**9.** *Neurospora crassa*

*Parasitology and Microbiology Research*

become favorable, usually the next spring.

**8.** *Trypanosoma brucei*

interaction leading to cell fusion [49].

*Volvox carteri* is in the Phylum *Chlorophyta*. It is a facultatively sexual species of colonial green algae. The *V. carteri* life cycle can include both a sexual and asexual phase. Under natural conditions, *V. carteri* reproduces asexually in temporary ponds during the spring. However, before the ponds dry up in the summer heat, it becomes sexual forming male and female gametes which can then undergo fertilization to form a desiccation resistant overwintering diploid zygospore. Germination of zygospores involves meiosis and takes place when environmental conditions

*V. carteri* can be induced by heat shock to undergo sexual reproduction [45]. Antioxidants can inhibit this induction, indicating that oxidative stress likely mediates the induction of sexual reproduction by heat shock [46]. Also implicating oxidative stress is the finding that an inhibitor of the mitochondrial electron transport chain, that causes an increase in reactive oxygen species, induces sex in *V. carteri* [47]. Thus the induction of facultative sex, even under natural conditions, may be due to oxidative stress, a condition that causes oxidative DNA damage [47].

Human African trypanosomiasis (sleeping sickness) is caused by *T. brucei* infection (**Figure 3**). *T. brucei* undergoes meiosis within the salivary glands of its tsetse fly vector. Meiosis appears to be a normal part of the developmental cycle of *T. brucei* [49–51]. Three proteins that are only known to express during meiosis, Dmc1, Mad1 and Hop1, are found to be expressed in the nucleus of a small fraction of dividing epimastigote trypanosomes in the salivary glands and nowhere else [51, 52]. Haploid gametes produced by meiosis can subsequently undergo pairwise

**7.** *Volvox carteri*

**142**

**Figure 3.**

Trypanosoma brucei *[48].*

The *Ascomycete Neurospora crassa* grows vegetatively as a haploid filamentous fungus. **Figure 4A** illustrates a segment of haploid hyphae which form a mass of

#### **Figure 4.**

*(A)* Neurospora crassa *hyphae [58], (B)* Neurospora crassa *life cycle. The haploid mycelium reproduces asexually by two processes: (1) simple proliferation of existing mycelium, and (2) formation of conidia (macroand micro-) which can be dispersed and then germinate to produce new mycelium. In the sexual cycle, mating can only occur between individual strains of different mating type, A and a. Fertilization occurs by the passage of nuclei of conidia or mycelium of one mating type into the protoperithecia of the opposite mating type through the trichogyne. Fusion of the nuclei of opposite mating types occurs within the protoperithecium to form a zygote (2N) nucleus [59].*

thread-like filaments comprising the mycelium which is the vegetative part of the fungus. The life cycle of *N. crassa* is outlined in **Figure 4B** which indicates the structures and events of sexual reproduction. Like *S. cerevisiae*, *N. crassa* has two mating types. Sexual interaction in *N. crassa* can only occur between individuals of opposite mating type. The diploid stage is very brief, occurring just prior to entry into meiosis. However, the brief diploid stage of *N. crassa* involves considerable complexity. The haploid vegetative multicellular filamentous stage, although longer lasting and larger than the diploid stage, has a relatively simple modular structure. In natural populations, recessive mutations specifically affecting the diploid stage are quite frequent [56]. Such diplophase specific mutations, when homozygous, can cause barren fruiting bodies (perithecia) and failure to form asci. Homozygous mutations can also lead to an abnormal meiosis with faulty pachytene or diplotene stages, or defective chromosome pairing [57]. At least 435 genes were estimated to affect the diploid stage [56]. This is at least 4% of the total 9730 genes of *N. crassa*. Thus it appears that the requirement for union of opposite mating types provides the adaptive benefit, in the diploid stage, of allowing the masking of deleterious recessive mutations (complementation) while also promoting the recombinational repair benefits of meiosis.

Species of *Neurospora*, including *N. crassa*, have life cycles adapted to ecosystems arising as the result of fire [60]. *Neurospora* species are common primary colonizers of trees and shrubs that have been killed by fire in Western North America. Fire appears to provide heat and chemical byproducts necessary for germination of ascospores that have been produced by sexual reproduction. Also fire can create a sterile environment with an abundance of nutrients derived from dead plant tissues upon which *Neurospora* can grow. The distribution of *Neurospora* growing at natural sites suggests that initial colonization by heat resistant ascospores is followed by vegetative growth, the production of conidia and then the dispersal of the conidia.

## **10.** *Amoebozoa*

The *Amoebozoa*, a phylum within the kingdom *Protozoa*, contains about 2400 described species. Amoebozoan species include a variety of lineages of polymorphic amoeboid forms that until recently were considered to be asexual. A recent study, however revealed that amoebozoans representing all major subclades possess most of the genes that function specifically in meiosis, as well as many of the genes involved in meiotic recombinational repair [61]. It was concluded that *Amoebozoa* is ancestrally sexual. Since the A*moebozoa* diverged in eukaryotic evolution before 720 million years ago [62], these findings suggest that meiotic sex was present early in eukaryotic evolution.

Another analysis of the *Amoebozoa* also supported the probable occurrence of syngamy (cell fusion) and meiotic processes in all major amoebozoan lineages [63]. This study concluded that most amoebozoans are likely capable of a canonical meiotic process. As one example, wild populations of the social amoeba *Dictyostelium discoideum* undergo widespread mating and sexual reproduction including meiosis when food is scarce [64, 65]. The evidence for the occurrence of meiotic sex among the amoebozoa is consistent with the general idea (see Section 1) that meiotic sex is likely a primitive characteristic of eukaryotes.

#### **11. Eukaryotic sexual processes likely arose in the archaea**

From about 3.4 billion to 570 million years ago, microbes were the only forms of life. The last common ancestor of all eukaryotes arose before 1.5 billion years ago [66].

**145**

**12. Conclusions**

are summarized below.

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

The eukaryotic common ancestor is considered to have arisen when an anaerobic host archaeal cell acquired an internalized aerobic bacterium [67]. The internalized aerobe eventually evolved into the mitochondrion, providing the capability for respiration. The ancestral archaeal genome appears to have contributed more important genes to the eukaryotic nuclear genome (such as those genes involved in transcription, translation and replication) than the internalized aerobe [68]. Meiotic sex appears to be a primordial characteristic of eukaryotes (see Section 1). This suggests that sexual processes may already have been present in the archaeal microbe from which eukaryotes arose. Extant archaeal species such as *Sulfolobus solfataricus* and *Sulfolobus acidocaldarius* as well as several other archaeal species undergo interactions that have

For instance, the hyperthermophilic archaeon *S. solfataricus* expresses the RadA protein, a homolog of the eukaryotic proteins Rad51 and Dmc1 that catalyze DNA pairing and strand exchange, central steps in recombinational repair during meiosis [70]. Exposure of *S. solfataricus* to DNA damaging UV irradiation or agents that cause DNA double-strand breaks induces pilus formation leading to cellular aggregation [71]. UV-induced cellular aggregation mediates high frequency chromosomal marker exchange between cells [72]. The DNA damage inducible DNA transfer process and subsequent homologous recombination were hypothesized to represent an important mechanism for providing increased repair of damaged DNA via homologous recombination in order to maintain genome integrity [71–73]. Van Wolferan and collaborators [74, 75] also obtained evidence with *S. acidocaldarius* that led them to propose that DNA transfer occurs in order to repair DNA damages by homologous recombination. Thus it appears likely that key elements of eukaryotic meiosis, namely the coming together and intimate alignment of chromosomes from different cells followed by repair of DNA damage by homologous recombination, already existed in the archaeal ancestors of eukaryotes. To some extent, these key elements are also present in many extant eubacteria, particularly in those species capable of natural genetic transformation [1]. This suggests that sexual processes

key features similar to sexual processes in microbial eukaryotes [69].

were even present in a common ancestor of both eubacteria and archaea.

Meiotic sex appears to be a primordial characteristic of microbial eukaryotes and has likely provided a continuous adaptive benefit for as long as 1.5 billion years in diverse lineages of microbial eukaryotes. Since eukaryotes appear to have evolved from an archaeal ancestor, the adaptive function of sexual processes, even in archaeal species, is relevant to understanding sexual processes in microbial eukaryotes. In the archaea, homologous recombinational repair of DNA damages appears to be the principal adaptive benefit of sexual processes. Conclusions bearing on the adaptive benefit of sexual processes (syngamy and meiosis) in microbial eukaryotes

The dikaryotic fungi (*Ascomycetes* and *Basidiomycetes*) include some of the most

well-studied microbial eukaryotic species with respect to sexual reproduction. Wallen and Perlin [76] concluded in a 2018 review of the function and maintenance of sexual reproduction in the dikaryotic fungi that sexual reproduction, including its central feature of homologous recombination, evolved to repair DNA damages that arise particularly from environmental stresses. In the ascomycete yeast *S. cerevisiae,* DNA repair by homologous recombination during mitosis is well established. Recombinational repair during meiosis is stimulated under starvation conditions and appears to be even more efficient than during mitosis. In natural populations of *S. cerevisiae* and *S. paradoxus*, the great majority of matings that occur are between

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

*Parasitology and Microbiology Research*

thread-like filaments comprising the mycelium which is the vegetative part of the fungus. The life cycle of *N. crassa* is outlined in **Figure 4B** which indicates the structures and events of sexual reproduction. Like *S. cerevisiae*, *N. crassa* has two mating types. Sexual interaction in *N. crassa* can only occur between individuals of opposite mating type. The diploid stage is very brief, occurring just prior to entry into meiosis. However, the brief diploid stage of *N. crassa* involves considerable complexity. The haploid vegetative multicellular filamentous stage, although longer lasting and larger than the diploid stage, has a relatively simple modular structure. In natural populations, recessive mutations specifically affecting the diploid stage are quite frequent [56]. Such diplophase specific mutations, when homozygous, can cause barren fruiting bodies (perithecia) and failure to form asci. Homozygous mutations can also lead to an abnormal meiosis with faulty pachytene or diplotene stages, or defective chromosome pairing [57]. At least 435 genes were estimated to affect the diploid stage [56]. This is at least 4% of the total 9730 genes of *N. crassa*. Thus it appears that the requirement for union of opposite mating types provides the adaptive benefit, in the diploid stage, of allowing the masking of deleterious recessive mutations (complementation) while also promoting the recombinational repair benefits of meiosis.

Species of *Neurospora*, including *N. crassa*, have life cycles adapted to ecosystems arising as the result of fire [60]. *Neurospora* species are common primary colonizers of trees and shrubs that have been killed by fire in Western North America. Fire appears to provide heat and chemical byproducts necessary for germination of ascospores that have been produced by sexual reproduction. Also fire can create a sterile environment with an abundance of nutrients derived from dead plant tissues upon which *Neurospora* can grow. The distribution of *Neurospora* growing at natural sites suggests that initial colonization by heat resistant ascospores is followed by vegetative growth, the production of conidia and then the dispersal of the conidia.

The *Amoebozoa*, a phylum within the kingdom *Protozoa*, contains about 2400 described species. Amoebozoan species include a variety of lineages of polymorphic amoeboid forms that until recently were considered to be asexual. A recent study, however revealed that amoebozoans representing all major subclades possess most of the genes that function specifically in meiosis, as well as many of the genes involved in meiotic recombinational repair [61]. It was concluded that *Amoebozoa* is ancestrally sexual. Since the A*moebozoa* diverged in eukaryotic evolution before 720 million years ago [62], these findings suggest that meiotic sex was present early in

Another analysis of the *Amoebozoa* also supported the probable occurrence of syngamy (cell fusion) and meiotic processes in all major amoebozoan lineages [63]. This study concluded that most amoebozoans are likely capable of a canonical meiotic process. As one example, wild populations of the social amoeba *Dictyostelium discoideum* undergo widespread mating and sexual reproduction including meiosis when food is scarce [64, 65]. The evidence for the occurrence of meiotic sex among the amoebozoa is consistent with the general idea (see Section 1) that meiotic sex is

From about 3.4 billion to 570 million years ago, microbes were the only forms of life. The last common ancestor of all eukaryotes arose before 1.5 billion years ago [66].

**144**

**10.** *Amoebozoa*

eukaryotic evolution.

likely a primitive characteristic of eukaryotes.

**11. Eukaryotic sexual processes likely arose in the archaea**

The eukaryotic common ancestor is considered to have arisen when an anaerobic host archaeal cell acquired an internalized aerobic bacterium [67]. The internalized aerobe eventually evolved into the mitochondrion, providing the capability for respiration. The ancestral archaeal genome appears to have contributed more important genes to the eukaryotic nuclear genome (such as those genes involved in transcription, translation and replication) than the internalized aerobe [68]. Meiotic sex appears to be a primordial characteristic of eukaryotes (see Section 1). This suggests that sexual processes may already have been present in the archaeal microbe from which eukaryotes arose. Extant archaeal species such as *Sulfolobus solfataricus* and *Sulfolobus acidocaldarius* as well as several other archaeal species undergo interactions that have key features similar to sexual processes in microbial eukaryotes [69].

For instance, the hyperthermophilic archaeon *S. solfataricus* expresses the RadA protein, a homolog of the eukaryotic proteins Rad51 and Dmc1 that catalyze DNA pairing and strand exchange, central steps in recombinational repair during meiosis [70]. Exposure of *S. solfataricus* to DNA damaging UV irradiation or agents that cause DNA double-strand breaks induces pilus formation leading to cellular aggregation [71]. UV-induced cellular aggregation mediates high frequency chromosomal marker exchange between cells [72]. The DNA damage inducible DNA transfer process and subsequent homologous recombination were hypothesized to represent an important mechanism for providing increased repair of damaged DNA via homologous recombination in order to maintain genome integrity [71–73]. Van Wolferan and collaborators [74, 75] also obtained evidence with *S. acidocaldarius* that led them to propose that DNA transfer occurs in order to repair DNA damages by homologous recombination. Thus it appears likely that key elements of eukaryotic meiosis, namely the coming together and intimate alignment of chromosomes from different cells followed by repair of DNA damage by homologous recombination, already existed in the archaeal ancestors of eukaryotes. To some extent, these key elements are also present in many extant eubacteria, particularly in those species capable of natural genetic transformation [1]. This suggests that sexual processes were even present in a common ancestor of both eubacteria and archaea.
