The Biogeography of Wine Yeasts in New Zealand

Abstract

The study of biogeography for flora and fauna has been crucial to the development of the Theory of Evolution and our understanding of a variety of biological processes. While macroorganisms can be observed and identified by eye, it is only relatively recently that we have developed the molecular tools allowing accurate identification of microbial species present in the environment. This has lead to an increase in the number of studies attempting to find out whether microorganisms display similar biogeographical patterns as macroorganisms. The research presented here aims to investigate the biogeographical patterns of wine yeasts within three regions of New Zealand. Wine yeasts are unicellular eukaryotic microorganisms which are commonly found associated with vine- yards, wineries and wine fermentations. Humans have a long history of association with one species in particular, Saccharomyces cerevisiae, which carries out the fermentation process to completion to produce what we know of as wine. This species is also used in other industries such as baking, brewing, distilling and as a supermodel laboratory organism. Spontaneous wine fermentations are conducted by a succession of yeast species, first non-Saccharomyces species followed by S. cerevisiae which dominates the later stages of fermentations through the production of ethanol, heat and carbon dioxide. All of the species in the succession can influence the final sensory properties of the wine produced. Previous studies on the presence and distributions of wine yeasts throughout regions, or globally, have mainly focussed on identifying which yeasts are present with little concern for objectively identifying patterns of biogeography. The research presented in this thesis builds on top of a recent study which found that New Zealand contains a unique population of S. cerevisiae compared with the rest of the world and that this population is subdivided in the local environment and dispersed locally by insects. This prompted the question of whether New Zealand contains distinct populations of both non-Saccharomyces yeast species and S. cerevisiae at the regional scale. The null hypotheses explicitly tested in this thesis are that: 1. There are no differences in the community structure of yeast species isolated from wine grapes between three regions in New Zealand. 2. There is no population structure in S. cerevisiae yeasts found in spontaneous fermentations from three regions in New Zealand. Extensive sampling of grapes from the three regions yielded 2,304 non-Saccharomyces yeasts which were grown using culture-based methods and identified to the species level. 384 S. cerevisiae isolates from fermentations were also grown using culture-based methods and their diversity and relatedness was determined using microsatellite analysis. The results indicated that the community structure of wine yeast species in New Zealand differs between the three regions sampled: West Auckland, Waiheke Island and Hawkes Bay. Waiheke Island was found to be least like the other two regions in terms of its community structure. The results for population structure showed that there are a number of distinct populations of S. cerevisiae isolates unique to a single region, indicating patterns of biogeography. The presence of some populations consisting of isolates from multiple regions indicates that there is gene flow between the regions. The null hypotheses have been rejected because both community structure and population structure were found to differ between regions. These results expand our knowledge of microbial biogeography in general, and provide evidence for the concept of microbial terroir.