The budding yeast, Saccharomyces cerevisiae, is a leading system in genetics, genomics and molecular biology and is becoming a powerful tool to illuminate ecological and evolutionary principles. However, little is known of the ecology and population structure of this species in nature. Here, we present a field survey of this yeast at an unprecedented scale and have performed population genetics analysis of Chinese wild isolates with different ecological and geographical origins. We also included a set of worldwide isolates that represent the maximum genetic variation of S. cerevisiae documented so far. We clearly show that S. cerevisiae is a ubiquitous species in nature, occurring in highly diversified substrates from human-associated environments as well as habitats remote from human activity. Chinese isolates of S. cerevisiae exhibited strong population structure with nearly double the combined genetic variation of isolates from the rest of the world. We identified eight new distinct wild lineages (CHN I-VIII) from a set of 99 characterized Chinese isolates. Isolates from primeval forests occur in ancient and significantly diverged basal lineages, while those from human-associated environments generally cluster in less differentiated domestic or mosaic groups. Basal lineages from primeval forests are usually inbred, exhibit lineage-specific karyotypes and are partially reproductively isolated. Our results suggest that greatly diverged populations of wild S. cerevisiae exist independently of and predate domesticated isolates. We find that China harbours a reservoir of natural genetic variation of S. cerevisiae and perhaps gives an indication of the origin of the species.
Three ascomycetous yeast strains, H-6 T , ZX-15 and ZX-20, isolated from the bark of two tree species of the family Fagaceae collected from different regions of China, formed unconjugated and persistent asci containing two to four globose ascospores. 26S rDNA D1/D2 domain and internal transcribed spacer (ITS) region (including 5.8S rDNA) sequence analysis showed that they were closely related to the currently accepted Saccharomyces species with strong support. Comparisons of the rDNA sequences, electrophoretic karyotypes and physiological characters indicated that the three strains represent a novel species in the genus Saccharomyces. The name Saccharomyces arboricolus sp. nov. was proposed for the novel species, with H-6 T (5AS 2.3317 T 5CBS 10644 T ) isolated from the bark of Quercus fabri as the type strain.Seven species are currently included in the genus Saccharomyces Meyen ex Reess as redefined recently by Kurtzman (2003), based on multigene sequence analysis (Kurtzman & Robnett, 2003). During the investigation of the diversity of ascomycetous yeasts associated with plant materials from China, three strains, H-6 T , ZX-15 and ZX-20, isolated from the bark of broadleaf trees, were found to represent a novel species of the genus Saccharomyces by physiological characterization, rRNA gene sequencing and electrophoretic karyotyping.Yeast strains living in bark were isolated by using the enrichment method. Pieces of bark with or without exudates from independent trees were cut with a sterile scalpel and placed into sterile plastic tubes containing 3 ml liquid enrichment medium. The medium was made according to Sniegowski et al. (2002) with minor modifications, containing (w/v) 0.3 % yeast extract, 0.3 % malt extract, 0.5 % peptone, 0.5 % sucrose, 0.5 % galactose, 7.6 % (v/v) ethanol, 200 mg chloramphenicol ml 21 and 1 ml 1 M HCl per litre. The cultures were incubated at room temperature without shaking for 7 to 14 days. Aliquots (100 ml) of the 10 22 to 10 24 diluted enrichment culture was spread on plate medium made by adding 2 % agar to the liquid enrichment medium. After 2 days incubation at room temperature or 25 u C, colonies with different morphological characters were transferred into malt extract agar slants for further purification and examination.
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