.1038/35007142. PMID 10766229.
  10. Bernstein H, Hopf FA, Michod RE (1987). "The molecular basis of the evolution of sex". Advances in Genetics. 24: 323–70. doi:10.1016/s0065-2660(08)60012-7. ISBN 9780120176243. PMID 3324702.
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