Quantifying variation in the ability of yeasts to attract Drosophila melanogaster

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. © 2013 Palanca et al

Attraction index (AI) of eight yeast isolates against grape juice correlated with the relative attraction index among the isolates from head-to-head comparisons.

<p>The relative attraction index is the number of times an isolate was more attractive to flies compared to the other seven isolates. These are significantly correlated, Pearson’s product-moment correlation = 0.95(<i>P</i> = 0.0004).</p

Comparisons of mean (±S.E) attraction indices among different levels of yeast taxonomic grouping.

<p>A: <i>Saccharomyces</i> isolates are significantly more attractive than non-<i>Saccharomyces</i> isolates (F_[1,41] = 9.5843, <i>P</i> = 0.0035). B: There is no significant difference in attraction among <i>Saccharomyces</i> species (F_[4,28] = 0.5636, <i>P</i> = 0.6910). C: There is significant difference in attraction among non-<i>Saccharomyces</i> species (F_[9,28] = 7.0988, <i>P</i> &lt; 0.0001); <i>H</i><i>. uvarum</i> and <i>P</i><i>. kluyveri</i> were the only two significantly attractive non-<i>Saccharomyces</i> species. Asterisks above/below bars indicate a less than 5% probability that the flies had no preference given the observed total proportion of flies on either side of the arm, summed from all replicate T-maze tests per strain, and was calculated using the binominal distribution assuming an underlying 1:1 proportion.</p

Comparisons of mean (±S.E) attraction indices when yeast isolates are partitioned according to their ecological origin.

<p>A: there is no significant difference in attraction according to whether yeast strains were directly isolated from fruits or not (F_[1,33] = 1.4618, <i>P</i> = 0.2352). B: there is a significant difference in attraction according to whether the strains were isolated from niches associated with fruit bearing plants or not (F_[1,33] = 6.1062, <i>P</i> = 0.0188).</p