The effect of Wolbachia on gene expression in Drosophila paulistorum and its implications for symbiont-induced host speciation

Abstract

Background: The Neotropical fruit fly Drosophila paulistorum (Diptera: Drosophilidae) is a species complex in statu nascendi comprising six reproductively isolated semispecies, each harboring mutualistic Wolbachia strains. Although wild type flies of each semispecies are isolated from the others by both pre- and postmating incompatibilities, mating between semispecies and successful offspring development can be achieved once flies are treated with antibiotics to reduce Wolbachia titer. Here we use RNA-seq to study the impact of Wolbachia on D. paulistorum and investigate the hypothesis that the symbiont may play a role in host speciation. For that goal, we analyze samples of heads and abdomens of both sexes of the Amazonian, Centro American and Orinocan semispecies of D. paulistorum.

Results: We identify between 175 and 1192 differentially expressed genes associated with a variety of biological processes that respond either globally or according to tissue, sex or condition in the three semispecies. Some of the functions associated with differentially expressed genes are known to be affected by Wolbachia in other species, such as metabolism and immunity, whereas others represent putative novel phenotypes involving muscular functions, pheromone signaling, and visual perception.

Conclusions: Our results show that Wolbachia affect a large number of biological functions in D. paulistorum, particularly when present in high titer. We suggest that the significant metabolic impact of the infection on the host may cause several of the other putative and observed phenotypes. We also speculate that the observed differential expression of genes associated with chemical communication and reproduction may be associated with the emergence of pre- and postmating barriers between semispecies, which supports a role for Wolbachia in the speciation of D. paulistorum.

Keywords: Drosophila paulistorum; Speciation; Wolbachia; host-symbiont interactions; symbiosis; transcriptome.

Fig. 1

Principal component analysis (a) and heatmap (b) of expression data of the OR semispecies. The PCA is based on all Drosophila genes in the analysis, while the heatmap shows only DE genes. F: female, M: male, WT: wild type, GFR: gut flora restored, abd: abdomen

Fig. 2

Number of genes up- (a) or downregulated (b) in one or multiple semispecies. Shared genes are identified as those included in the same OrthoMCL cluster. All conditions are pooled in this analysis and GFR is used as the reference

Fig. 3

Number of genes differentially expressed in one or multiple conditions of the OR semispecies. A black dot indicates the presence of DE genes for the condition named on the left side. Dots linked by lines represent DE genes in multiple conditions. Vertical bars above the dots correspond to the number of annotated (blue) and unannotated (grey) DE genes present in the condition(s) marked with a dot. Horizontal black bars on the lower left indicate how many genes are DE in each condition. F: female, M: male, WT: wild type, GFR: gut flora restored, abd: abdomen, up: upregulated, down: downregulated

Fig. 4

Metabolic map of differentially expressed genes in carbon and energy metabolism of Drosophila paulistorum. Upregulated genes are shown in green and downregulated genes in blue. DE genes from all conditions of the OR semispecies are represented. Green box: TCA cycle. Yellow box: Glycine and serine metabolism. Blue box: pentose phosphate pathway. The figure is redrawn based on KEGG map01200

Fig. 5

Principal component analysis of all semispecies mapped to the OR transcriptome. (a): Female abdomens, (b): Male abdomens, (c): WT male and female heads, (d): GFR male and female heads. F: female, M: male, WT: wild type, GFR: gut flora restored, abd: abdomen