Abstract
Yeasts that invade and colonise fruit significantly enhance the volatile chemical diversity of this ecosystem. These modified bouquets are thought to be more attractive to Drosophila flies than the fruit alone, but the variance of attraction in natural yeast populations is uncharacterised. Here we investigate how a range of yeast isolates affect the attraction of female D. melanogaster to fruit in a simple two choice assay comparing yeast to sterile fruit. Of the 43 yeast isolates examined, 33 were attractive and seven repellent to the flies. The results of isolate-versus-isolate comparisons provided the same relative rankings. Attractiveness varied significantly by yeast, with the strongly fermenting Saccharomyces species generally being more attractive than the mostly respiring non- Saccharomyces species (P = 0.0035). Overall the habitat (fruit or other) from which the isolates were directly sampled did not explain attraction (P = 0.2352). However, yeasts isolated from fruit associated niches were more attractive than those from non-fruit associated niches (P = 0.0188) regardless of taxonomic positioning. These data suggest that while attractiveness is primarily correlated with phylogenetic status, the ability to attract Drosophila is a labile trait among yeasts that is potentially associated with those inhabiting fruit ecosystems. Preliminary analysis of the volatiles emitted by four yeast isolates in grape juice show the presence/absence of ethanol and acetic acid were not likely explanations for the observed variation in attraction. These data demonstrate variation among yeasts for their ability to attract Drosophila in a pattern that is consistent with the hypothesis that certain yeasts are manipulating fruit odours to mediate interactions with their Drosophila dispersal agent.
Highlights
Saccharomycetes (Hemiascomycete) yeasts are a highly diverse class of microbial fungi that inhabit a variety of niches; many can establish and multiply on and in fruits [1]
Overall we found significant variance in D. melanogaster attraction among yeast isolates, with many yeast strains enhancing attraction over sterile grape juice
Attraction was strongly associated with the taxonomic classification of the strains of yeast
Summary
Saccharomycetes (Hemiascomycete) yeasts are a highly diverse class of microbial fungi that inhabit a variety of niches; many can establish and multiply on and in fruits [1]. Whilst rare in the fruit microbial community initially, the Saccharomyces sensu-stricto species, especially S. cerevisiae, typically come to dominant the mature ferments of ripe fruits [2,3] These Saccharomyces species opt for fermentation even in the presence of oxygen (the Crabtree effect), which is the least energetically efficient route to produce energy from the substrate sugars, but it does have a greater rate of ATP production [4]. This strategy, produces ethanol, heat and carbon dioxide, which in combination modifies the fruit niche and excludes other competing microbes from establishing and utilizing the resource [3,5,6]. One hypothesis to explain the function of volatile manipulation and production by yeasts is that these odours attract insects, and enhance the dispersal and survival of otherwise non-motile yeasts [9]
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