This lab report is based on the broad perspective of yeast mating.
In this experiment, we approached samples of growing yeast on agarose gel prepared for mating. Through observations, and punnett square calculations we were able to predict the offspring’s genotype and phenotype. Common on plant leaves, soil, and many natural habitats, yeasts are also found on skin and in the intestinal tracts of animals. Yeasts are unicellular fungi unlike the complex cells of eukaryotes, it is able to be grown on a growth medium/defined media. The elegance of yeast genetics and ease of manipulation contributed it to become an influential tool in our science and research advancement. It’s most crucial role was in early genetics discovery when Dr. Winge, 1935, first demonstrated its alternation of haplophase and diplophase.
(Hall and Linder,1992). Yeast’s unique form of life cycle gives heterothallic strains the ability to be stable as haploid (one set of 17 chromosomes), or diploid (two sets of the 17 chromosomes), whereas homothallic strains are only stable as diploids ( Schneiter, 2004). This enables yeasts to reproduce in two methods, asexual budding, and mating of haploid cells of opposite- mating type. Budding, the most common method of vegetative reproduction in yeasts occurs in both haploid (axial- budding) and diploid (bipolar budding) yeast cells.
This is initiated when mother cells reach a certain size during DNA synthesis, and is followed by the weakening of cell walls and extrusion of cytoplasm (Wiley, 2012). This allows the new yeast cell to “bud” apart from the parent cell by utilizing the process of mitosis as its sole source of reproduction. In contrast to, yeasts can also undergo sexual matting.
Haploid yeast cells exhibit the conjunction of two haploid cellular phenotypes, the mating types a or . Conjugation is then preceded by the “shmoo” formation that generates specific surface projects as contact regions (Wiley, 2012). Shmoo formation requires and involves some reorganization of the cytoskeleton and some components such as the bud site proteins that have been previously outlined for budding. Similar to haploid cells, a/ are also capable to reproduce by a mitotic cell cycle, however, when cells of opposite mating types are cocultured, it must exit the cell cycle and participate in the mating process. It requires the cells to fuse with their plasma membranes to form a cytoplasmic, and nuclear fission resulting in a diploid set of chromosomes producing a a/ diploid zygote. But, unlike a or cells, a/cannot mate (Guthrie and Fink, 1991). Alternatively, the cells are induced to undergo meiosis and sporulation under starvation conditions or growth nonfermentable carbon sources (acetate or ethanol).
Following the meiotic division, the diploid mother cell differentiates into an ascus, containing four haploid ascospores (2a and 2). Under rich media, the spores germinate and mate once again to form diploids. (Wiley, 2012). In addition, yeast sporulation phenomenon is tightly coupled with meiosis and is perceived as an important model system in eukaryotic cell differentiation. The sporulating cell undergoes morphogenetic processes that are controlled by many factors.
During sporulation, the diploid cell undergoes meiosis yielding 4 haploid cells in a spore structure (Tannenbaum, 2008). Even though the cell undergoes meiosis, it is still considerably different from the typical mitotic divisions due to formation of the plasma membranes for four daughter cells (haploid spores) within the mother cell cytoplasm. From our experimental observations, we have predicted that the dominant allele would be R and the recessive allele being r. As a result, in the presence of homozygous RR and heterozygous Rr, they would both exhibit the dominant white in genotypic and phenotypic expressions. However, in absence of that dominant allele, homozygous rr would exhibit the recessive trait, red. Therefore, we hypothesised that the phenotypes of the mated yeast strains are 3:1, derived from the genotypic dominance of the allele R of the parental yeast strains that were predicted to have ratio of 1-RR:2-Rr:1-rr.