**5.** *Candida albicans*

*Candida albicans*, a type of diploid yeast in the Division *Ascomycota*, is the most commonly encountered fungal pathogen in humans. The human infection candidiasis, resulting from overgrowth of *C. albicans*, often occurs in immunocompromised patients [32]. *C. albicans* can be induced to undergo a parasexual cycle that involves mating of diploids (syngamy) to form a tetraploid that subsequently appears to undergo a form of meiosis, followed by chromosome loss leading to approximately diploid cells with high levels of aneuploidy and homozygosity [33]. The *C. albicans* genome contains many genes that are homologous to genes in other species that function in meiosis [34]. One such gene, *Dmc1*, encodes a protein that has a central role in homologous recombination and is only known to express during meiosis [35]. Under appropriate conditions, *C. albicans* is capable of same sex mating and can undergo extensive genetic recombination between homologous chromosomes [36]. The two successive cell divisions that ordinarily occur subsequent to meiotic recombination in other organisms appear to be absent in *C. albicans* meiosis. Instead a reduction from four copies of the genome (tetraploidy) to two copies (diploidy) occurs by random chromosome loss during the mitotic cell division subsequent to meiotic recombination [37]. Although *C. albicans* populations are largely clonal, parasexual recombination can facilitate the evolution of resistance to the antifungal agent fluconazole upon exposure to the agent over successive generations [38].

The parasexual cycle appears to occur with greater frequency under environmental stress condition [33]. Glucose starvation and oxidative stress are environmental stresses that are commonly encountered by pathogenic *C. albicans,* and these stresses efficiently induce same-sex mating between cells from a single progenitor [39]. As suggested by Guan and collaborators [39], same sex mating in *C. albicans* may be an important mode of sexual reproduction that occurs often in nature. Oxidative stress, associated with an increase in reactive oxygen species, causes DNA damage and thus the induction of mating may reflect an adaptive DNA repair response.

Unlike *C. albicans,* several other *Candida* clade species have a sexual cycle that includes ordinary meiosis and formation of sexual spores [37]. It appears that *C. albicans* has retained meiotic homologous recombination, a principal feature of sexual reproduction that provides the adaptive benefit of DNA repair, while losing the ability to undergo successive cell divisions in an organized fashion to reduce ploidy.

#### **6.** *Paramecium tetraurelia*

*P. tetraurelia* is a unicellular eukaryotic ciliate in the Phylum *Ciliophora* (**Figure 2**). It has two diploid micronuclei and a polyploid macronucleus. The

**141**

repaired micronuclear DNA.

**Figure 2.**

Paramecium tetraurelia *[41].*

tion involves mating with another individual.

micronuclear chromosomal DNA contains the genetic information that is inherited from one generation to the next, whereas the macronucleus contains many chromosomal DNA copies that express cellular functions. *P. tetraurelia* is able to undergo both asexual and sexual reproduction. Asexual reproduction occurs by binary fission in which the micronuclei divide by mitosis and the macronucleus divides by amitotic division [40]. Sexual reproduction involves a meiotic process, either automixis or conjugation. Automixis is a kind of self-fertilization, whereas conjuga-

As *P. tetraurelia* undergoes asexual reproduction by binary fission over many successive generations, the vitality of the lineage declines (clonal aging) until the lineage reaches the end of its clonal lifespan at about 200 fissions [42]. However, if *P. tetraurelia* undergoes automixis or conjugation during clonal aging, vitality is restored. Several laboratories found that clonal aging is associated with a dramatic increase in DNA damage [42–44]. When clonally aged *P. tetraurelia* undergo automixis or conjugation, the micronuclei go through meiosis followed by pairwise nuclear fusion (syngamy) of haploid meiotic products either from the same individual (automixis) or from different individuals (conjugation) to form a new diploid micronucleus. Subsequent to the formation of a new diploid micronucleus, the old macronucleus disintegrates and the new micronucleus replicates to form a new macronucleus. The paramecia that undergo this process have their clonal lifespan restored and are rejuvenated. Clonal aging thus appears to be caused to a large extent by progressive accumulation of DNA damage and rejuvenation likely depends on the repair of such damages in the micronuclear DNA during meiosis followed by the reestablishment of macronuclear DNA by replication of the newly

*Sexual Processes in Microbial Eukaryotes DOI: http://dx.doi.org/10.5772/intechopen.88469* *Parasitology and Microbiology Research*

**5.** *Candida albicans*

successive generations [38].

**6.** *Paramecium tetraurelia*

repair response.

Rad51), a Rec2 protein (more distantly related to mammalian Rad51), and the Brh2 protein [that is related to the mammalian Breast Cancer 2 (Brca2) protein)] [31]. When any of these proteins is inactive, *U. maydis* becomes more sensitive to DNA damaging agents, mitotic recombination is reduced, and there is failure to complete meiosis [31]. Recombinational repair occurring during meiosis as teliospores are formed by the pathogen likely contributes to the maintenance of its genome integ-

*Candida albicans*, a type of diploid yeast in the Division *Ascomycota*, is the most commonly encountered fungal pathogen in humans. The human infection candidiasis, resulting from overgrowth of *C. albicans*, often occurs in immunocompromised patients [32]. *C. albicans* can be induced to undergo a parasexual cycle that involves mating of diploids (syngamy) to form a tetraploid that subsequently appears to undergo a form of meiosis, followed by chromosome loss leading to approximately diploid cells with high levels of aneuploidy and homozygosity [33]. The *C. albicans* genome contains many genes that are homologous to genes in other species that function in meiosis [34]. One such gene, *Dmc1*, encodes a protein that has a central role in homologous recombination and is only known to express during meiosis [35]. Under appropriate conditions, *C. albicans* is capable of same sex mating and can undergo extensive genetic recombination between homologous chromosomes [36]. The two successive cell divisions that ordinarily occur subsequent to meiotic recombination in other organisms appear to be absent in *C. albicans* meiosis. Instead a reduction from four copies of the genome (tetraploidy) to two copies (diploidy) occurs by random chromosome loss during the mitotic cell division subsequent to meiotic recombination [37]. Although *C. albicans* populations are largely clonal, parasexual recombination can facilitate the evolution of resistance to the antifungal agent fluconazole upon exposure to the agent over

The parasexual cycle appears to occur with greater frequency under environmental stress condition [33]. Glucose starvation and oxidative stress are environmental stresses that are commonly encountered by pathogenic *C. albicans,* and these stresses efficiently induce same-sex mating between cells from a single progenitor [39]. As suggested by Guan and collaborators [39], same sex mating in *C. albicans* may be an important mode of sexual reproduction that occurs often in nature. Oxidative stress, associated with an increase in reactive oxygen species, causes DNA damage and thus the induction of mating may reflect an adaptive DNA

Unlike *C. albicans,* several other *Candida* clade species have a sexual cycle that includes ordinary meiosis and formation of sexual spores [37]. It appears that *C. albicans* has retained meiotic homologous recombination, a principal feature of sexual reproduction that provides the adaptive benefit of DNA repair, while losing the ability to undergo successive cell divisions in an organized fashion to reduce

*P. tetraurelia* is a unicellular eukaryotic ciliate in the Phylum *Ciliophora* (**Figure 2**). It has two diploid micronuclei and a polyploid macronucleus. The

rity by removing DNA damages incurred during infection.

**140**

ploidy.

**Figure 2.** Paramecium tetraurelia *[41].*

micronuclear chromosomal DNA contains the genetic information that is inherited from one generation to the next, whereas the macronucleus contains many chromosomal DNA copies that express cellular functions. *P. tetraurelia* is able to undergo both asexual and sexual reproduction. Asexual reproduction occurs by binary fission in which the micronuclei divide by mitosis and the macronucleus divides by amitotic division [40]. Sexual reproduction involves a meiotic process, either automixis or conjugation. Automixis is a kind of self-fertilization, whereas conjugation involves mating with another individual.

As *P. tetraurelia* undergoes asexual reproduction by binary fission over many successive generations, the vitality of the lineage declines (clonal aging) until the lineage reaches the end of its clonal lifespan at about 200 fissions [42]. However, if *P. tetraurelia* undergoes automixis or conjugation during clonal aging, vitality is restored. Several laboratories found that clonal aging is associated with a dramatic increase in DNA damage [42–44]. When clonally aged *P. tetraurelia* undergo automixis or conjugation, the micronuclei go through meiosis followed by pairwise nuclear fusion (syngamy) of haploid meiotic products either from the same individual (automixis) or from different individuals (conjugation) to form a new diploid micronucleus. Subsequent to the formation of a new diploid micronucleus, the old macronucleus disintegrates and the new micronucleus replicates to form a new macronucleus. The paramecia that undergo this process have their clonal lifespan restored and are rejuvenated. Clonal aging thus appears to be caused to a large extent by progressive accumulation of DNA damage and rejuvenation likely depends on the repair of such damages in the micronuclear DNA during meiosis followed by the reestablishment of macronuclear DNA by replication of the newly repaired micronuclear DNA.
