Domain: Eukaryota Kingdom: Fungi Subkingdom: Dikarya Phylum: Ascomycota Subphylum: Taphrinomycotina Class: Schizosaccharomycetes Order: Schizosaccharomycetales Family: Schizosaccharomycetaceae Genus: Schizosaccharomyces Species: cryophilus
Background & Description
Fission yeast are unicellular eukaryotes that undergo division by medial fission rather than budding.1 Normally, propagation is mitotic as haploids, but under nitrogen starvation, sexual union between two haploid cells of opposite mating-types occurs, thus deriving the name fission yeast. The first physical step in the process is cellular elongation and the formation of conjugation tubes towards pheromones secreted by opposite mating-type cells.2 Conjugation of two opposite mating-types creates a diploid zygote which will undergo meiosis to produce four haploid nuclei which become encapsulated by spore walls.3
Schizosaccharomyces cryophilus is specific fission yeast of the genus Schizosaccharomyces that grows at lower temperatures than other known fission yeasts. Morphologically Schizosaccharomyces cryophilus is very similar to Schizosaccharomyces octosporus, but displays unique differences phenotypically and genotypically.
Schizosaccharomyces cryophilus contains a deviation of the D1/D2 divergent domain of the LSU rRNA gene, the RNA subunit of Ribonuclease P (Rnase P), and the ITS elements from Schizosaccharomyces octosporus. 5,6,7,8,9, Genome sequencing between the two species revealed the following genetic differences in the aforementioned loci:
D1/D2 divergent domain of the LSU rRNA gene: 25 nucleotide substitutions and 3 indels
RNA subunit on RNase P: 15 nucleotide substitutions and 3 indels
ITS elements: ITS1: 95 nucleotide substitutions and 66 indels ITS2: 84 nucleotide substitutions and 51 indels
Restriction enzyme digests can also be used in conjunction with gel electrophoresis to confirm differing band patterns between S. cryophilus and S. octosporus.
Metabolism, Physical Structure, & Function 
Unlike Saccharomycotina, fission yeasts cannot use ethanol as a primary carbon source, suggesting independent evolution. The gene content between budding yeasts and fission yeasts indicates that the glyoxylate cycle, loss of glycogen biosynthesis, fewer glycolytic paralogs, loss of phosphoenolpyruvate carboxykinase, lack of expanded adh genes, and lack of transcriptional regulators of glucose repression are what prohibit fission yeasts from using ethanol as a primary carbon source unlike Saccharomycotina.10 The loss of the phosphoenolpyruvate carboxykinase and adh genes prevents the use of pyruvate for respiration, producing ethanol not as a consumable by-product, but as a waste product. The expression of ald genes in fission yeast indicates there is an alternative pathway for acetyl-coA production in fission yeast and that fission yeast are highly dependent upon glucose rather than ethanol for acetyl-coA manufacture.11
S. cryophilus growth occurs at the optimal temperature of 25°C, compared to the optimal temperature of 32°C for Schizosaccharomyces japonicas, Schizosaccharomyces pombe, and S. octosporus. Additionally, S. cryophilus cannot proliferate at temperatures higher than 25°C, while S. japonicas, S. pombe, and S. octosporus have a growth range of 18°C to 36°C.
Colonies of S. cryophilus on agar are cream-colored and almost butter-like in appearance, which contrasts to the smooth, rounded colonies of S. pombe and the foam-like colonies of S. octosporus. Other differences between S. cryophilus and S. octosporus include the ability to ferment D-glucose, maltose, and sucrose in S. cryophilus as in S. pombe, but not D-galactose, as well as the ability to hydrolyze urea in S. cryophilus and S. pombe.
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