**2.** *Saccharomyces cerevisiae* **and** *Saccharomyces paradoxus*

The budding yeast *S. cerevisiae* (**Figure 1A**) is a microbial fungus in the Division *Ascomycota. S. cerevisiae* occurs in nature as haploid (n) or diploid (2n) cells (**Figure 1B**). Haploid vegetative cells can reproduce by mitosis under favorable conditions. Diploid cells can also reproduce by mitosis when nutrients are abundant. However, when quiescent *S. cerevisiae* are starved, they accumulate DNA damages that include double-strand breaks and apurinic/apyrimidinic sites [10]. *S. cerevisiae* cells maintained in a non-replicating quiescent state undergo chronological aging

#### **Figure 1.**

*(A) Budding yeast* Saccharomyces cerevisiae *[13] and (B) cycle of sexual and vegetative reproduction*  S. cerevisiae *[14].*

during which they accumulate DNA double-strand breaks and their ability to repair such damages declines [11]. When starving (and accumulating DNA damages), haploid cells can mate to form diploid cells that can undergo meiosis to produce four haploid spores that are contained within a sac-like structure, the ascus (tetrad) [12] (**Figure 1B**). Such spores are resistant to stress, but under favorable conditions can germinate to produce haploid descendants by mitosis. When haploid cells of mating type MATa and MATalpha come into contact with each other they can fuse to form a diploid cell (syngamy) that may then either reproduce by mitosis or, if stressed, initiate another sexual cycle by undergoing meiosis. Recombination between homologous chromosomes is a central feature of meiosis and involves the systematic intimate pairing of homologous chromosomes. This process facilitates recombinational repair of DNA damages [9].

Increased sensitivity to killing by DNA damaging radiation or DNA damaging chemicals is a general characteristic of *S. cerevisiae* mutants that are defective in genes necessary for meiotic and mitotic recombination [15]. As an example, *Rad52* mutants of *S. cerevisiae* are deficient in meiotic and mitotic recombination and have increased susceptibility to killing by X-rays, methyl methanesulfonate and agents that introduce DNA crosslinks [15–17]. Homologous recombination is also necessary for recovery from oxidative DNA damage [18]. Such results demonstrate that DNA damages caused by diverse agents can be removed by recombinational repair.

A major effect of X-irradiation is the introduction of double-strand breaks in DNA. *S. cerevisiae* diploid cells in mitotic G1 phase are unable to repair such lethal X-ray induced damages [19]. However, *S. cerevisiae* cells in the G1 phase of meiosis are more resistant to the lethal effect of X-rays than cells in the mitotic G1 phase [19]. This suggests that X-ray induced lethal DNA damages are more efficiently repaired when occurring in meiotic G1 compared to mitotic G1. The increased resistance of cells undergoing meiosis may be explained by the intimate pairing of homologous chromosomes during meiosis which facilitates the replacement of damaged sequence information in one homolog by intact information from the other homolog.

Another proposed benefit of meiotic recombination, aside from DNA repair, is the production of progeny of varied genetic constitution, as occurs in outcross matings between unrelated individuals.

A study of the ancestry of natural *S. cerevisiae* strains indicated that outcrossing to an unrelated strain occurs only about once every 50,000 cell divisions [20]. That is, in nature, *S. cerevisiae* outcrossing is rare and mating is ordinarily between closely related cells. In nature, matings of *S. cerevisiae* tend to be between close relatives for two reasons [20]. First, the products of individual meiotic events are contained within the sac-like ascus, and each ascus contains a tetrad of ascospores, two ascospores of each mating type. Cells of different mating type from the same ascus tend to mate with each other because of their proximity, and such matings are between closely related individuals, which may not yield much, if any, genetic variation among the progeny. The second reason that mating tends to occur between genetically close relatives is mating type switching. Here, a cell of one mating type, upon mitotic cell division, produces two cells, usually one of the same mating types as the original cell and, often, a second cell of the opposite mating type. These two cells are physically adjacent and can mate with each other. Thus, in nature, the sexual cycle of *S. cerevisiae* can provide the benefit of recombinational repair, but only infrequently provides genetic variation.

In natural populations of the species *Saccharomyces paradoxus,* a sister species of *S. cerevisiae,* the frequency of matings between meiotic products from the same tetrad is estimated to be about 94% [21]. Also about 5% of matings are between clone-mates after switching of mating type. Only 1% of matings appear to be

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

**3.** *Schizosaccharomyces pombe*

in a sac called an ascus.

meiotic recombination [27].

**4.** *Ustilago maydis*

spores [29].

maintaining meiotic sex is DNA repair [1, 22, 23].

outcrossings. Outcrossing, in principal, may provide the adaptive benefit of generating beneficial genetic variants. Nevertheless, the low frequency of outcrossing in natural populations of *S. cerevisiae* and *S. paradoxus* indicates that the production of genetic variation is unlikely to be the principal selective force maintaining meiotic sex in these organisms. On the other hand, meiosis facilitates homologous recombinational repair of DNA damages and such repair is especially beneficial under stressful conditions that are likely to be common in nature. This proposed benefit is compatible with the hypothesis that, in general, the principal selective force

*S. pombe***,** also referred to as "fission yeast," is a unicellular rod-shaped eukaryotic microorganism in the Division *Ascomycota*. It grows vegetatively primarily as a haploid organism. *S. pombe* is facultatively sexual, so that when nutrients are limiting cells of opposite mating type tend to undergo syngamy (union of gametes) to form diploid zygotes [24]. The zygote can then enter meiosis leading to the production of four haploid products (spores) initially enclosed

Several different types of experiments have shown that DNA damages induce the sexual cycle and meiotic recombination in *S. pombe*. First, exposure of *S. pombe* cells to hydrogen peroxide, a reactive chemical that causes oxidative DNA damage, was observed to lead to an increase in sexual reproduction associated with a 4- to 18-fold increase in the formation of meiotic spores [25]. Second, DNA damages, in which the base cytosine is deaminated to uracil, forming the inappropriate base pair dU:dG, stimulate meiotic recombination [26]. Third, faulty processing of DNA replication intermediates (referred to as Okazaki fragments) produces DNA damages, including single-strand breaks or gaps, that stimulate

The fission yeast *S. pombe*, like the budding yeast *S. cerevisiae* (see above)*,* switches mating type during vegetative growth, though they each use different mechanisms [28]. This provides *S. pombe* with increased mating opportunities with close relatives. The decreased opportunity for outcrossing in *S. pombe* indicates that the production of genetic variation is unlikely to be the principal selective force maintaining meiotic sex in these organisms. Overall, the findings with *S. pombe*, like those with the other yeasts, *S. cerevisiae* and *S. paradoxus,* suggest that meiotic

*U. maydis* is a fungus in the Division *Basidiomycota*. It is a plant pathogen that causes corn smut. *U. maydis* teliospores are thick-walled rounded melanized cells with diploid nuclei that are capable of tolerating extreme temperatures and desiccation. Before teliospores mature in the infected corn plant, meiosis is initiated [29]. As teliospores germinate they complete meiosis to produce four haploid basidio-

Plants often defend themselves from pathogenic microbial invasion by releasing

an oxidative burst that includes the production of reactive oxygen species [30]. *U. maydis* can protect against the host oxidative attack by an oxidative stress response [30]. In protecting against oxidative DNA damage, *U. maydis* employs a recombinational DNA repair system that includes the Rad51 protein (related to mammalian

recombination is primarily an adaptation for repairing DNA damage.

#### *Sexual Processes in Microbial Eukaryotes DOI: http://dx.doi.org/10.5772/intechopen.88469*

*Parasitology and Microbiology Research*

recombinational repair of DNA damages [9].

during which they accumulate DNA double-strand breaks and their ability to repair such damages declines [11]. When starving (and accumulating DNA damages), haploid cells can mate to form diploid cells that can undergo meiosis to produce four haploid spores that are contained within a sac-like structure, the ascus (tetrad) [12] (**Figure 1B**). Such spores are resistant to stress, but under favorable conditions can germinate to produce haploid descendants by mitosis. When haploid cells of mating type MATa and MATalpha come into contact with each other they can fuse to form a diploid cell (syngamy) that may then either reproduce by mitosis or, if stressed, initiate another sexual cycle by undergoing meiosis. Recombination between homologous chromosomes is a central feature of meiosis and involves the systematic intimate pairing of homologous chromosomes. This process facilitates

Increased sensitivity to killing by DNA damaging radiation or DNA damaging chemicals is a general characteristic of *S. cerevisiae* mutants that are defective in genes necessary for meiotic and mitotic recombination [15]. As an example, *Rad52* mutants of *S. cerevisiae* are deficient in meiotic and mitotic recombination and have increased susceptibility to killing by X-rays, methyl methanesulfonate and agents that introduce DNA crosslinks [15–17]. Homologous recombination is also necessary for recovery from oxidative DNA damage [18]. Such results demonstrate that DNA damages caused by diverse agents can be removed by recombinational repair. A major effect of X-irradiation is the introduction of double-strand breaks in DNA. *S. cerevisiae* diploid cells in mitotic G1 phase are unable to repair such lethal X-ray induced damages [19]. However, *S. cerevisiae* cells in the G1 phase of meiosis are more resistant to the lethal effect of X-rays than cells in the mitotic G1 phase [19]. This suggests that X-ray induced lethal DNA damages are more efficiently repaired when occurring in meiotic G1 compared to mitotic G1. The increased resistance of cells undergoing meiosis may be explained by the intimate pairing of homologous chromosomes during meiosis which facilitates the replacement of damaged sequence information in one homolog by intact information from the

Another proposed benefit of meiotic recombination, aside from DNA repair, is the production of progeny of varied genetic constitution, as occurs in outcross

A study of the ancestry of natural *S. cerevisiae* strains indicated that outcrossing to an unrelated strain occurs only about once every 50,000 cell divisions [20]. That is, in nature, *S. cerevisiae* outcrossing is rare and mating is ordinarily between closely related cells. In nature, matings of *S. cerevisiae* tend to be between close relatives for two reasons [20]. First, the products of individual meiotic events are contained within the sac-like ascus, and each ascus contains a tetrad of ascospores, two ascospores of each mating type. Cells of different mating type from the same ascus tend to mate with each other because of their proximity, and such matings are between closely related individuals, which may not yield much, if any, genetic variation among the progeny. The second reason that mating tends to occur between genetically close relatives is mating type switching. Here, a cell of one mating type, upon mitotic cell division, produces two cells, usually one of the same mating types as the original cell and, often, a second cell of the opposite mating type. These two cells are physically adjacent and can mate with each other. Thus, in nature, the sexual cycle of *S. cerevisiae* can provide the benefit of recombinational

In natural populations of the species *Saccharomyces paradoxus,* a sister species of *S. cerevisiae,* the frequency of matings between meiotic products from the same tetrad is estimated to be about 94% [21]. Also about 5% of matings are between clone-mates after switching of mating type. Only 1% of matings appear to be

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other homolog.

matings between unrelated individuals.

repair, but only infrequently provides genetic variation.

outcrossings. Outcrossing, in principal, may provide the adaptive benefit of generating beneficial genetic variants. Nevertheless, the low frequency of outcrossing in natural populations of *S. cerevisiae* and *S. paradoxus* indicates that the production of genetic variation is unlikely to be the principal selective force maintaining meiotic sex in these organisms. On the other hand, meiosis facilitates homologous recombinational repair of DNA damages and such repair is especially beneficial under stressful conditions that are likely to be common in nature. This proposed benefit is compatible with the hypothesis that, in general, the principal selective force maintaining meiotic sex is DNA repair [1, 22, 23].
