Heterothallism

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Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable of sexual reproduction from a single organism.

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In heterothallic fungi, two different individuals contribute nuclei to form a zygote. Examples of heterothallism are included for Saccharomyces cerevisiae, Aspergillus fumigatus, Aspergillus flavus , Penicillium marneffei and Neurospora crassa . The heterothallic life cycle of N. crassa is given in some detail, since similar life cycles are present in other heterothallic fungi.

Life cycle of Saccharomyces cerevisiae

Saccharomyces cerevisiae tetrad YeastTetrad2.png
Saccharomyces cerevisiae tetrad

The yeast Saccharomyces cerevisiae is heterothallic. This means that each yeast cell is of a certain mating type and can only mate with a cell of the other mating type. During vegetative growth that ordinarily occurs when nutrients are abundant, S. cerevisiae reproduces by mitosis as either haploid or diploid cells. However, when starved, diploid cells undergo meiosis to form haploid spores. [1] Mating occurs when haploid cells of opposite mating type, MATa and MATα, come into contact. Ruderfer et al. [2] pointed out that such contacts are frequent between closely related yeast cells for two reasons. The first is that cells of opposite mating type are present together in the same ascus, the sac that contains the tetrad of cells directly produced by a single meiosis, and these cells can mate with each other. The second reason is that haploid cells of one mating type, upon cell division, often produce cells of the opposite mating type with which they may mate.

Katz Ezov et al. [3] presented evidence that in natural S. cerevisiae populations clonal reproduction and a type of “self-fertilization” (in the form of intratetrad mating) predominate. Ruderfer et al. [2] analyzed the ancestry of natural S. cerevisiae strains and concluded that outcrossing occurs only about once every 50,000 cell divisions. Thus, although S. cerevisiae is heterothallic, it appears that, in nature, mating is most often between closely related yeast cells. The relative rarity in nature of meiotic events that result from outcrossing suggests that the possible long-term benefits of outcrossing (e.g. generation of genetic diversity) are unlikely to be sufficient for generally maintaining sex from one generation to the next.[ citation needed ] Rather, a short-term benefit, such as meiotic recombinational repair of DNA damages caused by stressful conditions such as starvation may be the key to the maintenance of sex in S. cerevisiae. [4] [5]

Life cycle of Aspergillus fumigatus

Aspergillus fumigatus , is a heterothallic fungus. [6] It is one of the most common Aspergillus species to cause disease in humans with an immunodeficiency. A. fumigatus, is widespread in nature, and is typically found in soil and decaying organic matter, such as compost heaps, where it plays an essential role in carbon and nitrogen recycling. Colonies of the fungus produce from conidiophores thousands of minute grey-green conidia (2–3 μm) that readily become airborne. A. fumigatus possesses a fully functional sexual reproductive cycle that leads to the production of cleistothecia and ascospores. [7]

Although A. fumigatus occurs in areas with widely different climates and environments, it displays low genetic variation and lack of population genetic differentiation on a global scale. [8] Thus the capability for heterothallic sex is maintained even though little genetic diversity is produced. As in the case of S. cereviae, above, a short-term benefit of meiosis may be the key to the adaptive maintenance of sex in this species.

Life cycle of Aspergillus flavus

A. flavus is the major producer of carcinogenic aflatoxins in crops worldwide. It is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals. In 2009, a sexual state of this heterothallic fungus was found to arise when strains of opposite mating type were cultured together under appropriate conditions. [9]

Sexuality generates diversity in the aflatoxin gene cluster in A. flavus, [10] suggesting that production of genetic variation may contribute to the maintenance of heterothallism in this species.

Life cycle of Talaromyces marneffei

Henk et al. [11] showed that the genes required for meiosis are present in T. marneffei, and that mating and genetic recombination occur in this species.

Henk et al. [11] concluded that T. marneffei is sexually reproducing, but recombination in natural populations is most likely to occur across spatially and genetically limited distances resulting in a highly clonal population structure. Sex is maintained in this species even though very little genetic variability is produced. Sex may be maintained in T. marneffei by a short-term benefit of meiosis, as in S. cerevisiae and A. fumigatus, discussed above.

Life cycle of Neurospora crassa

Neurospora crassa life cycle. The haploid mycelium reproduces asexually by two processes: (1) simple proliferation of existing mycelium, and (2) formation of conidia (macro- and micro-) which can be dispersed and then germinate to produce new mycelium. In the sexual cycle, mating can only occur between individual strains of different mating type, 'A' and 'a'. Fertilization occurs by the passage of nuclei of conidia or mycelium of one mating type into the protoperithecia of the opposite mating type through the trichogyne. Fusion of the nuclei of opposite mating types occurs within the protoperithecium to form a zygote (2N) nucleus. Neurospora crassa life cycle.jpg
Neurospora crassa life cycle. The haploid mycelium reproduces asexually by two processes: (1) simple proliferation of existing mycelium, and (2) formation of conidia (macro- and micro-) which can be dispersed and then germinate to produce new mycelium. In the sexual cycle, mating can only occur between individual strains of different mating type, ‘A’ and ‘a’. Fertilization occurs by the passage of nuclei of conidia or mycelium of one mating type into the protoperithecia of the opposite mating type through the trichogyne. Fusion of the nuclei of opposite mating types occurs within the protoperithecium to form a zygote (2N) nucleus.

The sexual cycle of N. crassa is heterothallic. Sexual fruiting bodies (perithecia) can only be formed when two mycelia of different mating type come together. Like other ascomycetes, N. crassa has two mating types that, in this case, are symbolized by ‘A’ and ‘a’. There is no evident morphological difference between the ‘A’ and 'a' mating type strains. Both can form abundant protoperithecia, the female reproductive structure (see figure, top of §). Protoperithecia are formed most readily in the laboratory when growth occurs on solid (agar) synthetic medium with a relatively low source of nitrogen. [12] Nitrogen starvation appears to be necessary for expression of genes involved in sexual development. [13] The protoperithecium consists of an ascogonium, a coiled multicellular hypha that is enclosed in a knot-like aggregation of hyphae. A branched system of slender hyphae, called the trichogyne, extends from the tip of the ascogonium projecting beyond the sheathing hyphae into the air. The sexual cycle is initiated (i.e. fertilization occurs) when a cell (usually a conidium) of opposite mating type contacts a part of the trichogyne (see figure, top of §). Such contact can be followed by cell fusion leading to one or more nuclei from the fertilizing cell migrating down the trichogyne into the ascogonium. Since both ‘A’ and ‘a’ strains have the same sexual structures, neither strain can be regarded as exclusively male or female. However, as a recipient, the protoperithecium of both the ‘A’ and ‘a’ strains can be thought of as the female structure, and the fertilizing conidium can be thought of as the male participant.

The subsequent steps following fusion of ‘A’ and ‘a’ haploid cells, have been outlined by Fincham and Day, [14] and by Wagner and Mitchell. [15] After fusion of the cells, the further fusion of their nuclei is delayed. Instead, a nucleus from the fertilizing cell and a nucleus from the ascogonium become associated and begin to divide synchronously. The products of these nuclear divisions (still in pairs of unlike mating type, i.e. ‘A’ / ‘a’) migrate into numerous ascogenous hyphae, which then begin to grow out of the ascogonium. Each of these ascogenous hypha bends to form a hook (or crozier) at its tip and the ‘A’ and ‘a’ pair of haploid nuclei within the crozier divide synchronously. Next, septa form to divide the crozier into three cells. The central cell in the curve of the hook contains one ‘A’ and one ‘a’ nucleus (see figure, top of §). This binuclear cell initiates ascus formation and is called an “ascus-initial” cell. Next the two uninucleate cells on either side of the first ascus-forming cell fuse with each other to form a binucleate cell that can grow to form a further crozier that can then form its own ascus-initial cell. This process can then be repeated multiple times.

After formation of the ascus-initial cell, the ‘A’ and ‘a’ nucleus fuse with each other to form a diploid nucleus (see figure, top of §). This nucleus is the only diploid nucleus in the entire life cycle of N. crassa. The diploid nucleus has 14 chromosomes formed from the two fused haploid nuclei that had 7 chromosomes each. Formation of the diploid nucleus is immediately followed by meiosis. The two sequential divisions of meiosis lead to four haploid nuclei, two of the ‘A’ mating type and two of the ‘a’ mating type. One further mitotic division leads to four ‘A’ and four ‘a’ nuclei in each ascus. Meiosis is an essential part of the life cycle of all sexually reproducing organisms, and in its main features, meiosis in N. crassa seems typical of meiosis generally.

As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium, becomes impregnated with melanin, and blackens. The mature perithecium has a flask-shaped structure.

A mature perithecium may contain as many as 300 asci, each derived from identical fusion diploid nuclei. Ordinarily, in nature, when the perithecia mature the ascospores are ejected rather violently into the air. These ascospores are heat resistant and, in the lab, require heating at 60 °C for 30 minutes to induce germination. For normal strains, the entire sexual cycle takes 10 to 15 days. In a mature ascus containing 8 ascospores, pairs of adjacent spores are identical in genetic constitution, since the last division is mitotic, and since the ascospores are contained in the ascus sac that holds them in a definite order determined by the direction of nuclear segregations during meiosis. Since the four primary products are also arranged in sequence, the pattern of genetic markers from a first-division segregation can be distinguished from the markers from a second-division segregation pattern.

See also

Related Research Articles

<span class="mw-page-title-main">Basidiomycota</span> Division of fungi

Basidiomycota is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basidiomycota includes these groups: agarics, puffballs, stinkhorns, bracket fungi, other polypores, jelly fungi, boletes, chanterelles, earth stars, smuts, bunts, rusts, mirror yeasts, and Cryptococcus, the human pathogenic yeast. Basidiomycota are filamentous fungi composed of hyphae and reproduce sexually via the formation of specialized club-shaped end cells called basidia that normally bear external meiospores. These specialized spores are called basidiospores. However, some Basidiomycota are obligate asexual reproducers. Basidiomycota that reproduce asexually can typically be recognized as members of this division by gross similarity to others, by the formation of a distinctive anatomical feature, cell wall components, and definitively by phylogenetic molecular analysis of DNA sequence data.

<span class="mw-page-title-main">Ascomycota</span> Division or phylum of fungi

Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defining feature of this fungal group is the "ascus", a microscopic sexual structure in which nonmotile spores, called ascospores, are formed. However, some species of the Ascomycota are asexual, meaning that they do not have a sexual cycle and thus do not form asci or ascospores. Familiar examples of sac fungi include morels, truffles, brewers' and bakers' yeast, dead man's fingers, and cup fungi. The fungal symbionts in the majority of lichens such as Cladonia belong to the Ascomycota.

<span class="mw-page-title-main">Ascus</span> Spore-bearing cell in ascomycete fungi

An ascus is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores, produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or species can occur in numbers of one, two, four, or multiples of four. In a few cases, the ascospores can bud off conidia that may fill the asci with hundreds of conidia, or the ascospores may fragment, e.g. some Cordyceps, also filling the asci with smaller cells. Ascospores are nonmotile, usually single celled, but not infrequently may be coenocytic, and in some cases coenocytic in multiple planes. Mitotic divisions within the developing spores populate each resulting cell in septate ascospores with nuclei. The term ocular chamber, or oculus, refers to the epiplasm that is surrounded by the "bourrelet".

<i>Saccharomyces cerevisiae</i> Species of yeast

Saccharomyces cerevisiae is a species of yeast. The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been originally isolated from the skin of grapes. It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism behind the most common type of fermentation. S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. It reproduces by budding.

<span class="mw-page-title-main">Karyogamy</span> Fusion of the nuclei of two haploid eukaryotic cells

Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronuclei. Once the cell membranes, cytoplasm, and pronuclei fuse, the resulting single cell is diploid, containing two copies of the genome. This diploid cell, called a zygote or zygospore can then enter meiosis, or continue to divide by mitosis. Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.

<i>Neurospora crassa</i> Species of ascomycete fungus in the family Sordariaceae

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<span class="mw-page-title-main">Heterokaryon</span>

A heterokaryon is a multinucleate cell that contains genetically different nuclei. Heterokaryotic and heterokaryosis are derived terms. This is a special type of syncytium. This can occur naturally, such as in the mycelium of fungi during sexual reproduction, or artificially as formed by the experimental fusion of two genetically different cells, as e.g., in hybridoma technology.

<i>Aspergillus</i> Genus of fungi

Aspergillus is a genus consisting of several hundred mould species found in various climates worldwide.

<span class="mw-page-title-main">Mating of yeast</span> Biological process

The yeast Saccharomyces cerevisiae is a simple single-celled eukaryote with both a diploid and haploid mode of existence. The mating of yeast only occurs between haploids, which can be either the a or α (alpha) mating type and thus display simple sexual differentiation. Mating type is determined by a single locus, MAT, which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of genetic recombination, haploid yeast can switch mating type as often as every cell cycle.

<i>Neurospora</i> Genus of fungi

Neurospora is a genus of Ascomycete fungi. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores that resemble axons.

A meiocyte is a type of cell that differentiates into a gamete through the process of meiosis. Through meiosis, the diploid meiocyte divides into four genetically different haploid gametes. The control of the meiocyte through the meiotic cell cycle varies between different groups of organisms.

<span class="mw-page-title-main">Saccharomycotina</span> Subdivision of fungi

Saccharomycotina is a subdivision (subphylum) of the division (phylum) Ascomycota in the kingdom Fungi. It comprises most of the ascomycete yeasts. The members of Saccharomycotina reproduce by budding and they do not produce ascocarps.

<i>Aspergillus nidulans</i> Species of fungus

Aspergillus nidulans is one of many species of filamentous fungi in the phylum Ascomycota. It has been an important research organism for studying eukaryotic cell biology for over 50 years, being used to study a wide range of subjects including recombination, DNA repair, mutation, cell cycle control, tubulin, chromatin, nucleokinesis, pathogenesis, metabolism, and experimental evolution. It is one of the few species in its genus able to form sexual spores through meiosis, allowing crossing of strains in the laboratory. A. nidulans is a homothallic fungus, meaning it is able to self-fertilize and form fruiting bodies in the absence of a mating partner. It has septate hyphae with a woolly colony texture and white mycelia. The green colour of wild-type colonies is due to pigmentation of the spores, while mutations in the pigmentation pathway can produce other spore colours.

<span class="mw-page-title-main">Mating in fungi</span> Combination of genetic material between compatible mating types

Fungi are a diverse group of organisms that employ a huge variety of reproductive strategies, ranging from fully asexual to almost exclusively sexual species. Most species can reproduce both sexually and asexually, alternating between haploid and diploid forms. This contrasts with many eukaryotes such as mammals, where the adults are always diploid and produce haploid gametes which combine to form the next generation. In fungi, both haploid and diploid forms can reproduce – haploid individuals can undergo asexual reproduction while diploid forms can produce gametes that combine to give rise to the next generation.

Microbial genetics is a subject area within microbiology and genetic engineering. Microbial genetics studies microorganisms for different purposes. The microorganisms that are observed are bacteria, and archaea. Some fungi and protozoa are also subjects used to study in this field. The studies of microorganisms involve studies of genotype and expression system. Genotypes are the inherited compositions of an organism. Genetic Engineering is a field of work and study within microbial genetics. The usage of recombinant DNA technology is a process of this work. The process involves creating recombinant DNA molecules through manipulating a DNA sequence. That DNA created is then in contact with a host organism. Cloning is also an example of genetic engineering.

Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct sexes. They also occur in macro-organisms such as fungi.

The mating-type locus is a specialized region in the genomes of some yeast and other fungi, usually organized into heterochromatin and possessing unique histone methylation patterns. The genes in this region regulate the mating type of the organism and therefore determine key events in its life cycle, such as whether it will reproduce sexually or asexually. In fission yeast such as S. pombe, the formation and maintenance of the heterochromatin organization is regulated by RNA-induced transcriptional silencing, a form of RNA interference responsible for genomic maintenance in many organisms. Mating type regions have also been well studied in budding yeast S. cerevisiae and in the fungus Neurospora crassa.

<span class="mw-page-title-main">Tetrad (meiosis)</span>

The tetrad is the four spores produced after meiosis of a yeast or other Ascomycota, Chlamydomonas or other alga, or a plant. After parent haploids mate, they produce diploids. Under appropriate environmental conditions, diploids sporulate and undergo meiosis. The meiotic products, spores, remain packaged in the parental cell body to produce the tetrad.

The parasexual cycle, a process restricted to fungi and single-celled organisms, is a nonsexual mechanism of parasexuality for transferring genetic material without meiosis or the development of sexual structures. It was first described by Italian geneticist Guido Pontecorvo in 1956 during studies on Aspergillus nidulans. A parasexual cycle is initiated by the fusion of hyphae (anastomosis) during which nuclei and other cytoplasmic components occupy the same cell. Fusion of the unlike nuclei in the cell of the heterokaryon results in formation of a diploid nucleus (karyogamy), which is believed to be unstable and can produce segregants by recombination involving mitotic crossing-over and haploidization. Mitotic crossing-over can lead to the exchange of genes on chromosomes; while haploidization probably involves mitotic nondisjunctions which randomly reassort the chromosomes and result in the production of aneuploid and haploid cells. Like a sexual cycle, parasexuality gives the species the opportunity to recombine the genome and produce new genotypes in their offspring. Unlike a sexual cycle, the process lacks coordination and is exclusively mitotic.

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

References

  1. ↑ Herskowitz I (December 1988). "Life cycle of the budding yeast Saccharomyces cerevisiae". Microbiol. Rev. 52 (4): 536–53. doi:10.1128/MMBR.52.4.536-553.1988. PMC   373162 . PMID   3070323.
  2. 1 2 Ruderfer DM, Pratt SC, Seidel HS, Kruglyak L (September 2006). "Population genomic analysis of outcrossing and recombination in yeast". Nat. Genet. 38 (9): 1077–81. doi:10.1038/ng1859. PMID   16892060. S2CID   783720.
  3. ↑ Katz Ezov T, Chang SL, Frenkel Z, Segrè AV, Bahalul M, Murray AW, Leu JY, Korol A, Kashi Y (January 2010). "Heterothallism in Saccharomyces cerevisiae isolates from nature: effect of HO locus on the mode of reproduction". Mol. Ecol. 19 (1): 121–31. doi:10.1111/j.1365-294X.2009.04436.x. PMC   3892377 . PMID   20002587.
  4. ↑ Birdsell JA, Wills C (2003). The evolutionary origin and maintenance of sexual recombination: A review of contemporary models. Evolutionary Biology Series >> Evolutionary Biology, Vol. 33 pp. 27–137. MacIntyre, Ross J.; Clegg, Michael, T (Eds.), Springer. ISBN   978-0306472619
  5. ↑ Elvira Hörandl (2013). Meiosis and the Paradox of Sex in Nature, Meiosis, ISBN   978-953-51-1197-9, InTech, DOI: 10.5772/56542
  6. ↑ Sugui JA, Losada L, Wang W, Varga J, Ngamskulrungroj P, Abu-Asab M, Chang YC, O'Gorman CM, Wickes BL, Nierman WC, Dyer PS, Kwon-Chung KJ (2011). "Identification and characterization of an Aspergillus fumigatus "supermater" pair". mBio. 2 (6): e00234–11. doi:10.1128/mBio.00234-11. PMC   3225970 . PMID   22108383.
  7. ↑ O'Gorman CM, Fuller H, Dyer PS (January 2009). "Discovery of a sexual cycle in the opportunistic fungal pathogen Aspergillus fumigatus". Nature. 457 (7228): 471–4. Bibcode:2009Natur.457..471O. doi:10.1038/nature07528. PMID   19043401. S2CID   4371721.
  8. ↑ Rydholm C, Szakacs G, Lutzoni F (April 2006). "Low genetic variation and no detectable population structure in aspergillus fumigatus compared to closely related Neosartorya species". Eukaryotic Cell. 5 (4): 650–7. doi:10.1128/EC.5.4.650-657.2006. PMC   1459663 . PMID   16607012.
  9. ↑ Horn BW, Moore GG, Carbone I (2009). "Sexual reproduction in Aspergillus flavus". Mycologia. 101 (3): 423–9. doi:10.3852/09-011. PMID   19537215. S2CID   20648447.
  10. ↑ Moore GG, Elliott JL, Singh R, Horn BW, Dorner JW, Stone EA, Chulze SN, Barros GG, Naik MK, Wright GC, Hell K, Carbone I (2013). "Sexuality generates diversity in the aflatoxin gene cluster: evidence on a global scale". PLOS Pathog. 9 (8): e1003574. doi: 10.1371/journal.ppat.1003574 . PMC   3757046 . PMID   24009506.
  11. 1 2 Henk DA, Shahar-Golan R, Devi KR, Boyce KJ, Zhan N, Fedorova ND, Nierman WC, Hsueh PR, Yuen KY, Sieu TP, Kinh NV, Wertheim H, Baker SG, Day JN, Vanittanakom N, Bignell EM, Andrianopoulos A, Fisher MC (2012). "Clonality despite sex: the evolution of host-associated sexual neighborhoods in the pathogenic fungus Penicillium marneffei". PLOS Pathog. 8 (10): e1002851. doi: 10.1371/journal.ppat.1002851 . PMC   3464222 . PMID   23055919.
  12. ↑ Westergaard M, Mitchell HK (1947). "Neurospora. Part V. A synthetic medium favoring sexual reproduction". American Journal of Botany. 34: 573–577. doi:10.1002/j.1537-2197.1947.tb13032.x.
  13. ↑ Nelson MA, Metzenberg RL (September 1992). "Sexual development genes of Neurospora crassa". Genetics. 132 (1): 149–162. doi:10.1093/genetics/132.1.149. PMC   1205113 . PMID   1356883.
  14. ↑ Fincham J RS, Day PR (1963). Fungal Genetics. Oxford, UK: Blackwell Scientific Publications. ASIN   B000W851KO.
  15. ↑ Wagner RP, Mitchell HK (1964). Genetics and Metabolism. New York, NY: John Wiley and Sons. ASIN   B00BXTC5BO.
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