Mating of yeast
The mating of yeast, also known as yeast sexual reproduction, is a biological process that promotes genetic diversity and adaptation in yeast species. Yeast species, such as Saccharomyces cerevisiae, are single-celled eukaryotes that can exist as either haploid cells, which contain a single set of chromosomes, or diploid cells, which contain two sets of chromosomes. Haploid yeast cells come in two mating types, a and α, each producing specific pheromones to identify and interact with the opposite type, thus displaying simple sexual differentiation. A yeast cell's mating type is determined by a specific genetic locus known as MAT, which governs its mating behaviour. Haploid yeast can switch mating types through a form of genetic recombination, allowing them to change mating type as often as every cell cycle. When two haploid cells of opposite mating types encounter each other, they undergo a complex signaling process that leads to cell fusion and the formation of a diploid cell. Diploid cells can reproduce asexually, but under nutrient-limiting conditions, they undergo meiosis to produce new haploid spores.
The differences between a and α cells, driven by specific gene expression patterns regulated by the MAT locus, are crucial for the mating process. Additionally, the decision to mate involves a highly sensitive and complex signaling pathway that includes pheromone detection and response mechanisms. In nature, yeast mating often occurs between closely related cells, although mating type switching and pheromone signaling allow for occasional outcrossing to enhance genetic variation. Certain yeast species have unique mating behaviors, demonstrating the diversity and adaptability of yeast reproductive strategies.
Mating types
Yeast cells can stably exist in either a diploid or a haploid form. Both haploid and diploid yeast cells reproduce by mitosis, in which daughter cells bud from mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions like nutrient depletion, can undergo meiosis to produce four haploid spores: two a spores and two α spores.Differences between a and α cells
a cells produce a-factor, a mating pheromone which signals the presence of an a cell to neighbouring α cells. a cells respond to α-factor, the α cell mating pheromone, by growing a projection towards the source of α-factor. Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone. The selective response of haploid cells 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.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 which binds to α-factor and triggers signaling within the cell. a cells also repress the genes associated with being an α cell. Conversely, α cells activate genes which produce α-factor and produce a cell surface receptor which binds and responds to a-factor, and α cells repress the genes associated with being an a cell.
''MAT'' locus
The different sets of transcriptional repression and activation, which characterize a and α cells, are caused by the presence of one of two alleles for a mating-type locus called MAT: MATa or MATα located on chromosome III. The MAT locus is usually divided into five regions based on the sequences shared among the two mating types. The difference lie in the Y region, which contains most of the genes and promoters.The MATa allele of MAT encodes a gene called a1, which directs the a-specific transcriptional program that defines an a haploid cell. The MATα allele of MAT encodes the α1 and α2 genes, which directs the α-specific transcriptional program that defines an α haploid cell. S. cerevisiae has an a2 gene with no apparent function that shares much of its sequence with α2; however, other yeast species like Candida albicans do have a functional and distinct MATa2 gene.
Differences between haploid and diploid cells
cells are one of two mating types and respond to the mating pheromone produced by haploid cells of the opposite mating type. Haploid cells cannot undergo meiosis. Diploid cells do not produce or respond to either mating pheromone and do not mate, but they can undergo meiosis to produce four haploid cells.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. In addition to the transcriptional patterns of a and α cells, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes and represses diploid-specific genes. Conversely, diploid cells activate diploid-specific genes and repress haploid-specific genes.
The different gene expression patterns of haploid and diploid cells are attributable to the MAT locus. Haploid cells only contain one copy of each of the 16 chromosomes and therefore only possess one MAT allele, which determines their mating type. Diploid cells result from the mating of an a cell and an α cell, and they possess 32 chromosomes, including one chromosome bearing the MATa allele and another chromosome bearing the MATα allele. The combination of the information encoded by the MATa allele and the MATα allele triggers the diploid transcriptional program. Conversely, the presence of only one MAT allele, either MATa or MATα, triggers the haploid transcriptional program.
Through genetic engineering, a MATa allele can be added to a MATα haploid cell, causing it to behave like a diploid cell. The cell will not produce or respond to mating pheromones, and when starved, the cell will unsuccessfully attempt to undergo meiosis with fatal results. Similarly, deletion of one copy of the MAT locus in a diploid cell, leaving either a MATa or MATα allele, will cause a diploid cell to behave like a haploid cell of the associated mating type.
a-like faker cells
α cells with inactivated α1 and α2 genes at the MAT locus will exhibit the mating behavior of a cells. When an a-like faker cell mates with an α cell, they form a diploid cell lacking an active copy of the a1 gene. As a result, these diploid cells cannot form the a1-α2 protein complex needed to repress haploid-specific genes. This diploid cell will act like a haploid α cell, producing α pheromones to mate with an a haploid cell, resulting in aneuploidy.Since α cells do not ordinarily mate with each other, the presence of a-like faker cells in a population of α cells can be detected in an a-like faker assay. This test exposes the MATα population, which lacks an active copy of the HIS3 gene, to a tester strain like YPH316 yeast, which lack a HIS1 gene, on YEPD agar. After transferring the pairs of yeast strains onto Sabouraud agar, only those that formed diploid cells by having a-like faker cells mate with the tester strain will be capable of synthesizing the amino acid histidine to survive. The extent of chromosome instability can be inferred from the proportion of surviving pairs since a-like faker cells naturally arise from damage to Chromosome III in yeast cells.
Decision to mate
Mating in yeast is stimulated by a cells' a-factor or α cells' α-factor pheromones binding the Ste3 receptor of α cells or Ste2 receptor of a cells, respectively, activating a heterotrimeric G protein. The dimeric portion of this G-protein recruits Ste5 and its MAPK cascade to the membrane, resulting in the phosphorylation of Fus3.The switching mechanism arises as a result of competition between the Fus3 protein and the phosphatase Ptc1. These proteins both attempt to control the four phosphorylation sites of Ste5, a scaffold protein, with Fus3 attempting to phosphorylate the phosphosites and Ptc1 attempting to dephosphorylate them.
Presence of α-factor induces recruitment of Ptc1 to Ste5 via a four-amino acid motif located within the Ste5 phosphosites. Ptc1 then dephosphorylates Ste5, resulting in the dissociation of the Fus3-Ste5 complex. Fus3 dissociates in a switch-like manner, dependent on the phosphorylation state of the four phosphosites. All four phosphosites must be dephosphorylated in order for Fus3 to dissociate. Fus3's ability to compete with Ptc1 decreases as Ptc1 is recruited, and thus the rate of dephosphorylation increases with the presence of pheromone.
Kss1, a homologue of Fus3, does not affect shmooing, and does not contribute to the switch-like mating decision.
In yeast, mating as well as the production of shmoos occur via an all-or-none, switch-like mechanism. This switch-like mechanism allows yeast cells to avoid making an unwise commitment to a highly demanding procedure. The decision to mate must balance being energy-conservative and fast enough to avoid losing the potential mate.
Yeast maintain an ultra-sensitivity to mating through:
- Multi-site phosphorylation – Fus3 only dissociates from Ste5 and becomes fully active when all four of the phosphosites are dephosphorylated. Even one phosphorylated site will result in immunity to α-factor.
- Two-stage binding – Fus3 and Ptc1 bind to separate docking sites on Ste5. Only after docking can they act on the phosphosites.
- Steric hindrance – competition between Fus3 and Ptc1 to control the four phosphosites on Ste3