A Microbial Biorealm page on the genus Saccharomyces paradoxus
Higher Order Taxa
Description and significance
Saccharomyces paradoxus is a yeast species which can be found living on the bark of deciduous trees worldwide. It is the closest relative of Saccharomyces cerevisiae, the most widely studied eukaryotic model organism. The two species diverged from a common ancestor species approximately 5 million years ago . Unlike its sister species, which has been used for brewing and baking for millenia, S. paradoxus has no domestic uses and consequently has been relatively unaffected by human interaction. Nevertheless, the two species are very similar, and S. paradoxus can easily be cultured in a laboratory setting in a manner similar to S. cerevisiae. The non-domesticated S. paradoxus therefore is an attractive model organism for the study of the population genetics and genomics of wild yeast .
In its haploid form, S. paradoxus has 16 linear chromosomes, with a total genome length of approximately 12 million base pairs. Analysis of the genome sequence found an anticipated 8,908 protein-encoding genes. The genome is highly conserved when compared to its sister species, S. cerevisiae. In coding regions, its genome shares 90% of its identity with the genome of S. cerevisiae; in intergenic regions, 80% .
Artificially-created diploid F1 hybrids of S. cerevisiae and S. paradoxus are able to survive in culture; however, the haploid gamete daughter cells of the hybrid are not viable .
Cell structure, metabolism & life cycle
S. paradoxus is almost phenotypically indistinguishable from its closely-related counterpart, S. cerevisiae, in nearly all aspects of morphology, metabolism, and life cycle. However, it is a phylogenetically distinct species, as haploid gametes of S. paradoxus and S. cerevisiae cannot combine into a viable diploid hybrid organism .
S. paradoxus is a unicellular fungus with a chitinous cell wall. Externally, it appears spherical or ellipsoid in shape, with a diameter of 1-5 micrometers, dependent upon life cycle phase. Internally, it possesses a membrane-bound nucleus, a cytoskeleton, and mitochondria, as is typical for any eukaryotic organism .
S. paradoxus is a chemoorganotrophic organism, preferring glucose as a carbon source (though it possesses enzymes to convert certain other sugars into glucose.) It is capable of aerobic respiration via glycolysis and the TCA cycle, or ethanolic fermentation of glucose directly .
S. paradoxus can exist in both haploid or diploid forms. In either state, asexual reproduction can occur through a mitotic process called budding, in which a daughter cell "bubbles out" from the mother cell. In the diploid state, sporulation can occur, a meiotic process in which a single diploid cell divides into four haploid spore. The spores themselves are capable of reproducing via budding in their haploid state, or fusing with other spores to form a diploid yeast cell . This is rare, however, as gamete fusion occurs only about once in every 1000 generatons; reproduction is primarily asexual .
Ecology (including pathogenesis)
S. paradoxus is found in exudates of oak tree bark, and in samples of oak-associated soil . It shares the same habitat with several other members of genus Saccharomyces (including S. cerevisiae,) all of which possess nearly identical metabolic requirements. It is theorized that competitive exclusion does not occur because each species possesses specialized glycolytic enzymes with differing optimal temperature ranges . S. paradoxus grows optimally around 27-30 °C, markedly lower than the preferred temperature of 37 °C for S. cerevisiae .
It is not a known pathogen, though S. cerevisiae is known to cause disease in humans in rare instances .
In a controlled study, the taste profile of Chardonnay wines fermented with wild strains of S. paradoxus compared favorably with a batch fermented with traditional brewer's yeast (S. cerevisiae), though S. cerevisiae was able to achieve a slightly higher terminal alcohol concentration .
 Naumov, GI; Naumova, ES; Sniegowski, PD. "Saccharomyces paradoxus and Saccharomyces cerevisiae are associated with exudates of North American oaks." Canadian Journal of Microbiology. 1998 Nov; 44(11):1045-50.
 Sweeney, JY; Kuehne, HA; Sniegowski, PD. "Sympatric natural Saccharomyces cerevisiae and S. paradoxus populations have different thermal growth profiles." FEMS Yeast Research. 2004 Jan; 4(4-5):521-5.
 Dickinson, JR; Schweizer, M. The Metabolism and Molecular Physiology of Saccharomyces cerevisiae. 2004. London: Taylor & Francis. Print.