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Sexual reproduction enables genetic exchange in eukaryotic sex as diverse o fungi, animals, plants, and ciliates. Given its ubiquity, sex is thought to have evolved once, possibly concomitant with or shortly after the origin of eukaryotic organisms themselves. The basic principles of sex are conserved, including ploidy changes, the formation of gametes via meiosis, mate recognition, and cell-cell fusion leading to the production of a zygote.

Although the basic tenants are shared, sex determination and sexual reproduction occur in myriad forms throughout nature, including outbreeding systems with more than two mating types or sexes, unisexual selfing, and even examples in which organisms switch mating type. As robust and diverse genetic models, fungi provide insights into the molecular nature of sex, sexual specification, and evolution to advance our understanding of sexual reproduction and its impact throughout sex eukaryotic tree of life.

Sexual reproduction drives genetic recombination throughout eukaryotic organisms and serves to purge deleterious mutations, producing better-adapted progeny. The processes of sex involve mate recognition, cell-cell fusion yielding a zygote, generation of gametes via meiosis, and ploidy changes.

Although these basic tenants are shared, a diversity of sexual reproduction sex is encountered throughout mzt. The fungal kingdom includes outbreeding systems with up to thousands of mating types at one end of the spectrum and self-fertile, inbreeding systems at the other, and provides a window to investigate the molecular nature of sex, sexual specification, and evolution.

Our aim in this review is to investigate these specific aspects of mar in fungi. In addition, we discuss the connections between sexual development and other aspects of the life cycle, including evolution of sex chromosomes, mitochondrial inheritance, and genome defense mechanisms.

An early key step in sexual reproduction is mate recognition. Both yeast and filamentous fungi have evolved systems to detect mating partners via mating type-specific peptide pheromones and receptors. Mating type-specific pheromone receptors decorate the cell surface and sense these reciprocal signals to activate the pheromone-sensing mitogen-activated protein MAP kinase signal transduction pathway common to both mating types.

Mate recognition through pheromone sensing is widespread throughout the fungal kingdom. In the filamentous ascomycete Neurospora crassaa pheromone-like protein sex recently identified that participates in mating, ascospore formation, and vegetative growth Interestingly, pheromone-receptor interactions exhibit considerable plasticity between species.

In the filamentous ascomycete Aspergillus nidulanspheromone receptors are required for self-fertilization, whereas the outcrossing of pheromone receptor mutants results in the production of fertile ascospores but reduced levels of cleistothecia, the sexual structures bearing the spores Alby et al.

Basidiomycetes have only a factor—like pheromones and pheromone receptors, l are encoded by genes in the Mat locus. In the human pathogenic basidiomycete Cryptococcus neoformanspheromones and their receptors contribute to both opposite- mat same-sex mating 85, We can therefore conclude that pheromone-receptor systems are integral for both heterothallism and homothallism.

Indeed, the presence of pheromones and their receptors in a given genome suggests an ability to mate, even in the absence of sex known extant sexual cycle. As the pheromone system is a common element of sexual differentiation in both basidiomycetes and ascomycetes, it is likely that their last common ancestor possessed a pheromone-receptor system that governed the sexual cycle in either a homothallic or heterothallic fashion.

However, the molecular nature of the pheromone system is not universal in the fungal kingdom. In basal matt such as the zygomycetes, trisporic acid derivatives serve as mating pheromones. In chytridiomycetes, another basal fungal lineage, the pheromone system involves mat compounds with structures completely unrelated to those of the zygomycete, ascomycete, or basidiomycete pheromones.

In the aquatic chytrid Allomyces macrogynusthe pheromone produced by the female is named sirenin, and the male produces parisin. Following cell recognition via pheromone sensing, mating cells undergo cell-cell fusion, resulting in a dikaryotic state that prepares the cells for nuclear fusion and meiosis.

Under nat environmental conditions e. The S. This system is common in the closely related Candida genus and in the rather distantly related archiascomycete Schizosaccharomyces pombe, where both haploid and diploid states of the sexual cycle are characterized by yeast cells.

However, some ascomycetes and the majority of basidiomycetes are filamentous, and sexual reproduction occurs during the hyphal state. In filamentous fungi, cell fusion can occur between a hyphal and a specialized cell, or between two hyphal partners. In filamentous ascomycetes such as N. Following cell-hypha fusion, the nucleus mat the male cell migrates through the hypha aex the female reproductive structure.

The nuclei of both mating types proliferate, pair, and migrate to the dikaryotic ascus where they fuse and undergo meiosis.

Mitosis then produces four sex of recombinant ascospores A similar mating process is observed in o Aspergilli and filamentous basidiomycetes, where the mating partners are two hyphae of the same or opposite mating type.

An interesting example is the model mushroom Coprinopsis cinereawhere monokaryotic hyphae fuse through hyphal anastomosis If monokaryons have different mating types, they establish a fertile dikaryotic mycelium in which nuclear exchange and reciprocal migration of the nuclei occur within the hyphae. At the tip of the hyphae, the opposite mating-type nuclei pair, proliferate, and migrate in the hyphae.

Under certain environmental conditions, the dikaryon produces the maat body that contains the basidia, where nuclear fusion occurs and subsequent meiosis and sporulation produce basidiospores An interesting feature of basidiomycetes is mat long delay between cell fusion and nuclear fusion.

In addition, the absence of the pheromone-receptor interplay in mating partner recognition in mushrooms is striking, as fusion can occur between any type of hyphae. However, genetic compatibility of mating partners is required for sexual development to continue.

Hypha-hypha fusion zex common in basidiomycetes considering that the majority are filamentous at some point during their life cycle. However, some exist as haploid yeasts in nature and fuse upon stimulation by pheromones, at which mat the dikaryon undergoes a dimorphic transition from yeast to hypha.

An interesting example is C. The cells fuse and establish a dikaryotic state that initiates hyphal formation. The basidium is formed at the tip of the hypha, where nuclear fusion, meiosis, and sporulation occur The three morphological states of mating—yeast cells, hyphae, and the dimorphic transition from yeast to hyphae—are common in both phyla of the Dikarya; therefore, it is challenging to determine the ancestral morphological condition as there is evidence to support all three.

However, species of the distantly related, basal zygomycete lineage mate through hyphal fusion in a manner similar to that described above.

Phycomyces blakesleeanus mycelia of opposite mating types produce trisporic acid chemical signals that induce the formation of fruiting bodies with thicker hyphae at the mat, which are known as zygophores o Following physical contact, zygophores twist to form a circle where at the top, two cells of opposite mating types fuse to form a zygospore.

Nuclear mat mag occurs and gives rise to a sporangium filled with spores Based on these observations and previous phylogenetic analyses that support a filamentous species as a possible precursor of the Dikarya, it is conceivable that the ancestral mating process involved the presence of hyphae.

Alternatively, the sexual cycle of A. Which of these is the ancestral state, or the possibility of both being ancestral states, invites future investigation. Sexual reproduction is central to eukaryotic evolution via mqt ability to increase genetic diversity and eliminate deleterious mutations. Fungi have evolved two paradigmatic sexual systems: heterothallism and homothallism Figure 1.

Heterothallic fungi require two compatible partners for mating to occur, whereas homothallic fungi are self fertile with mat single individual capable of sexual reproduction even in solo culture.

Both modes of sexual reproduction share key features e. Transitions between heterothallic and homothallic patterns of sexuality are common throughout the fungal kingdom, and both modes can be observed concomitantly in different species of the same genus and sometimes even within the same species reviewed in 55 Modes sex sexual reproduction in fungi.

Bipolar: sez mating-type MAT locus regulates sexual development, and two isolates need xex possess opposite MAT alleles to mate. Tetrapolar: two MAT loci regulate sexual development and are often multiallelic, and two isolates need to possess opposite alleles at both loci for sexual reproduction. Both heterothallic and homothallic sexual reproduction modes are dynamic and have evolved to fulfill the mating requirements of each fungal species.

Heterothallic fungi require two partners of opposite mating types with compatible MAT idiomorphs, which contain genes controlling cell identity, cell fusion, and the formation of the dikaryotic zygote state that leads to nuclear fusion, meiosis, and sporulation. In homothallic fungi, the same genes are often required for successful mating; however, there is no mating partner, the partners sex of the same mating type, both mating type idiomorphs are present or fused or one partner switches mating types.

The canonical model of homothallism is the MAT switching paradigm in ascomycetes, in which a MAT cassette system enables mother cells to switch mating type to mate with a daughter cell. Studies of mating-type switching in S. Mating-type switching may also have independently arisen in basidiomycetes, based on a report on Agrocybe aegeritaindicating that this form of homothallism has evolved repeatedly and independently 78 Mating-type switching in Saccharomyces cerevisiaeKluyveromyces lactisand Schizosaccharomyces pombe.

Homologous recombination repairs the break via gene conversion. Initially, the silent cassettes were independently acquired in S. The Saccharomycotina gained the HO gene, which increased switching efficiency. However, in the diverged lineage of K. In several types of homothallism, the two MAT idiomorphs mta for mating coexist in the same genome and are either fused into one MAT locus, as observed in Cochliobolus spp. Homothallic Sex spp. The pattern of sexual reproduction can be readily altered by genetically manipulating the MAT locus, and thus the two patterns of sexuality may share a common evolutionary origin The mat organization of MAT in Cochliobolus spp.

Similarly, the homothallic species A. The recent discovery of an extant heterothallic A. Remarkably, some homothallic fungal species contain only one MAT idiomorph and yet exhibit sex complete sexual cycle.

Some heterothallic fungi exhibit homothallism under sex environmental conditions. The heterothallic basidiomycete C. The heterothallic ascomycete C. Homothallism has also been observed in the basal fungal zygomycete and chytridiomycete lineages.

In the homothallic species A. The transition from heterothallism to homothallism appears to be a choice between outcrossing and inbreeding in a population, and the frequency with which they occur is under the control mag specific environmental and genetic conditions that favor one or m other breeding strategy.

Conversions between sex two sexual modes are common mat the same genus and even in isolates of the same species. Whether homothallism evolved from heterothallism or vice versa is not known, as there is evidence to support both hypotheses. The pervasiveness of heterothallism may favor this as the ancestral mode of reproduction, and examples of heterothallic species with recently derived closely aligned homothallic species provides further support for heterothallism as ancestral.

However, the finding that some homothallic species involve just one mating type same-sex mating suggests that homothallism could represent an original sexual mat of fungi with only one mating type. One way to envision this is as a primitive homothallic fungus with a self-fertile unisexual mode of reproduction, possibly similar to that of C. Heterothallism could have been derived later, with two distinct idiomorphs evolving to govern the sexual cycle and identity. If this is the case, the pervasiveness of heterothallism throughout the fungal kingdom may be due sez its advantages in response to distinct evolutionary pressures.

What then could have been an ancestral homothallic state?

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Some fungal species, specifically the ascomycetes S. These systems involve a cassette mechanism with one active expression locus, two silent MAT allele copies, and machinery for DNA lesion-promoted recombination Figure 2. Although the mechanisms differ between species, full genome sequences and molecular genetic studies have revealed conserved features and evidence of independent origins of the cassettes and switching mechanisms.

The opposite MAT silent cassettes are an essential part of the system, functioning as templates for mating-type switching. The presence of the cassettes in the genome underlies the ability of the organism to switch mating types; the species that feature them switch mating types at various frequencies. Interestingly, the cassettes are not present in closely related Candida spp. These observations support an independent acquisition of the silent cassettes in the genomes of S.

The cassette MAT switching model proposed by Herskowitz and colleagues has been extensively studied in S. MAT switching occurs through a DSB repair mechanism known as synthesis-dependent strand annealing using the silent cassette of the opposite mating type as a template The Ho endonuclease is cell cycle-controlled and only expressed in mother cells during late G1.

Thus, only mother cells cells that have undergone at least one mitotic division are licensed to switch. The HO gene or pseudogene is present in all members of the Saccharomycotina that harbor silent cassettes, and all of these members have been reported to exhibit mating-type switching.

Interestingly, K. In addition, K. Recent studies revealed a novel alternative Ho-independent mechanism of high efficiency switching in K. Barsoum et al. The DSB is repaired through homologous recombination employing the silent cassettes as templates and results in mating-type switching via gene conversion Moreover, Booth et al.

These results suggest a common evolutionary path of mating-type switching in S. A first step in the evolution of mating-type switching involved the acquisition of the silent cassettes. Initially, the switching efficiency was likely very low; however, the acquisition of the HO gene into the regulatory circuit improved the efficiency of mating-type switching.

Nevertheless, K. Although S. The nature of the imprint has been the subject of active debate, but recent evidence supports the hypothesis that a protected single-strand nick is introduced on the leading strand template, possibly during maturation of Okazaki fragments in the prior replication cycle, by an unknown nuclease , During replication, the incoming replication fork is delayed at the imprint, and the newly synthesized leading strand invades the opposite MAT donor cassette, using it as a replication template for the mat region This mechanism is unique among known homothallic species, indicating that the mating-type switching mechanism in S.

Initially, the acquisition of the silent MAT cassettes may have allowed mating-type switching with low efficiency in an ancestral species of S. However, a subsequent, unknown event, possibly the evolution or acquisition of a nuclease, increased the efficiency of mating-type switching, similar to S.

A previous review from our group covered mating-type systems in filamentous ascomycetes, using the genus Aspergillus as an example Both S. They are exemplary genetic models to study fungal sexual development and meiosis because their sexual spores are arranged linearly in fruiting bodies 76 , In addition, P. All three genome sequences are available 40 , 48 , Sexual development of these fungal species Figure 3 a begins with the germination of ascospores, followed by the growth of vegetative mycelium with the formation of an ascogonium female gametangia , which further develops into a perithecium the fruiting body.

Inside the perithecium, two nuclei fuse to generate a diploid nucleus, which undergoes meiosis followed by a postmeiotic mitosis, resulting in the formation of eight haploid, linearly arranged ascospores in N. Mating of heterothallic N. Sexual development of pseudohomothallic P.

The mycelia of each mating type develop into spermatia or ascogonia, and fertilization occurs between a spermatium and an ascogonium of opposite mating types. Figures were modified from Figure 1 with permission from the author Patrick Shiu Abbreviations: HMG, high-mobility group domain; PPF, the domain containing conserved proline, proline, and phenylalanine residues.

The binucleate ascospores germinate to form self-fertile, heterokaryotic mycelia carrying nuclei of both mating types that can complete the sexual cycle in solo culture pseudohomothallic inbreeding or segregate hyphae of opposite mating type to enable outcrossing. The mycelia of each mating type develop into spermatia male or ascogonia female , and fertilization occurs between a spermatium and an ascogonium of opposite mating type Unlike many other ascomycetes, spermatia asexual spores of P.

In contrast to the two species discussed above, N. In contrast to P. The fertilization process is similar to that of P. As shown in Figure 3 a , all three fungi produce perithecia with linearly arranged ascospores, thus providing excellent model systems in which meiotic drive elements can be studied. These three fungi share a similar genetic organization at the MAT locus Figure 3 b. Deletion of the PPF or HMG domain gene does not confer any apparent phenotype, whereas the deletion of both genes dramatically decreases fertility Instead, four genes are located within the S.

These genes are all transcribed during the sexual life cycle in both S. The deletion of the S. In contrast, the deletion of the PPF domain gene causes an arrest of sexual development after fertilization, suggesting a postfertilization role 7 , As these fungi and their closely related species have similar processes of sexual development but differ in MAT locus configuration and reproductive modes, this group of fungi provides a rich resource to study links between the evolution of the MAT and reproductive mode.

It is unknown whether heterothallism evolved from homothallism or vice versa However, the homothallic Neurospora species Neurospora africana , Neurospora dodgei , Neurospora galapagonensis , and Neurospora lineolata have only the MAT1 — 1 idiomorph and are unisexual 51 , , indicating that the fused MAT loci of the homothallic species S. A recent Neurospora phylogenetic analysis further supports this conclusion and predicts that homothallism might have evolved independently more than six times within the Neurospora genus and involved both MAT fusion and unisexual reproduction The molecular mechanisms responsible for the fusion event have not been elucidated.

As shown in Figure 3 b , it is likely that a recombination event occurred between conserved regions of MAT1 — 1 and MAT1 — 2 , supporting the model that homothallism evolved from heterothallism.

In plants and animals, sex is often determined by distinguishable sex chromosomes reviewed in Sex chromosomes share a common feature: Their recombination is inhibited and restricted to a pseudoautosomal region The sex-determining MAT locus in fungi is generally shorter and composed of key cell identity genes, as discussed above. Nevertheless, recent studies have revealed that the chromosome regions determining mating-type identity in several fungi span large chromosomal distances and share more features with the complex sex chromosomes of animals and plants.

A notable example of fungal sex chromosomes is the ascomycete Neurospora tetrasperma , which has a pseudohomothallic sexual life cycle similar to P. Sharing features of sex chromosomes with animals and plants, the sex chromosomes of N.

As the N. By examining preferred codon usage in genes on the N. In basidiomycetes, there exist both bipolar mating systems where mating type is determined by one locus, and two strains must possess different alleles at the locus to be sexually compatible and tetrapolar mating systems where mating type is determined by two loci, and two strains must possess different alleles at both loci to be sexually compatible , even among closely related species 45 , 96 , The number of alleles of the MAT locus also varies among species, ranging from two e.

Based on phylogenetic analyses, the common ancestor of all fungi was most likely bipolar. How bipolar mating systems evolve from tetrapolar mating systems is an intriguing question. Equally important is whether any extant bipolar species represents a hypothetical ancestral basidiomycete bipolar sexual state. Raper proposed three hypotheses on the emergence of bipolar mating systems from tetrapolar ancestors in the basidiomycota.

In the first model, one of the two original MAT loci of the tetrapolar ancestor loses the cell identity function, possibly because of mutations that result in self-compatibility, such that the mating type is now determined only by the other original MAT locus.

Examples supporting this hypothesis include studies of the mushrooms C. It should be noted that this scenario is not likely to instantaneously establish a complete reproductive barrier between the bipolar and tetrapolar lineages, and subsequent introgression between the two lineages likely still occurs see discussion below. As a result, the two loci now function and are inherited as one single unit, thus giving rise to a bipolar system.

One example supporting this hypothesis is the MAT locus of Ustilago hordei , a fungal pathogen of small-grain cereals 9 , Another example is the human pathogenic fungus C. A recent study discovered an extant sexual cycle in Cryptococcus heveanensis , a species that is closely related to C.

Further analyses revealed that the MAT locus of C. In addition, current evidence suggests that the physical linkage between the two MAT loci, similar to C. Although theoretically possible, there is little empirical evidence to support this last hypothesis. Fraser et al. This hypothesis is similar to the first hypothesis that was proposed by Raper see discussion above.

Further studies are necessary to test this specific molecular hypothesis. Because of introgression, the transition from a tetrapolar to a bipolar mating system may not be a sharply demarcated process, and mating systems representing intermediate stages could exist. This is supported by the findings in which a tetrapolar mating system was genetically engineered from the bipolar species C. The red yeast Sporidiobolus salmonicolor has a bipolar mating system; however, a recent study revealed that its mating system has certain features that resemble the tetrapolar system, namely that the MAT locus is multiallelic, and recombination can occur within MAT to generate novel MAT alleles Additionally, the MAT locus of S.

Further chromosomal rearrangements e. Interestingly, a recent study of the mushroom Schizophyllum commune details a mating system that differs from both bipolar and tetrapolar mating systems Specifically, S.

However, the matA locus of S. Therefore, S. Among fungal species that have been examined, a majority exhibit uniparental mitochondrial inheritance, i. Examples of species in which uniparental mitochondrial inheritance is observed include A. In contrast, in some fungal species, including S. In these species, mating between isogamous sexual partners results in an equal contribution of organelles from the two gametes into the zygote, and the coexistence of two different mitochondria often results in recombination 13 , Even in cases of biparental inheritance, homoplasy is rapidly established after the initial heteroplasmic zygote, such that each daughter cell possesses the mitochondrion of one parental genotype or a recombinant of the two parental genotypes 10 , The predominance of uniparental mitochondrial inheritance suggests that coexistence of two different mitochondria within one cell may be disadvantageous, possibly due to conflicts or competition between mitochondria with different genotypes.

Indeed, avoiding potential conflicts between genetically different mitochondria has been proposed as one selection pressure that maintains uniparental inheritance of organelles in many fungal species, as well as in other organisms, including plants and animals.

Another possible and not mutually exclusive hypothesis is that uniparental inheritance of mitochondria as well as other organelles such as chloroplasts could prevent the spread of selfish or deleterious genetic elements arising within these organelles in the population 54 , 60 , 61 , 68 , It should be noted that the selection pressure for uniparental inheritance of organelles could vary among different fungal species because of a variety of biological factors.

For example, during inbreeding modes of sexual reproduction, genetic conflicts and competition between mitochondria from the two mating partners would be less likely to occur. Even in these scenarios, uniparental mitochondrial inheritance may still be favored to limit the spread of potentially selfish deleterious mitochondrial mutations during sexual reproduction. In most fungal species, mating occurs between either two isogametes or between two compatible mycelia.

Unlike in plants and animals, where size differences between the gametes i. In filamentous fungi, mating between two compatible mycelia is achieved by the mutual migration of the two nuclei while all of the cytoplasm, including the organelles, is left behind, and the inheritance of two different mitochondria is avoided 59 , In unicellular fungi in which mating occurs between two isogametous mating partners, there is evidence indicating that active degradation of organelles from one gamete occurs, thus ensuring homoplasy in the zygote.

One example is the human pathogenic fungus C. Another example in which MAT controls mitochondrial inheritance is U. The biallelic a locus a1 and a2 is involved in pheromone and pheromone receptor-based cell recognition and fusion, whereas the multiallelic b locus encodes the homeodomain transcription factors.

Although evidence supports an active lga2 -and rga2 -mediated selective elimination process, the finding that deletion of rga2 reverses the inheritance in favor of the a1 -type mitochondria rather than resulting in a biparental pattern indicates that an rga2 -independent mechanism may also be involved in the control of mitochondrial inheritance It should be noted that in heterokaryons of dimorphic ascomycetes and basidiomycetes, biparental inheritance of the mitochondrion, as well as mitochondrial recombination, frequently occurs 33 , , Additionally, even in species where the mitochondrion is typically uniparentally inherited, mitochondrial leakage i.

Mobile genetic elements populate the genomes of virtually all eukaryotic organisms. Although transposable elements can confer beneficial effects for their hosts, unchecked transposon activity can be detrimental by challenging genetic integrity.

Many organisms have developed control mechanisms that effectively limit the activity of these selfish DNA elements and thus establish a more peaceful symbiotic relationship between transposons and their hosts. Transposon control is especially critical during sexual reproduction, a time at which transposons could be more threatening because meiosis may trigger new transposon exchange between genomes or lead to heterologous chromosome alignment and recombination or translocation due to the presence of ectopic copies of preexisting transposons 17 , Therefore, it is not surprising that many organisms have evolved specific sex-related silencing mechanisms that are activated during the sexual cycle to suppress transposable elements.

Early, strong evidence for active sex-related silencing emerged from studies of the filamentous fungus N. This organism has developed a number of complex genome defenses operating at different stages of its life cycle, including DNA methylation, quelling, repeat-induced point mutation RIP , and meiotic silencing of unpaired DNA MSUD 29 , 48 , 50 , 71 , Among these, quelling is an RNAi-dependent posttranscriptional gene-silencing pathway that is active in the vegetative phase of the life cycle, whereas RIP and MSUD function during the sexual cycle but via different molecular mechanisms.

RIP acts premeiotically and efficiently detects duplicated sequences present in the haploid genomes destined to participate in meiosis and inactivates these sequences by extensive C to T G to A base mutations Therefore, RIP serves as a powerful genome defense mechanism against repetitive transposable elements, as evidenced by the presence of numerous nonfunctional transposon relics in the N.

MSUD is another genome defense mechanism occurring during meiosis that operates as a failsafe mechanism against any transposons that escape RIP. This process is mediated by trans -sensing that identifies the unsuccessful pairing of discrete DNA regions during homolog pairing The failure to sense an equivalent region in the opposite chromosome triggers an RNAi-mediated silencing mechanism that posttranscriptionally silences all genes contained in the loop of unpaired DNA 71 , Given these rigorous genome defense mechanisms, it is not surprising that duplicated elements and active transposons have been largely eradicated in the N.

As a result, N. Transposon families are typically heterogeneous. Indeed, small RNA pathways play a prominent role in transposon control in many eukaryotic organisms A surprisingly large number of small RNAs mapping to specific transposable elements have been identified in plants, insects, nematodes, and most recently in the budding yeast Saccharomyces castellii through the application of next-generation sequencing technologies 6 , 38 , These processes frequently share obvious mechanistic overlap involving RNAi pathways and are considered to function in parallel with or to have evolved as specialized adaptions of RNAi pathways.

In addition, a similar requirement for factors linked to RNAi during meiotic silencing has also been demonstrated in ciliates, Caenorhabditis elegans , and mammals 5 , 24 , Recently, a novel sex-induced RNAi genome defense system has been reported in the human fungal pathogen C. Most interestingly, RNAi machinery components were more abundant during mating, supporting a model in which increased expression of RNAi machinery may function to silence potentially overexpressed transposons during mating Although the mechanistic details of the initiation of SIS are not yet understood, this discovery brings a fresh perspective to meiotic silencing involving the upregulation of RNAi pathways as a strategy to guard genomic integrity during the sexual cycle.

Similar mechanisms may be conserved and operate in other fungal species, especially in those that contain the RNAi component machinery yet lack known meiotic silencing pathways 71 , such as the fission yeast S.

In fungi such as U. Aneuploidy refers to changes in chromosome copy number that do not correspond to increases or decreases of the entire haploid or diploid genome suite of chromosomes. In humans, aneuploidy can frequently occur during meiosis in both oogenesis and spermatogenesis 67 and also in the genesis of cancer cells Aneuploidy is also generated during sexual reproduction in several fungal species.

For example, C. During this process, Spodependent recombination occurs between homologous chromosomes. However, many of the progeny are trisomic for one or more chromosomes, conferring phenotypic and genotypic plasticity Aneuploidy is generated during both same- and opposite-sex mating in C. Ni, M. Feretzaki, W. Li, Y-L. Chen, A. Floyd, unpublished results. During same-sex mating, approximately seven percent 6 of 90 of the progeny exhibit phenotypic changes such as temperature-sensitive growth, drug sensitivity or resistance, or enhanced melanin production.

Comparative genomic hybridization analyses revealed that many of these variant progeny are aneuploid and carry an extra chromosome. Aneuploidy appears to be responsible for the observed phenotypic variance, as the return to euploidy following chromosome loss restores wild-type phenotypes M. The generation of aneuploidy during sexual reproduction may be a common feature in fungi, and genotypic analysis of progeny using advanced genomic techniques in more species is necessary to explore this hypothesis.

Why do fungi generate aneuploidy at a high rate during meiosis? Several studies have suggested that aneuploidy may provide phenotypic and genotypic plasticity for natural selection during evolution. Rancanti et al. In addition, aneuploidy has been found to evoke transcriptomic and proteomic changes that orchestrate phenotypic diversity in S. A single nucleotide mutation in a deubiquitinating enzyme of S. A potential explanation for this is that the mutation leads to the activation of the proteosome and the degradation of protein subunits present in unbalanced ratios, thereby contributing to the restoration of normal growth.

Aneuploid isolates of C. Therefore, aneuploidy generated from sexual development may yield a diverse genetic pool upon which natural selection acts. Compared with spontaneous mutations, aneuploidy may be beneficial because its effects can be rapidly reversed through the loss or gain of entire chromosomes, thereby returning to the euploid state when the environment is again favorable.

Why do many organisms maintain sexual reproduction? The advantages of sex have been among the most debated questions in biology. The advantage of asexual development is to propagate rapidly while expending less energy, whereas the advantage of sexual development is to generate genetic diversity to accelerate adaptation to novel and changing environments.

There are many obligately sexual species, including mammals, plants, and possibly even fungi such as S. Certain fungi and plants are able to reproduce both asexually and sexually, and choose one or the other strategy depending on the environment.

For instance, the filamentous fungus A. Are there any obligately asexual species? Many fungal species, such as A. However, recent genomic studies have revealed that all contain the MAT locus and the entire suite of genes needed for sexual development 49 , 88 , , , Furthermore, sexual development of these fungi has been shown to occur under unusual lab conditions, e.

The most common pathogenic fungus, C. Further lab experiments discovered that C. In addition to heterothallic mating, same-sex mating has been reported in both C. Interestingly, aneuploidy is generated at a high rate during same-sex mating in C. An interesting question remains as to whether any obligately asexual fungi exist that are entirely lacking the MAT locus or mating pathway genes, or key meiotic genes, and if so, how and why were they lost?

In an other view, certain unknown loci may be responsible for sexual development. One possible example is that Lodderomyces elongisporus lacks a MAT -like locus MTL and an a -factor pheromone, receptor, and transporter but is thought to have an extant homothallic sexual cycle based on the production of asci harboring single spores monads Further study is necessary to establish whether this is a true sexual cycle or an asexual mode of sporulation.

Sex in fungi is still mysterious in many aspects and open questions remain. Did the ancestral mating process involve filamentous fungi or yeast? What are the driving forces for the evolution of MAT loci, and why are the molecular components so plastic throughout the fungal kingdom?

This may require a better understanding of the ecology and population genetics of different fungi species. Is the ancestral sexual state homothallic or heterothallic, or possibly both? Are there mating systems that differ from both bipolar and tetrapolar mating systems that exist, and if so how common are they? Are there any obligately asexual fungal species? Further study of sex in fungi provides fertile ground to explore and solve these and other mysteries and thereby advance our understanding of the evolution of sex in both unicellular and multicellular eukaryotes Figure 4.

Modes of sexual reproduction. Sex typically involves two genetically divergent partners of opposite sex or mating type. Specialized cells 2n in adult gonads undergo meiosis to form haploid n gametes sperm or egg. Haploid gametes fuse to form the diploid zygote 2n , which undergoes repeated mitosis, differentiation, and growth to become multicellular organisms juvenile, 2n.

The mature organism is diploid; gametes are the only haploid cells. Saccharomyces cerevisiae can grow as haploid yeasts by asexual budding. The mature organism can be diploid or haploid; gametes are haploid cells. Many fungi maintain haploid life cycles, and only become diploid following fertilization.

Sex is initiated by secretion of peptide pheromones or chemical compounds and following mate recognition cells and nuclei fuse, and meiosis and sporulation occur. Mating-type switching evolved independently in S. Homothallism and heterothallism are present in all phyla of the fungal kingdom, and transitions from one pattern to the other are common, and can even coexist in the same species. In basidiomycetes, both bipolar and tetrapolar mating systems exist.

In addition, recent studies suggest the existence of mating systems that represent possible transitions between the two. In fungi, both biparental and uniparental mitochondrial inheritance occur. Current evidence suggests the underlying mechanisms of uniparental mitochondrial inheritance differ in different species.

Sex-related silencing mechanisms function during fungal sexual development, suppressing transposon activity and defending genome integrity. What are the driving forces for the evolution of MAT and the genes resident therein? We thank Cecelia A. National Center for Biotechnology Information , U. Annu Rev Genet. Author manuscript; available in PMC Mar Author information Copyright and License information Disclaimer. Copyright notice. The publisher's final edited version of this article is available at Annu Rev Genet.

See other articles in PMC that cite the published article. Abstract Sexual reproduction enables genetic exchange in eukaryotic organisms as diverse as fungi, animals, plants, and ciliates.

Open in a separate window. Figure 1. Figure 2. Figure 3. Figure 4. Aneuploidy generated during mating enables genotypic plasticity and rapid adaption.

Is the ancestral sexual state homothallic or heterothallic? What is the nuclease that induces mating-type switching in S. Are there mating systems that differ from both bipolar and tetrapolar mating systems? What are the mechanisms underlying uniparental mitochondrial inheritance? How is sex-induced RNAi-dependent silencing initiated? Identification and linkage mapping of the genes for the putative homeodomain protein hox1 and the putative pheromone receptor protein homologue rcb1 in a bipolar basidiomycete, Pholiota nameko.

Alby K, Bennett RJ. Interspecies pheromone signaling promotes biofilm formation and same-sex mating in Candida albicans. Homothallic and heterothallic mating in the opportunistic pathogen Candida albicans. Infrequent genetic exchange and recombination in the mitochondrial genome of Candida albicans.

The small RNA profile during Drosophila melanogaster development. What is a bona fide mating-type gene? Internuclear complementation of mat mutants in Podospora anserina.

Mutations in mating-type genes of the heterothallic fungus Podospora anserina lead to self-fertility. Bakkeren G, Kronstad JW. Linkage of mating-type loci distinguishes bipolar from tetrapolar mating in basidiomycetous smut fungi. Inheritance and recombination of mitochondrial genomes in plants, fungi and animals. New Phytol. Genes Dev. Completion of a parasexual cycle in Candida albicans by induced chromosome loss in tetraploid strains. EMBO J. Birky CW. The inheritance of genes in mitochondria and chloroplasts: laws, mechanisms, and models.

The Neurospora crassa pheromone precursor genes are regulated by the mating type locus and the circadian clock. Intercalation of a new tier of transcription regulation into an ancient circuit. Cellular and Molecular Biology of Filamentous Fungi. Aneuploid chromosomes are highly unstable during DNA transformation of Candida albicans. Bourc'his D, Bestor TH. Meiotic catastrophe and retrotransposon reactivation in male germ cells lacking Dnmt3L. Evolution of the MAT locus and its Ho endonuclease in yeast species.

Evolution of pathogenicity and sexual reproduction in eight Candida genomes. Recurrence of repeat-induced point mutation RIP in Neurospora crassa. The origin of multiple mating types in the model mushrooms Coprinopsis cinerea and Schizophyllum commune. See Ref. Cerda-Olmedo E. Phycomyces and the biology of light and color. FEMS Microbiol. Nongenic, bidirectional transcription precedes and may promote developmental DNA deletion in Tetrahymena thermophila.

Chang S, Staben C. Directed replacement of mt A by mt a-1 effects a mating type switch in Neurospora crassa. Loading recommendations for you. Recommendations for you. Adding to Cart Added to Cart. Not Added. Item is in your Cart. View Cart Proceed to checkout. Sorry, we're having trouble showing recommendations right now. Please try again later. Your Amazon Music account is currently associated with a different marketplace.

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Metin B(1), Findley K, Heitman J. The C. neoformans MAT locus is unusually large (> kb), contains >20 genes, and enhances virulence. Li TV LE S T L VER Li T TL F S I Lv ER 39 MONMOUTH K C8 1 * 2 K 12 3, 7 MAD ISON TOWNSHIP MAT AWAN MIDDLE SEx K 12

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