Parasexual cycle

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The parasexual cycle, a process peculiar to fungi and single-celled organisms, is a nonsexual mechanism of parasexuality for transferring genetic material without meiosis or the development of sexual structures. [1] It was first described by Italian geneticist Guido Pontecorvo in 1956 during studies on Aspergillus nidulans (also called Emericella nidulans when referring to its sexual form, or teleomorph). A parasexual cycle is initiated by the fusion of hyphae (anastomosis) during which nuclei and other cytoplasmic components occupy the same cell (heterokaryosis and plasmogamy). 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.

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The parasexual cycle resembles sexual reproduction. In both cases, unlike hyphae (or modifications thereof) may fuse (plasmogamy) and their nuclei will occupy the same cell. The unlike nuclei fuse (karyogamy) to form a diploid (zygote) nucleus. In contrast to the sexual cycle, in the parasexual cycle recombination takes place during mitosis followed by haploidization (but without meiosis). The recombined haploid nuclei appear among vegetative cells, which differ genetically from those of the parent mycelium.

Both heterokaryosis and the parasexual cycle are very important for those fungi that have no sexual reproduction. Those cycles provide for somatic variation in the vegetative phase of their life cycles. This is also true for fungi where the sexual phase is present, although in this case, additional and significant variation is incorporated through the sexual reproduction.

Stages

Diploidization

Occasionally, two haploid nuclei fuse to form a diploid nucleus—with two homologous copies of each chromosome. The mechanism is largely unknown, and it seems to be a relatively rare event, but once a diploid nucleus has been formed it can be very stable and divide to form further diploid nuclei, along with the normal haploid nuclei. Thus the heterokaryon consists of a mixture of the two original haploid nuclear types as well as diploid fusion nuclei. [2]

Mitotic chiasma formation

Chiasma formation is common in meiosis, where two homologous chromosomes break and rejoin, leading to chromosomes that are hybrids of the parental types. It can also occur during mitosis but at a much lower frequency because the chromosomes do not pair in a regular arrangement. Nevertheless, the result will be the same when it does occur—the recombination of genes. [2]

Haploidization

Occasionally, nondisjunction of chromosomes occurs during division of a diploid nucleus, so that one of the daughter nuclei has one chromosome too many (2n+1) and the other has one chromosome too few (2n–1). Such nuclei with incomplete multiples of the haploid number are termed aneuploid, as they do not have even chromosome number sets such as n or 2n. They tend to be unstable and to lose further chromosomes during subsequent mitotic divisions, until the 2n+1 and 2n-1 nuclei progressively revert to n. Consistent with this, in E. nidulans (where normally, n=8) nuclei have been found with 17 (2n+1), 16 (2n), 15 (2n–1), 12, 11, 10, and 9 chromosomes. [2]

Each of these events is relatively rare, and they do not constitute a regular cycle like the sexual cycle. But the outcome would be similar. Once a diploid nucleus has formed by fusion of two haploid nuclei from different parents, the parental genes can potentially recombine. And, the chromosomes that are lost from an aneuploid nucleus during its reversion to a euploid could be a mixture of those in the parental strain. [2]

Organisms

The potential to undergo a parasexual cycle under laboratory conditions has been demonstrated in many species of filamentous fungi, including Fusarium monoliforme , [3] Penicillium roqueforti [4] (used in making blue cheeses [5] ), Verticillium dahliae , [6] [7] Verticillium alboatrum , [8] Pseudocercosporella herpotrichoides , [9] Ustilago scabiosae , [10] Magnaporthe grisea , [11] Cladosporium fulvum , [12] [13] and the human pathogens Candida albicans [14] and Candida tropicalis . [15]

Significance

Parasexuality has become a useful tool for industrial mycologists to produce strains with desired combinations of properties. Its significance in nature is largely unknown and will depend on the frequency of heterokaryosis, determined by cytoplasmic incompatibility barriers and it is also useful in rDNA technology. [2]

Related Research Articles

Meiosis Type of cell division in sexually-reproducing organisms used to produce gametes

Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each paternal and maternal chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.

Ploidy Number of sets of chromosomes in a cell

Ploidy is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Somatic cells, tissues, and individual organisms can be described according to the number of sets of chromosomes present : monoploid, diploid, triploid, tetraploid, pentaploid, hexaploid, heptaploid or septaploid, etc. The generic term polyploid is often used to describe cells with three or more chromosome sets.

Zygote Single diploid eukaryotic cell formed by a fertilization event between two gametes

A zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual. In multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.

Basidiomycota Division of fungi

Basidiomycota is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya within the kingdom Fungi. More specifically, Basidiomycota includes these groups: mushrooms, puffballs, stinkhorns, bracket fungi, other polypores, jelly fungi, boletes, chanterelles, earth stars, smuts, bunts, rusts, mirror yeasts, and the human pathogenic yeast Cryptococcus. 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.

Ascomycota 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, brewer's yeast and baker's yeast, dead man's fingers, and cup fungi. The fungal symbionts in the majority of lichens such as Cladonia belong to the Ascomycota.

Gametogenesis Biological process

Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes, or by mitosis. For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations.

Biological life cycle

In biology, a biological life cycle is a series of changes in form that an organism undergoes, returning to the starting state. "The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.

Ascospore

An ascospore is a spore contained in an ascus or that was produced inside an ascus. This kind of spore is specific to fungi classified as ascomycetes (Ascomycota).

Karyogamy

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 together, 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

Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" in Greek, refers to the characteristic striations on the spores. The first published account of this fungus was from an infestation of French bakeries in 1843.

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.

Heterokaryon

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.

An oogonium is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female gametangium of certain thallophytes.

Mating of yeast 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>Aspergillus nidulans</i>

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.

Mating in fungi Combination of genetic material between compatible mating types

Mating in fungi is a complex process governed by mating types. Research on fungal mating has focused on several model species with different behaviour. Not all fungi reproduce sexually and many that do are isogamous; thus, the terms "male" and "female" do not apply to many members of the fungal kingdom. Homothallic species are able to mate with themselves, while in heterothallic species only isolates of opposite mating types can mate.

Sporogenesis is the production of spores in biology. The term is also used to refer to the process of reproduction via spores. Reproductive spores were found to be formed in eukaryotic organisms, such as plants, algae and fungi, during their normal reproductive life cycle. Dormant spores are formed, for example by certain fungi and algae, primarily in response to unfavorable growing conditions. Most eukaryotic spores are haploid and form through cell division, though some types are diploid or dikaryons and form through cell fusion.

Pathogenic fungi are fungi that cause disease in humans or other organisms. Approximately 300 fungi are known to be pathogenic to humans. The study of fungi pathogenic to humans is called "medical mycology". Although fungi are eukaryotic, many pathogenic fungi are microorganisms. The study of fungi and other organisms pathogenic to plants is called plant pathology.

Sexual reproduction Reproduction process that creates a new organism by combining the genetic material of two organisms

Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete with a single set of chromosomes (haploid) combines with another to produce an organism composed of cells with two sets of chromosomes (diploid). Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. Sexual reproduction does not occur in prokaryotes, but they have processes with similar effects such as bacterial conjugation, transformation and transduction, which may have been precursors to sexual reproduction in early eukaryotes.

Brachymeiosis

Brachymeiosis was a hypothesized irregularity in the sexual reproduction of ascomycete fungi, a variant of meiosis following an "extra" karyogamy step. The hypothesized process would have transformed four diploid nuclei into eight haploid ones. The current scientific consensus is that brachymeiosis does not occur in any fungi.

References

  1. ↑ Alexopolous (1996), et al., pp. 196–97.
  2. 1 2 3 4 5 Deacon J. (2005). Fungal Biology. Cambridge, MA: Blackwell Publishers. pp. 167–68. ISBN   1-4051-3066-0.
  3. ↑ Sidhu GS. (1983). "Sexual and parasexual variability in soil fungi with special reference to Fusarium moniliforme". Phytopathology. 73 (6): 952–55. doi:10.1094/phyto-73-952.
  4. ↑ Durand N, Reymond P, Fevre M (1992). "Transmission and modification of transformation markers during an induced parasexual cycle in Penicillium roqueforti". Current Genetics. 21 (4–5): 377–83. doi:10.1007/bf00351698. S2CID   30871714.
  5. ↑ Alexopolous (1996), et al., p. 12.
  6. ↑ Pulhalla JE, Mayfield JE (1974). "The mechanism of heterokaryotic growth in Verticillium dahliae". Genetics. 76 (3): 411–422. PMC   1213075 . PMID   17248647.
  7. ↑ O'Garro LW, Clarkson JM (1992). "Variation for pathogenicity on tomato among parasexual recombinants of Verticillium dahliae". Plant Pathology. 41 (2): 141–47. doi:10.1111/j.1365-3059.1992.tb02331.x.
  8. ↑ Hastie AC. (1964). "The parasexual cycle in Verticillium albo-atrum". Genetics Research. 5 (2): 305–15. doi: 10.1017/s0016672300001245 .
  9. ↑ Hocart MJ, Lucas JA, and Peberdy JF. "Parasexual recombination between W and R pathotypes of Pseudocercosporella herpotrichoides through protoplast fusion." Mycological Research. 1993 August;97(8):977-983.
  10. ↑ Garber ED, Ruddat M (1992). "The parasexual cycle in Ustilago scabiosae (Ustilaginales)". International Journal of Plant Sciences. 153: 98–101. doi:10.1086/297010.
  11. ↑ Zeigler RS, Scott RP, Leung H, Bordeos AA, Kumar J, Nelson RJ (1997). "Evidence of parasexual exchange of DNA in the rice blast fungus challenges its exclusive clonality". Phytopathology. 87 (3): 284–94. doi: 10.1094/phyto.1997.87.3.284 . PMID   18945171.
  12. ↑ Higgins VJ, Miao V, Hollands J (1987). "The use of benomyl and cycloheximide resistance markers in studies of race development by the leaf mold pathogen Cladosporium fulvum". Canadian Journal of Plant Pathology. 9: 14–19. doi:10.1080/07060668709501905.
  13. ↑ Arnaru J, Oliver RP (1993). "Inheritance and alteration of transformed DNA during an induced parasexual cycle in the imperfect fungus Cladosporium fulvum". Current Genetics. 23 (5–6): 508–11. doi:10.1007/bf00312643. PMID   8319310. S2CID   25780981.
  14. ↑ Bennett RJ and Johnson AD. "Completion of a parasexual cycle in Candida albicans by induced chromosome loss in tetraploid strains." EMBO J. 2003 May 15;22(10):2505-15.
  15. ↑ Seervai RNH, Knox SKJ, Hirakawa MK, Porman AM, and Bennett RJ. "Parasexuality and Ploidy Change in Candida tropicalis." Eukaryotic Cell. 2013 Dec; 12(12): 1629–1640.

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