Drosophila suzukii avoidance of microbes in oviposition choice

Abstract

While the majority of Drosophila species lays eggs onto fermented fruits, females of Drosophila suzukii pierce the skin and lay eggs into ripening fruits using their serrated ovipositors. The changes of oviposition site preference must have accompanied this niche exploitation. In this study, we established an oviposition assay to investigate the effects of commensal microbes deposited by conspecific and heterospecific individuals and showed that the presence of microbes on the oviposition substrate enhances egg laying of Drosophila melanogaster and Drosophila biarmipes, but discourages that of D. suzukii. This result suggests that a drastic change has taken place in the lineage leading to D. suzukii in how females respond to chemical cues produced by microbes. We also found that hardness of the substrate, resembling that of either ripening or damaged and fermenting fruits, affects the response to microbial growth, indicating that mechanosensory stimuli interact with chemosensory-guided decisions to select or avoid oviposition sites.

Keywords: Acetobacter; Gluconobacter; acetic acid bacteria; decision-making; mechanosensory stimulus; spotted-wing Drosophila.

Figure 1.

Experimental scheme of the oviposition assay to quantify response to microbes deposited by flies on the surface of media. (a) Washed water collected from the surface of inoculated and control plates. (b) Oviposition assay using media inoculated with solutions from (a) for 24 h.

Figure 2.

Comparisons of the preference indices (PIs) of D. melanogaster, D. biarmipes and D. suzukii for oviposition substrates treated with inoculant from conspecific (open boxplots) or heterospecific (filled boxplots in grey) flies. (a) The PIs assayed on soft substrate (1% agar medium) with and without inoculant treatment (microbial growth). (b) The PIs assayed on 1% agar medium for substrates treated with sterile filtered solutions of inoculant. (c) The PIs assayed on hard oviposition substrate (3% agar medium) with and without inoculant treatment (microbial growth). Control substrates were treated with solutions from non-exposed (non-inoculated) substrate in all assays. Species used for heterospecific inoculations were conducted using D. suzukii for D. melanogaster assay, and D. melanogaster for D. biarmipes and D. suzukii assays. Results from assays with fewer than 10 eggs on either substrate were excluded from the analysis. Box signifies the upper and lower quartiles and horizontal bar indicates median. Upper and lower whiskers represent maximum and minimum 1.5 × interquartile range, respectively. The difference from PI = 0 (no preferences) was tested by Wilcoxon signed-rank test with Bonferroni correction for multiple comparisons (six tests). *p < 0.05, ns p ≥ 0.05.

Figure 3.

Preference indices (PIs) for the soft substrate with and without microbes. (a) The substrate placement in the chambers for the oviposition assay. ‘1%' and ‘3%' indicate soft (1% agar medium) and hard (3% agar medium) oviposition substrates, respectively. The microbe (+) chambers have been treated with inoculant collected from substrate surface exposed to D. melanogaster; microbial (−) chambers were treated with inoculant from non-exposed surfaces. (b) The preference indices (PIs) for soft oviposition substrate in the absence (open boxplots) and presence (filled boxplots in grey) of microbes. Results from assays with fewer than 10 eggs on either substrate were excluded from the analysis. Box signifies the upper and lower quartiles and horizontal bar indicates median. Upper and lower whiskers represent maximum and minimum 1.5 × interquartile range, respectively. Statistical significance was tested by permutation test with Bonferroni correction for multiple comparisons (six tests). *p < 0.05, ns p ≥ 0.05.