Figure 4. Processing by Drosophila larvae increased the similarity of yeast communities on banana fruits. Bars represent the mean, pairwise Jaccard’s similarity between replicate yeast communities in each of the four treatment groups. The mean, pairwise Jaccard’s similarity provides a relative measure of the multivariate dispersion within each treatment group; i.e., larger bars represent communities that are more consistent in community composition than shorter bars. Error bars represent 61 SE. doi:10.1371/journal.pone.0042238.g004 4. Palatability of Yeasts Collected from Fly-conditioned Fruits We found that the strains of C. californica, C. zemplinina and P. kluyveri isolated from fruits exposed to both adults and larvae in Experiment 2 were highly palatable to young larvae. When clean one day old larvae were placed on ‘lawns’ of these yeasts, they quickly began feeding; by the end of an hour they had fed to satiation, as indicated by the cessation of feeding and locomotory activity. These larvae later deposited fecal pools of their own that were, 12 h later, filled with yeast colonies. The number and size of Drosophila Effects on Yeast Communities the fecal pools deposited by these larvae were comparable to those of larvae fed fly-conditioned banana (see above), although they also varied as a function of yeast diet (pools/h: C. califor- nica = 7.2660.8, C. zemplinina = 10.561.54, P. kluyveri = 11.761.3, Welsh statistic = 4.476, df = 2, 62.27, P = 0.015, n = 38 larvae per group); pool size: C. californica = 0.7860.05 mm, n = 33; C. zemplinia = 1.0260.08 mm, n = 31; P. kluvyeri = 1.2860.08 mm, n = 33; Welsh statistic = 13.97, df = 2, 58.17, P,0.0005). Discussion Drosophila melanogaster adults and larvae, but especially larvae, had profound effects on the densities and community structure of yeasts that developed in banana substrates. Virgin adult females previously fed conditioned fruit increased yeast abundance in fruits that supported low yeast densities in the absence of flies, and affected yeast species diversity in bananas, supporting the hypothesis that adult Drosophila are able to vector yeasts to new fruit substrates. However, vectoring is not the whole story, because larvae alone had dramatic effects on yeast density and species composition. Larvae increased yeast densities in fruits that supported relatively low densities of yeast in the absence of exposure to flies, and regulated yeast densities within relatively narrow limits in fruits that supported a much wider range of yeast densities in the absence of flies. Larvae also had dramatic effects on yeast species composition, dramatically reducing species diversity across fruits, and reducing variation in yeast communities from one fruit to the next (beta diversity). In particular, larvae encouraged the consistent development of a yeast community comprised of three yeast species (Candida californica, C. zemplinina and Pichia kluvyeri). Together, adult females and larvae encouraged the growth and maintenance of a simple, predictable yeast community in banana fruits which, in the absence of flies, supported a much wider range of yeast species, and yeast communities that differed widely from one fruit to the next. Conversely, these results indicate that a small subset (C. californica, Figure 5. Regulation of yeast densities across fruits by adults and larvae. Where d indicates yeast density [log (CFU/g)], (dadults 2 dno flies ) was regressed against dno flies (a), (dlarvae 2 dno flies ) was regressed against dno flies (b), and (dadults and larvae 2 dno flies ) was regressed against dno flies (c). The dashed line indicates the hypothesis in which treatment yeast densities are identical to yeast densities in the matched ‘‘no flies’’ controls. The highly significant negative slopes for all three analyses indicate that virgin females, larvae alone, and virgin females and larvae acting together regulated yeast densities across different banana fruits. The intersection of the dashed and solid lines indicates the equilibrium yeast density on the abscissa; i.e., populations to the left of this intersection along the regression line would be expected to show positive population growth, while populations to the right of this intersection along the regression line would be expected to show negative population growth due to the fly treatment. doi:10.1371/journal.pone.0042238.g005 PLoS ONE | www.plosone.org 7 July 2012 | Volume 7 | Issue 7 | e42238