**1. Introduction**

Different microbial eukaryotic species are capable of a variety of sexual processes. Basically, however, the different sexual processes have, as a central element, syngamy and meiosis. Syngamy is the fusion of two cells or two nuclei. Meiosis is ordinarily initiated in a diploid cell that contains a pair of homologs, which is two copies of each chromosome. In meiosis, generally, first the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Next, homologous chromosomes undergo intimate pairing with each other and exchange genetic information by homologous recombination. Recombination is succeeded by two cycles of cell division to yield four haploid daughter cells each having half the number of chromosomes as the original diploid cell. Some microbial eukaryotes, however, use a similar process, parasexual meiosis. This is where ploidy (the number of complete sets of chromosomes in a cell) is determined, both before and after homologous recombination, by processes other than those in standard meiosis. One of the microbial eukaryotes we discuss, below, *Candida albicans,* uses parasexual meiosis.

There appears to be broad agreement among geneticists that the key to understanding why sex exists is to understand the adaptive benefit of meiotic homologous recombination, the molecular event that syngamy and meiosis seem designed to promote. The evidence reviewed here on microbial eukaryotes, we think, supports the general view that the meiotic recombination mechanism is maintained by natural selection at each generation because of the benefit of DNA repair [1]. Recombinational repair is especially beneficial as an adaptation for responding to stressful conditions, such as starvation or oxidative stress, that cause DNA damage.

Meiotic sex appears to be very widespread among microbial eukaryotes. In 1999, Dacks and Roger [2] proposed, on the basis of phylogenetic evidence, that the common ancestor of all known eukaryotes was likely facultatively sexual. Since this proposal was presented, sex has been reported in several microbial eukaryotes that had previously been considered to be asexual. Examples of organisms recently recognized to be sexual are *Giardia intestinalis* (syn. *G. lamblia*) and *Trichomonas vaginalis*. These microbial eukaryotes were found to possess a core set of genes that function in meiosis, including genes that encode proteins that are specific to meiosis and act in homologous recombination [3, 4]. Both *G. intestinalis* and *T. vaginalis* are descended from ancient lineages that diverged from each other early in the evolution of eukaryotes, thus indicating that core genes necessary for meiosis, and hence sex, were likely present in an early ancestor of both species. Parasitic protozoans of the genus *Leishmania* are another example of eukaryotic microbes once considered to be asexual, but subsequently found upon further investigation, to have a sexual cycle [5]. Also, evidence for meiotic sex has recently been reported for the phylum *Amoebozoa*, another early diverging lineage in eukaryotic evolution (see Section 10). Fungi, a diverse group of eukaryotic microorganisms, also appear to be anciently sexual [6]. Recent findings on additional species, reviewed by Speijer et al. [7], also tend to substantiate the concept that sex is an ancient, ubiquitous and fundamental feature of eukaryotic life. Such varied reports have contributed to the current understanding that meiotic sex is likely a fundamental and primordial property of eukaryotes (e.g. [3, 4, 8]).

We describe here the typical stages of the sexual cycles of eukaryotic microbes, although the amount of time spent in each stage is variable among species. The stages are: (1) Haploid cells reproduce by mitosis (vegetative growth). (2) Haploid cell undergo cellular fusion (syngamy) to form a heterokaryon that may undergo further mitotic divisions (vegetative growth). (3) A diploid cell is formed when two haploid nuclei fuse. Diploid cells may also undergo additional mitotic divisions (vegetative growth). (4) The meiotic process is initiated in the nucleus of a diploid cell by undergoing a round of DNA replication without cell division, so that the nucleus has four copies of its genome. Conventionally, the nucleus at this stage is described as having two sets of homologous chromosomes where each chromosome is composed of two sister chromatids (a chromatid being equivalent to a long DNA molecule bound with appropriate histone proteins). (5) Homologous chromatids undergo intimate pairing (synapsis) including pairing of non-sister homologous chromatids. (6) Genetic information is exchanged between the paired homologous chromatids by a process of recombination. Recombination may involve breakage and exchange between paired chromatids, but in most cases information is exchanged without breakage and exchange by a process referred to as synthesis dependent strand annealing [9]. (7) Meiosis is completed by two successive cell divisions whereby a cell nucleus, starting with four copies of the genome, produces four cell nuclei each having a single copy of the genome. During the first meiotic division chromosome segregation occurs so that after completion of the division there is one set of chromosomes (each with two chromatids) in each cell nucleus. During the second meiotic division there is only one set of chromatids (each

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**Figure 1.**

S. cerevisiae *[14].*

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

chromatid, now renamed as a chromosome, in each cell nucleus). That is, haploidy is restored. In parasexual meiosis, control of ploidy both before and after homologous recombination may occur by processes other than those in "standard" meiosis

The budding yeast *S. cerevisiae* (**Figure 1A**) is a microbial fungus in the Division

(**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

(see Section 5). The life cycle may now be repeated starting at stage (1).

*Ascomycota. S. cerevisiae* occurs in nature as haploid (n) or diploid (2n) cells

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

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

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

*Parasitology and Microbiology Research*

property of eukaryotes (e.g. [3, 4, 8]).

There appears to be broad agreement among geneticists that the key to understanding why sex exists is to understand the adaptive benefit of meiotic homologous recombination, the molecular event that syngamy and meiosis seem designed to promote. The evidence reviewed here on microbial eukaryotes, we think, supports the general view that the meiotic recombination mechanism is maintained by natural selection at each generation because of the benefit of DNA repair [1]. Recombinational repair is especially beneficial as an adaptation for responding to stressful conditions, such as starvation or oxidative stress, that cause DNA damage. Meiotic sex appears to be very widespread among microbial eukaryotes. In 1999, Dacks and Roger [2] proposed, on the basis of phylogenetic evidence, that the common ancestor of all known eukaryotes was likely facultatively sexual. Since this proposal was presented, sex has been reported in several microbial eukaryotes that had previously been considered to be asexual. Examples of organisms recently recognized to be sexual are *Giardia intestinalis* (syn. *G. lamblia*) and *Trichomonas vaginalis*. These microbial eukaryotes were found to possess a core set of genes that function in meiosis, including genes that encode proteins that are specific to meiosis and act in homologous recombination [3, 4]. Both *G. intestinalis* and *T. vaginalis* are descended from ancient lineages that diverged from each other early in the evolution of eukaryotes, thus indicating that core genes necessary for meiosis, and hence sex, were likely present in an early ancestor of both species. Parasitic protozoans of the genus *Leishmania* are another example of eukaryotic microbes once considered to be asexual, but subsequently found upon further investigation, to have a sexual cycle [5]. Also, evidence for meiotic sex has recently been reported for the phylum *Amoebozoa*, another early diverging lineage in eukaryotic evolution (see Section 10). Fungi, a diverse group of eukaryotic microorganisms, also appear to be anciently sexual [6]. Recent findings on additional species, reviewed by Speijer et al. [7], also tend to substantiate the concept that sex is an ancient, ubiquitous and fundamental feature of eukaryotic life. Such varied reports have contributed to the current understanding that meiotic sex is likely a fundamental and primordial

We describe here the typical stages of the sexual cycles of eukaryotic microbes, although the amount of time spent in each stage is variable among species. The stages are: (1) Haploid cells reproduce by mitosis (vegetative growth). (2) Haploid cell undergo cellular fusion (syngamy) to form a heterokaryon that may undergo further mitotic divisions (vegetative growth). (3) A diploid cell is formed when two haploid nuclei fuse. Diploid cells may also undergo additional mitotic divisions (vegetative growth). (4) The meiotic process is initiated in the nucleus of a diploid cell by undergoing a round of DNA replication without cell division, so that the nucleus has four copies of its genome. Conventionally, the nucleus at this stage is described as having two sets of homologous chromosomes where each chromosome is composed of two sister chromatids (a chromatid being equivalent to a long DNA molecule bound with appropriate histone proteins). (5) Homologous chromatids undergo intimate pairing (synapsis) including pairing of non-sister homologous chromatids. (6) Genetic information is exchanged between the paired homologous chromatids by a process of recombination. Recombination may involve breakage and exchange between paired chromatids, but in most cases information is exchanged without breakage and exchange by a process referred to as synthesis dependent strand annealing [9]. (7) Meiosis is completed by two successive cell divisions whereby a cell nucleus, starting with four copies of the genome, produces four cell nuclei each having a single copy of the genome. During the first meiotic division chromosome segregation occurs so that after completion of the division there is one set of chromosomes (each with two chromatids) in each cell nucleus. During the second meiotic division there is only one set of chromatids (each

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chromatid, now renamed as a chromosome, in each cell nucleus). That is, haploidy is restored. In parasexual meiosis, control of ploidy both before and after homologous recombination may occur by processes other than those in "standard" meiosis (see Section 5). The life cycle may now be repeated starting at stage (1).
