Saccharomyces paradoxus

Lore & Background

Saccharomyces paradoxus is mostly isolated from deciduous trees such as oak, maple, and birch, and in rare occasions from insects and fruits. It is often found in sympatry with other Saccharomyces species. Like Saccharomyces cerevisiae, it has a worldwide distribution and 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 the British islands and Eastern Canada.

Unlike most other Saccharomyces species, there is no evidence that Saccharomyces paradoxus has been domesticated by humans. Its biogeography is mostly marked by natural processes like limited migration, glacial refugia, and adaptation to climate. At least four genetically and phenotypically distinct populations have been identified, corresponding to 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). A fifth population is represented by a singleton isolate from Hawaii. Some strains from the European population are found in North America and New Zealand, likely resulting from recent colonization events. Two isolates from South America, described as Saccharomyces cariocanus, are genetically indistinguishable but exhibit post-zygotic isolation when crossed to strains from the American population, due to chromosomal translocations.

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. This purges recessive deleterious mutations that accumulated during clonal expansion, in a process known as 'genome renewal.' Post-zygotic isolation between strains is commonly observed and could be due to genetic divergence between populations or to chromosomal changes within populations. Like in other Saccharomyces species, heterothallism can be restored using standard genetic tools to obtain stable haploid strains for experimental purposes.

Reader's Guide

Saccharomyces paradoxus holds significance primarily as a wild counterpart to the domesticated Saccharomyces cerevisiae, serving as a model for population genomics and phylogenetic studies. Its lack of domestication allows researchers to examine natural evolutionary processes, including limited migration, glacial refugia, and climate adaptation, without human influence. The identification of at least four distinct populations—European, Far East Asian, North American, and North-East American—along with a singleton from Hawaii, provides a framework for studying genetic divergence and post-zygotic isolation. The observation that some European strains have colonized North America and New Zealand highlights recent natural dispersal events. Additionally, the existence of South American isolates (Saccharomyces cariocanus) that are genetically identical but reproductively isolated due to chromosomal translocations underscores the role of chromosomal changes in speciation. Its reproductive strategy, with 99% of sexual reproduction occurring between spores from the same ascus, facilitates genome renewal by purging recessive deleterious mutations. The ability to restore heterothallism using standard genetic tools makes it experimentally tractable. Overall, Saccharomyces paradoxus provides a natural baseline for understanding yeast evolution, population structure, and reproductive isolation, distinct from laboratory and domesticated strains.

Did You Know?

• Saccharomyces paradoxus is the closest known species to the baker's yeast Saccharomyces cerevisiae.
• It is mostly found as diploid in the environment and reproduces clonally, with 99% of sexual reproduction between spores from the same ascus.
• At least four genetically distinct populations have been identified, corresponding to Europe, Far East Asia, North America, and North-East America.