The mating of yeast, also known as yeast sexual reproduction, is an important biological process integral to the lifecycle of various yeast species, facilitating genetic diversity and adaptation. Yeasts such as Saccharomyces cerevisiae (bakers yeast) are simple single-celled eukaryotes with both a diploid and haploid mode of existence. Mating of yeast only occurs between haploids, which can be either the a or α (alpha) mating type and thus display simple sexual differentiation. [1] Mating type is determined by a single locus, MAT, which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of genetic recombination, haploid yeast can switch mating type as often as every cell cycle.
Yeasts can stably exist as either a diploid or a haploid. Both haploid and diploid yeast cells reproduce by mitosis, with daughter cells budding from mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type (an a cell can only mate with an α cell, and vice versa) to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions such as nutrient depletion, can undergo meiosis to produce four haploid spores: two a spores and two α spores.[ citation needed ]
a cells produce 'a-factor', a mating pheromone which signals the presence of an a cell to neighbouring α cells. [2] a cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo, due to its distinctive shape resembling the Al Capp cartoon character Shmoo) towards the source of α-factor. [3] Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone. [4] The response of haploid cells only to the mating pheromones of the opposite mating type allows mating between a and α cells, but not between cells of the same mating type. [5]
These phenotypic differences between a and α cells are due to a different set of genes being actively transcribed and repressed in cells of the two mating types. a cells activate genes which produce a-factor and produce a cell surface receptor (Ste2) which binds to α-factor and triggers signaling within the cell. [6] [7] a cells also repress the genes associated with being an α cell. Similarly, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to a-factor, and α cells repress the genes associated with being an a cell. [8]
The different sets of transcriptional repression and activation which characterize a and α cells are caused by the presence of one of two alleles of a mating-type locus called MAT: MATa or MATα located on chromosome III. [9] The MAT locus is usually divided into five regions (W, X, Y, Z1, and Z2) based on the sequences shared among the two mating types. [10] The difference lies in the Y region (Ya and Yα), which contains most of the genes and promoters. [6]
The MATa allele of MAT encodes a gene called a1, which in haploids direct the transcription of the a-specific transcriptional program (such as expressing STE2 and repressing STE3) that defines an a cell. The MATα allele of MAT encodes the α1 and α2 genes, which in haploids direct the transcription of the α-specific transcriptional program (such as expressing STE3, repressing STE2, producing prepro-α-factor) which causes the cell to be an α cell. [6] S. cerevisiae has an a2 gene with no apparent function that shares much of its sequence with α2; however, other yeasts like Candida albicans do have a functional and distinct MATa2 gene. [9] [5]
Haploid cells are one of two mating types (a or α), and respond to the mating pheromone produced by haploid cells of the opposite mating type, and can mate with cells of the opposite mating type. [3] Haploid cells cannot undergo meiosis. [11] Diploid cells do not produce or respond to either mating pheromone and do not mate, but can undergo meiosis to produce four haploid cells. [12]
Like the differences between haploid a and α cells, different patterns of gene repression and activation are responsible for the phenotypic differences between haploid and diploid cells. [13] In addition to the specific a and α transcriptional patterns, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes (such as HO) and represses diploid-specific genes (such as IME1). [14] Similarly, diploid cells activate diploid-specific genes and repress haploid-specific genes. [15]
The different gene expression patterns of haploids and diploids are again due to the MAT locus. Haploid cells only contain one copy of each of the 16 chromosomes and thus can only possess one allele of MAT (either MATa or MATα), which determines their mating type. [16] Diploid cells result from the mating of an a cell and an α cell, and thus possess 32 chromosomes (in 16 pairs), including one chromosome bearing the MATa allele and another chromosome bearing the MATα allele. [17] The combination of the information encoded by the MATa allele (the a1 gene) and the MATα allele (the α1 and α2 genes) triggers the diploid transcriptional program. [18] Similarly, the presence of only a single allele of MAT, whether it is MATa or MATα, triggers the haploid transcriptional program.[ citation needed ]
The alleles present at the MAT locus are sufficient to program the mating behaviour of the cell. For example, using genetic manipulations, a MATa allele can be added to a MATα haploid cell. [19] Despite having a haploid complement of chromosomes, the cell now has both the MATa and MATα alleles, and will behave like a diploid cell: it will not produce or respond to mating pheromones, and when starved will attempt to undergo meiosis, with fatal results. [20] Similarly, deletion of one copy of the MAT locus in a diploid cell, leaving only a single MATa or MATα allele, will cause a cell with a diploid complement of chromosomes to behave like a haploid cell. [21] [22]
Mating in yeast is stimulated by the presence of a pheromone which binds to either the Ste2 receptor (in a-cells) or the Ste3 receptor (in α-cells). [23] [24] The binding of this pheromone then leads to the activation of a heterotrimeric G protein. [25] The dimeric portion of this G-protein recruits Ste5 (and its related MAPK cascade components) to the membrane, and ultimately results in the phosphorylation of Fus3. [26]
The switching mechanism arises as a result of competition between the Fus3 protein (a MAPK protein) and the phosphatase Ptc1. [27] These proteins both attempt to control the 4 phosphorylation sites of Ste5, a scaffold protein with Fus3 attempting to phosphorylate the phosphosites, and Ptc1 attempting to dephosphorylate them. [28]
Presence of α-factor induces recruitment of Ptc1 to Ste5 via a 4 amino acid motif located within the Ste5 phosphosites. [29] Ptc1 then dephosphorylates Ste5, ultimately resulting in the dissociation of the Fus3-Ste5 complex. [30] Fus3 dissociates in a switch-like manner, dependent on the phosphorylation state of the 4 phosphosites. [31] All 4 phosphosites must be dephosphorylated in order for Fus3 to dissociate. [32] [33] Fus3's ability to compete with Ptc1 decreases as Ptc1 is recruited, and thus the rate of dephosphorylation increases with the presence of pheromone. [34]
Kss1, a homologue of Fus3, does not affect shmooing, and does not contribute to the switch-like mating decision. [35] [36]
In yeast, mating as well as the production of shmoos occur via an all-or-none, switch-like mechanism. [37] This switch-like mechanism allows yeast cells to avoid making an unwise commitment to a highly demanding procedure. [38] However, not only does the mating decision need to be conservative (in order to avoid wasting energy), but it must also be fast to avoid losing the potential mate. [39]
The decision to mate is extremely sensitive. There are 3 ways in which this ultrasensitivity is maintained:
a and α yeast share the same mating response pathway, with the only difference being the type of receptor each mating type possesses. [42] Thus the above description, given for a-type yeast stimulated with α-factor, works equally well for α-type yeast stimulated with a-factor. [43] [44]
Wild type haploid yeast are capable of switching mating type between a and α. [45] Consequently, even if a single haploid cell of a given mating type founds a colony of yeast, mating type switching will cause cells of both a and α mating types to be present in the population. [46] [47] Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony. [48] The vast majority of yeast strains studied in laboratories have been altered such that they cannot perform mating type switching (by deletion of the HO gene; [49] see below); this allows the stable propagation of haploid yeast, as haploid cells of the a mating type will remain a cells (and α cells will remain α cells), and will not form diploids.
Haploid yeast switch mating type by replacing the information present at the MAT locus. [50] For example, an a cell will switch to an α cell by replacing the MATa allele with the MATα allele. [51] This replacement of one allele of MAT for the other is possible because yeast cells carry an additional silenced copy of both the MATa and MATα alleles: the HML (homothallic mating left) locus typically carries a silenced copy of the MATα allele, and the HMR (homothallic mating right) locus typically carries a silenced copy of the MATa allele. [6] The silent HML and HMR loci are often referred to as the silent mating cassettes, as the information present there is 'read into' the active MAT locus. [52]
These additional copies of the mating type information do not interfere with the function of whatever allele is present at the MAT locus because they are not expressed, so a haploid cell with the MATa allele present at the active MAT locus is still an a cell, despite also having a (silenced) copy of the MATα allele present at HML. [53] Only the allele present at the active MAT locus is transcribed, and thus only the allele present at MAT will influence cell behaviour. [5] Hidden mating type loci are epigenetically silenced by SIR proteins, which form a heterochromatin scaffold that prevents transcription from the silent mating cassettes. [54]
The process of mating type switching is a gene conversion event initiated by the HO gene. [55] The HO gene is a tightly regulated haploid-specific gene that is only activated in haploid cells during the G1 phase of the cell cycle. [56] The protein encoded by the HO gene is a DNA endonuclease, which physically cleaves DNA, but only at the MAT locus (due to the DNA sequence specificity of the HO endonuclease). [57]
Once HO cuts the DNA at MAT, exonucleases are attracted to the cut DNA ends and begin to degrade the DNA on both sides of the cut site. [58] This DNA degradation by exonucleases eliminates the DNA which encoded the MAT allele; however, the resulting gap in the DNA is repaired by copying in the genetic information present at either HML or HMR, filling in a new allele of either the MATa or MATα gene. Thus, the silenced alleles of MATa and MATα present at HML and HMR serve as a source of genetic information to repair the HO-induced DNA damage at the active MAT locus. [6]
The repair of the MAT locus after cutting by the HO endonuclease almost always results in a mating type switch. [6] [57] When an a cell cuts the MATa allele present at the MAT locus, the cut at MAT will almost always be repaired by copying the information present at HML. [5] This results in MAT being repaired to the MATα allele, switching the mating type of the cell from a to α. [59] Similarly, an α cell which has its MATα allele cut by the HO endonuclease will almost always repair the damage using the information present at HMR, copying the MATa gene to the MAT locus and switching the mating type of α cell to a. [60]
This is the result of the action of a recombination enhancer (RE) located on the left arm of chromosome III. [61] Deletion of this region causes a cells to incorrectly repair using HMR. [62] In a cells, Mcm1 binds to the RE and promotes recombination of the HML region. [63] In α cells, the α2 factor binds at the RE and establishes a repressive domain over RE such that recombination is unlikely to occur. [64] An innate bias means that the default behaviour is repair from HMR. The exact mechanisms of these interactions are still under investigation. [65]
Ruderfer et al. [66] analyzed the ancestry of natural S. cerevisiae strains and concluded that matings involving out-crossing occur only about once every 50,000 cell divisions. Thus it appears that, in nature, mating is most often between closely related yeast cells. Mating occurs when haploid cells of opposite mating type MATa and MATα come into contact. Ruderfer et al. [66] pointed out that such contacts are frequent between closely related yeast cells for two reasons. The first is that cells of opposite mating type are present together in the same ascus, the sac that contains the cells directly produced by a single meiosis, and these cells can mate with each other. The second reason is that haploid cells of one mating type, upon cell division, often produce cells of the opposite mating type with which they can mate (see section "Mating type switching", above). The relative rarity in nature of meiotic events that result from out-crossing appears to be inconsistent with the idea that production of genetic variation is the primary selective force maintaining mating capability in this organism. However this finding is consistent with the alternative idea that the primary selective force maintaining mating capability is enhanced recombinational repair of DNA damage during meiosis, [67] since this benefit is realized during each meiosis subsequent to a mating, whether or not out-crossing occurs.
Schizosaccharomyces pombe is a facultative sexual yeast that can undergo mating when nutrients are limiting. [68] Exposure of S. pombe to hydrogen peroxide, an agent that causes oxidative stress leading to oxidative DNA damage, strongly induces mating, meiosis, and formation of meiotic spores. [69] This finding suggests that meiosis, and particularly meiotic recombination, may be an adaptation for repairing DNA damage. [70] The overall structure of the MAT locus is similar to that in S. cerevisiae. The mating-type switching system is similar, but has evolved independently. [5]
Cryptococcus neoformans is a basidiomycetous fungus that grows as a budding yeast in culture and in an infected host. C. neoformans causes life-threatening meningoencephalitis in immune compromised patients. It undergoes a filamentous transition during the sexual cycle to produce spores, the suspected infectious agent. The vast majority of environmental and clinical isolates of C. neoformans are mating type α. Filaments ordinarily have haploid nuclei, but these can undergo a process of diploidization (perhaps by endoduplication or stimulated nuclear fusion) to form diploid cells termed blastospores. [71] The diploid nuclei of blastospores can then undergo meiosis, including recombination, to form haploid basidiospores that can then be dispersed. [71] This process is referred to as monokaryotic fruiting. Required for this process is a gene designated dmc1, a conserved homologue of genes RecA in bacteria, and RAD51 in eukaryotes. Dmc1 mediates homologous chromosome pairing during meiosis and repair of double-strand breaks in DNA (see Meiosis; also Michod et al. [72] ). Lin et al. suggested that one benefit of meiosis in C. neoformans could be to promote DNA repair in a DNA damaging environment that could include the defensive responses of the infected host. [71]
Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome again.
Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.
Saccharomyces cerevisiae is a species of yeast. The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been originally isolated from the skin of grapes. It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism which causes many common types of fermentation. S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. It reproduces by budding.
Schizosaccharomyces pombe, also called "fission yeast", is a species of yeast used in traditional brewing and as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically measure 3 to 4 micrometres in diameter and 7 to 14 micrometres in length. Its genome, which is approximately 14.1 million base pairs, is estimated to contain 4,970 protein-coding genes and at least 450 non-coding RNAs.
In biology, mating is the pairing of either opposite-sex or hermaphroditic organisms for the purposes of sexual reproduction. Fertilization is the fusion of two gametes. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization. Mating may also lead to external fertilization, as seen in amphibians, fishes and plants. For most species, mating is between two individuals of opposite sexes. However, for some hermaphroditic species, copulation is not required because the parent organism is capable of self-fertilization (autogamy); for example, banana slugs.
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronuclei. Once the cell membranes, cytoplasm, and pronuclei fuse, the resulting single cell is diploid, containing two copies of the genome. This diploid cell, called a zygote or zygospore can then enter meiosis, or continue to divide by mitosis. Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.
Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable of sexual reproduction from a single organism.
A meiocyte is a type of cell that differentiates into a gamete through the process of meiosis. Through meiosis, the diploid meiocyte divides into four genetically different haploid gametes. The control of the meiocyte through the meiotic cell cycle varies between different groups of organisms.
Fungi are a diverse group of organisms that employ a huge variety of reproductive strategies, ranging from fully asexual to almost exclusively sexual species. Most species can reproduce both sexually and asexually, alternating between haploid and diploid forms. This contrasts with most multicellular eukaryotes such as mammals, where the adults are usually diploid and produce haploid gametes which combine to form the next generation. In fungi, both haploid and diploid forms can reproduce – haploid individuals can undergo asexual reproduction while diploid forms can produce gametes that combine to give rise to the next generation.
Mitotic recombination is a type of genetic recombination that may occur in somatic cells during their preparation for mitosis in both sexual and asexual organisms. In asexual organisms, the study of mitotic recombination is one way to understand genetic linkage because it is the only source of recombination within an individual. Additionally, mitotic recombination can result in the expression of recessive alleles in an otherwise heterozygous individual. This expression has important implications for the study of tumorigenesis and lethal recessive alleles. Mitotic homologous recombination occurs mainly between sister chromatids subsequent to replication. Inter-sister homologous recombination is ordinarily genetically silent. During mitosis the incidence of recombination between non-sister homologous chromatids is only about 1% of that between sister chromatids.
Sister chromatid exchange (SCE) is the exchange of genetic material between two identical sister chromatids.
Microbial genetics is a subject area within microbiology and genetic engineering. Microbial genetics studies microorganisms for different purposes. The microorganisms that are observed are bacteria and archaea. Some fungi and protozoa are also subjects used to study in this field. The studies of microorganisms involve studies of genotype and expression system. Genotypes are the inherited compositions of an organism. Genetic Engineering is a field of work and study within microbial genetics. The usage of recombinant DNA technology is a process of this work. The process involves creating recombinant DNA molecules through manipulating a DNA sequence. That DNA created is then in contact with a host organism. Cloning is also an example of genetic engineering.
Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct sexes. They also occur in multicellular organisms such as fungi.
The mating-type locus is a specialized region in the genomes of some yeast and other fungi, usually organized into heterochromatin and possessing unique histone methylation patterns. The genes in this region regulate the mating type of the organism and therefore determine key events in its life cycle, such as whether it will reproduce sexually or asexually. In fission yeast such as S. pombe, the formation and maintenance of the heterochromatin organization is regulated by RNA-induced transcriptional silencing, a form of RNA interference responsible for genomic maintenance in many organisms. Mating type regions have also been well studied in budding yeast S. cerevisiae and in the fungus Neurospora crassa.
The tetrad is the four spores produced after meiosis of a yeast or other Ascomycota, Chlamydomonas or other alga, or a plant. After parent haploids mate, they produce diploids. Under appropriate environmental conditions, diploids sporulate and undergo meiosis. The meiotic products, spores, remain packaged in the parental cell body to produce the tetrad.
Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.
Homothallic refers to the possession, within a single organism, of the resources to reproduce sexually; i.e., having male and female reproductive structures on the same thallus. The opposite sexual functions are performed by different cells of a single mycelium.
The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.
SilentInformationRegulator (SIR) proteins are involved in regulating gene expression. SIR proteins organize heterochromatin near telomeres, ribosomal DNA (rDNA), and at silent loci including hidden mating type loci in yeast. The SIR family of genes encodes catalytic and non-catalytic proteins that are involved in de-acetylation of histone tails and the subsequent condensation of chromatin around a SIR protein scaffold. Some SIR family members are conserved from yeast to humans.
Biparental inheritance is a type of biological inheritance where the progeny inherits a maternal and a paternal allele for one gene. It is one of the criteria for Mendelian inheritance. Sexual reproduction, where offspring result from the fusion of gametes from two parents, is the most common form of biparental inheritance. While less common, cases of biparental inheritance in extranuclear genes have been documented, such as biparental inheritance of mitochondrial DNA, or chloroplast DNA in plants. Biparental inheritance of nuclear DNA by way of sexual reproduction can allow for new combinations of alleles from each contributing parent. The production of gametes through meiosis can sometimes include recombination, or crossing-over, which is a possibility for novel combinations of alleles.