8.2 Mating in budding yeast
The life cycle of the yeast Saccharomyces cerevisiae features an alternation of a haploid phase with a true diploid phase (Fig. 1), and in this respect differs from filamentous Ascomycota in which the growth phase after anastomosis is a heterokaryon.
|Fig. 1. Life cycle (left hand panel) and mating process of the yeast Saccharomyces cerevisiae. Yeast can reproduce asexually by budding. Haploid cells of different mating types fuse to form dumbbell-shaped zygotes, which can themselves bud to establish a diploid clone. Well-nourished diploid cells, which are exposed to starvation conditions, enter meiosis, forming a 4-spored ascus. Ascospores germinate by budding. In the laboratory, ascospores can be separated to form haploid clones but in nature ascospores usually mate immediately, so the haploid phase is greatly reduced. The right hand panel depicts pheromone interaction, agglutination and the mating process in a little more detail. Modified from Chapter 2 in Moore & Novak Frazer, 2002.|
There are two mating types: haploid yeast cells may be of mating type a, or α. Karyogamy (nuclear fusion) follows the fusion of cells of opposite mating type and then the next daughter cell that is budded off contains a diploid nucleus. Most natural yeast populations are diploid because the haploid meiotic products mate while they are still close together immediately after the meiosis is completed. Diploid cells reproduce vegetatively by mitosis and budding until particular environmental conditions (deficiency in nitrogen and carbohydrate, but well aerated and with acetate or other carbon sources which favour the glyoxylate shunt) induce sporulation. When that happens, the entire cell becomes an ascus mother cell; meiosis occurs and haploid ascospores are produced. Ascospore germination re-establishes the haploid phase, which is itself maintained by mitosis and budding if the spores are separated from one another (by laboratory experiment, or by some disturbance in nature) to prevent immediate mating.
Mating type factors of yeast specify peptide hormones; these are called pheromones (the term originally applied to mate-attracting hormones of insects and mammals) and there are both pheromone α- (alpha-) and a-factors (Fig. 2), and corresponding receptors specific for each pheromone.
|Fig. 2. Simplified chemical structures of yeast pheromones.|
Pheromones organise the mating process; they have no effect on cells of the same mating type or on diploids but their binding to pheromone receptors on the surface of cells of opposite mating type (Fig. 1) act through GTP binding proteins to alter metabolism and:
- cause recipient cells to produce an agglutinin that enables cells of opposite mating type to adhere;
- stop growth in the G1 stage of the cell cycle;
- change wall structure to alter the shape of the cell into elongated projections.
Fusion eventually occurs between the projections.
The mating process of S. cerevisiae is controlled by a complex genetic locus called MAT at which two linked genes are located (a1, a2 for mating type a and α1, α2 for mating type α (alpha)). The MATa locus encodes a1 and a2 polypeptides, the messengers for which are transcribed in opposite directions (Fig. 3), and MATα encodes polypeptides α1 and α2.
|Fig. 3. Functional domains in mating type factors of Saccharomyces cerevisiae. Region Y is the location of the mating type idiomorphs, which have very little homology with each other. Ya is 642 bp long, Yα is 747 bp long. Regions W, X, and Z1 and Z2 are homologous terminal regions. The arrows indicate direction of transcription and the legends beneath the arrows indicate functions of the gene products. In S. cerevisiae of mating type a, a general transcription activator is responsible for production of a-pheromone and the membrane-bound α-pheromone receptor. In a/α diploids, the MATa1/MATα2 heterodimer protein activates meiotic and sporulation functions, and represses haploid functions (turning off α-specific functions by repressing MATα1, a-specific functions being repressed by MATα2 alone). Modified from Chapter 2 in Moore & Novak Frazer, 2002.|
Heterozygosity at MAT is a sign of diploidy and eligibility to sporulate; even partial diploids carrying MATa/MATα will attempt to sporulate. In haploid cells, the α2 polypeptide represses transcription of a-factor in α-cells, whilst a1 represses α-specific genes in a-cells. The α1 protein activates transcription of genes coding for α -pheromone and the surface receptor for the a-factor. In a/α diploids, interaction occurs between a1 and α2 polypeptides to form a heterodimer, which represses genes specific for the haploid phases, including a gene called RME1, which itself suppresses meiosis and sporulation.
Updated December 17, 2016