Evolutionary ecology and long-term maintenance of phenotypic discontinuities in Saccharomyces kudriavzevii

SAMPAIO JP,; VALERIO, E.; CORREIA, C.; LIBKIND, DIEGO; IMANISHI, Y.; HITTINGER, C; GONCALVES, P

Jornada; Jornadas de Biología de Leveduras, “Prof. Nicolau van Uden”; 2009

In spite of the impressive wealth of knowledge that has been gathered on multiple aspects of the biology of Saccharomyces yeasts, with emphasis on S. cerevisiae, little is known on two important topics: the ecology of natural populations and the evolutionary processes that shaped present-day species. In a study aiming at identifying Saccharomyces habitats and detecting natural populations, we employed low temperature incubations at 10 C, which is contrary to standard practice but allowed us to isolate an unprecedented variety of Saccharomyces species from oak bark and soil samples in Europe (Portugal and Germany), North America (Canada - British Columbia and Ontario), South America (Argentina) and Asia (Japan). S. kudriavzevii, a species for which up to now only four isolates from Japan were known, was repeatedly isolated in Portugal and in Japan in the enrichments carried out at 10 C but not in those carried out at 30 C. Other species also exhibited temperature preferences. The results of this study suggest that Saccharomyces habitats are simultaneously occupied by more than one Saccharomyces species and that niche overlapping seems to be avoided by different temperature adaptations of the intervening species. In addition, the new isolates provided new insights into geographic distribution of species and populations. For example, the newly uncovered European population of S. kudriavzevii exhibits a moderate level of genetic divergence to the Far East strains (intermediate between S. paradoxus and S. cerevisiae). However, a notable difference concerning galactose utilization was found. While Japanese strains do not possess a single functional gene in the GAL pathway and are therefore unable to utilize this sugar, the European counterparts maintain functional genes. This marked polymorphism is in sharp contrast with unequivocal evidence for recent gene flow between the two populations. This divergence is unique because it involves a network of six genes scattered in different chromosomes. Moreover this polymorphism seems to have been maintained in two alternate states almost since the divergence of the S. kudriavzevii lineage and we suggest that it represents a new form of balancing selection with yet unknown ecological implications.