Saccharomyces paradoxus
Scientific classification
Domain:	Eukaryota
Kingdom:	Fungi
Division:	Ascomycota
Class:	Saccharomycetes
Order:	Saccharomycetales
Family:	Saccharomycetaceae
Genus:	Saccharomyces
Species:	S. paradoxus
Binomial name
Saccharomyces paradoxus Bachinskaya, A.A., 1914

Saccharomyces paradoxus is a wild yeast and the closest known species to the baker's yeast Saccharomyces cerevisiae. It is used in population genomics and phylogenetic studies to compare its wild characteristics to laboratory yeasts.^[1]

Ecology

Saccharomyces paradoxus is mostly isolated from deciduous trees (oak, maple, birch), and in some rare occasions on insects and fruits.^[2][3][4][5] It is often found in sympatry with other Saccharomyces species.^[6][7][8] Like Saccharomyces cerevisiae, it has a worldwide distribution and it is mesophilic, which limits its natural distribution to low latitudes. However, Saccharomyces paradoxus typically grows at lower temperatures than Saccharomyces cerevisiae, resulting in a slight shift in its distribution toward cooler regions, like British islands and Eastern Canada.^[2][8][9]

Biogeography

Saccharomyces paradoxus worldwide distribution. Populations are represented in different colours. Green asterisks indicate recent introductions of the European type.

Phylogeny of main Saccharomyces paradoxus populations.^[10]

Unlike most other Saccharomyces species, there is no evidence that Saccharomyces paradoxus has been domesticated by humans.^[3][11] Accordingly, its biogeography is mostly marked by natural processes like limited migration,^[3] glacial refugia^[12] and adaptation to climate.^[10] At least four genetically and phenotypically distinct populations of Saccharomyces paradoxus have been identified, corresponding to main geographical divisions: Europe (including West Siberia), Far East Asia (Japan, Eastern Siberia), North America (North American East and West coasts, Great Lakes region) and North-East America (Gaspé Peninsula, Saint Lawrence Valley and Appalaches), respectively.^{[3][9][10][11]} Representative strains of these populations exhibit partial post-zygotic isolation.^[12][13] A fifth population is represented by a singleton isolate from Hawaii.^[3][11] Some strains from the European population are found in North America and New Zealand and likely result from recent colonization events.^[14][15] Two isolates from South America, described as Saccharomyces cariocanus,^[16] are genetically indistinguishable but exhibit post-zygotic isolation when crossed to strains from the American population, due to chromosomal translocations.^[13]

Reproduction

Saccharomyces paradoxus is naturally homothallic, and is mostly found as diploid in the environment. Reproduction is mostly clonal and 99% of sexual reproduction occurs between spores from the same ascus.^[17] This purges recessive deleterious mutations that accumulated during clonal expansion, in a process known as "genome renewal".^[18][19] Post-zygotic isolation between strains of Saccharomyces paradoxus is commonly observed and could be either due to genetic divergence between populations or to chromosomal changes within populations.^[12][13]

Like in other Saccharomyces species, heterothallism can be restored using standard genetic tools, to obtain stable haploid strains for experimental purposes.

References