Drosophila Effects on Yeast Communities Figure 1. Regulation of yeast densities by adult females. Where d indicates yeast density [log (CFU/g)], (dvirgin females 2 dno flies ) was regressed against dno flies (a), and (dmated females 2 dno flies ) was regressed against dno flies (b). The dashed line indicates the hypothesis in which treatment yeast densities are identical to yeast densities in the matched ‘‘no flies’’ controls. Negative slopes for both analyses suggest that virgin females and mated females regulated yeast densities across different banana fruits. doi:10.1371/journal.pone.0042238.g001 Virgin adult females had effects on yeast densities similar to those observed in Experiment 1 (compare Figures 1a and 5a; see also below). However, larvae alone significantly increased yeast densities in fruits that supported , = 108 CFU/g of yeast in the absence of flies (larvae versus no flies: paired t test, t = 2.70, df = 7, P = 0.03, Figure 5b), a first indication that larvae were not simply consumers of yeast in banana fruits. Instead, our results indicated that, if anything, larvae regulated yeast densities across fruits more efficiently than was the case for vectoring females. When we regressed the yeast densities in fly- exposed pieces against the yeast densities for the pieces not exposed to flies, the slopes of all three regressions were negative ((dadults 2 dno flies) versus dno flies: slope = 20.50, t = 6.56; (dlarvae 2 dno flies) versus dno flies: slope = 20.85, t = 10.46; (dadults and larvae 2 dno flies) versus dno flies: slope = 20.92, t = 8.75, all three slopes significantly different from 0.0 at P,1.0e-6, after FDR)(Figure 5). In addition, although all four treatment groups ended up with approximately the same mean yeast density (mean density (CFU/ g): adults = 108.3, larvae = 108.0, adults and larvae = 108.5, no flies = 108.5), the variance in density across fruits was substantially and significantly lower for fruits exposed to larvae than for fruits not exposed to flies (variance in log CFU/g: adults = 0.28, larvae = 0.16, adults and larvae = 0.19, no flies = 0.63; adults versus no flies, Levene’s statistic = 2.50, df = 1,70, P = 0.12; larvae versus no flies, Levene’s statistic = 8.36, df = 1, 70, P,0.005; adults and larvae versus no flies, Levenes’s statistic = 7.28, df = 1, 70, P,0.009; the last two findings both significant at P,0.025 after correction for three tests via FDR)(Figure 6). At the mechanistic level, one way that larvae might have affected yeast communities was by suppressing the growth of filamentous fungi (molds), which may compete with yeast for space or nutrients [31]. Previous studies have demonstrated that D. melanogaster larvae can discourage the growth of molds [32], even in substrates without yeasts [33], perhaps by physically breaking up mold hyphae as they crawl around a substrate. We found that the proportion of banana pieces with mold was dramatically lower in pieces with larvae than without (no flies: 0.94, adults: 0.78, larvae: 0.08, adults and larvae: 0.06; effects of larvae on mold (logistic regression, Wald = 24.2, df = 1, P,0.000), effects of females on mold (Wald = 0.21, df 1, P = 0.645), with no significant interaction between them (Wald = 0.83, df = 1, P = 0.36). Figure 2. Effects of virgin females and mated females on yeast densities. Exposure to virgin females (a) and mated females (b) increased mean yeast densities and reduced the variance in yeast densities across different banana fruits. doi:10.1371/journal.pone.0042238.g002 PLoS ONE | www.plosone.org 5 July 2012 | Volume 7 | Issue 7 | e42238