The parasexual cycle, a process restricted to
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and
single-celled organisms, is a nonsexual mechanism of parasexuality for transferring genetic material without
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
or the development of sexual structures. It was first described by Italian geneticist
Guido Pontecorvo
Guido Pellegrino Arrigo Pontecorvo FRS FRSE (29 November 1907 – 25 September 1999) was an Italian-born Scottish geneticist.
Life
Guido Pontecorvo was born on 29 November 1907 in Pisa into a family of wealthy Italian industrialists. He was on ...
in 1956 during studies on ''
Aspergillus nidulans
''Aspergillus nidulans'' (also called ''Emericella nidulans'' when referring to its sexual form, or teleomorph) is one of many species of filamentous fungi in the phylum Ascomycota. It has been an important research organism for studying eukaryot ...
'' (also called ''Emericella nidulans'' when referring to its sexual form, or
teleomorph
In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota:
*Teleomorph: the sexual reproductive stage (morph), typically a fruiting body.
*Anamorph: an ase ...
). A parasexual cycle is initiated by the fusion of
hypha
A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.
Structure
A hypha consists of one or ...
e (
anastomosis
An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf#Veins, leaf veins, or streams. Such a connection m ...
) during which nuclei and other cytoplasmic components occupy the same cell (heterokaryosis and
plasmogamy
Plasmogamy is a stage in the sexual reproduction of fungi, in which the protoplasm of two parent cells (usually from the mycelia) fuse without the fusion of nuclei, effectively bringing two haploid nuclei close together in the same cell. This sta ...
). Fusion of the unlike nuclei in the cell of the heterokaryon results in formation of a diploid nucleus (
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 ...
), 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
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s; while haploidization probably involves mitotic nondisjunctions which randomly reassort the chromosomes and result in the production of
aneuploid
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
and haploid cells. Like a sexual cycle, parasexuality gives the species the opportunity to recombine the genome and produce new
genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
s in their offspring. Unlike a sexual cycle, the process lacks coordination and is exclusively mitotic.
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
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
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
Somatic may refer to:
* Somatic (biology), referring to the cells of the body in contrast to the germ line cells
** Somatic cell, a non-gametic cell in a multicellular organism
* Somatic nervous system, the portion of the vertebrate nervous sys ...
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.
[
]
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.[
]
Haploidization
Occasionally, nondisjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I ...
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.[
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.][
]
Organisms
The potential to undergo a parasexual cycle under laboratory conditions has been demonstrated in many species of filamentous fungi, including ''Fusarium monoliforme
''Fusarium'' is a large genus of filamentous fungi, part of a group often referred to as hyphomycetes, widely distributed in soil and associated with plants. Most species are harmless saprobes, and are relatively abundant members of the soil mic ...
'',[ '']Penicillium roqueforti
''Penicillium roqueforti'' is a common saprotrophic fungus in the genus ''Penicillium''. Widespread in nature, it can be isolated from soil, decaying organic matter, and plants.
The major industrial use of this fungus is the production of blue ...
''[ (used in making blue cheeses), '']Verticillium dahliae
''Verticillium dahliae'' is a fungal plant pathogen. It causes verticillium wilt in many plant species, causing leaves to curl and discolor. It may cause death in some plants. Over 400 plant species are affected by ''Verticillium'' complex.
M ...
'',[ '']Verticillium alboatrum
''Verticillium albo-atrum'' is a plant pathogen with many hosts.
Infected plants
See:
* List of potato diseases
* List of alfalfa diseases
* List of African daisy diseases
* List of beet diseases
* List of caneberries diseases
* List of t ...
'',[ '']Pseudocercosporella herpotrichoides
''Tapesia yallundae''
is the causal agent for a variety of cereal and forage grass diseases. The anamorph of ''T. yallundae'' is the W-type strain of ''Pseudocercosporella herpotrichoides''. The R-type strain of ''Pseudocercosporella herpotr ...
'', ''Ustilago scabiosae
''Ustilago'' is a genus of approximately 200 smut fungi parasitic on grasses.
Uses
''Ustilago maydis'' is eaten as a traditional Mexican food in many parts of the country, and is even available canned. Farmers have even been known to spread th ...
'',[ '']Magnaporthe grisea
''Magnaporthe grisea'', also known as rice blast fungus, rice rotten neck, rice seedling blight, blast of rice, oval leaf spot of graminea, pitting disease, ryegrass blast, Johnson spot, neck blast, wheat blast, and Imochi (Japanese:稲熱) is a ...
'',[ '']Cladosporium fulvum
''Passalora fulva'' is a fungal plant pathogen that causes tomato leaf mold
''Cladosporium fulvum'' is an Ascomycete called '' Passalora fulva'', a non-obligate pathogen that causes the disease on tomato known as the tomato leaf mold.Cladospo ...
'',[ and the human pathogens '']Candida albicans
''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is us ...
'' and ''Candida tropicalis
''Candida tropicalis'' is a species of yeast in the genus '' Candida''. It is a common pathogen in neutropenic hosts, in whom it may spread through the bloodstream to peripheral organs. For invasive disease, treatments include amphotericin B, ec ...
''.[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.]
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.[
]
References
Cited text
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{{Sex (biology)
Mycology
Molecular genetics
Cellular processes