Deciphering the olfactory repertoire of the tiger mosquito Aedes albopictus

Abstract

Background: The Asian tiger mosquito Aedes albopictus is a highly invasive species and competent vector of several arboviruses (e.g. dengue, chikungunya, Zika) and parasites (e.g. dirofilaria) of public health importance. Compared to other mosquito species, Ae. albopictus females exhibit a generalist host seeking as well as a very aggressive biting behaviour that are responsible for its high degree of nuisance. Several complex mosquito behaviours such as host seeking, feeding, mating or oviposition rely on olfactory stimuli that target a range of sensory neurons localized mainly on specialized head appendages such as antennae, maxillary palps and the mouthparts.

Results: With the aim to describe the Ae. albopictus olfactory repertoire we have used RNA-seq to reveal the transcriptome profiles of female antennae and maxillary palps. Male heads and whole female bodies were employed as reference for differential expression analysis. The relative transcript abundance within each tissue (TPM, transcripts per kilobase per million) and the pairwise differential abundance in the different tissues (fold change values and false discovery rates) were evaluated. Contigs upregulated in the antennae (620) and maxillary palps (268) were identified and relative GO and PFAM enrichment profiles analysed. Chemosensory genes were described: overall, 77 odorant binding proteins (OBP), 82 odorant receptors (OR), 60 ionotropic receptors (IR) and 30 gustatory receptors (GR) were identified by comparative genomics and transcriptomics. In addition, orthologs of genes expressed in the female/male maxillary palps and/or antennae and involved in thermosensation (e.g. pyrexia and arrestin1), mechanosensation (e.g. piezo and painless) and neuromodulation were classified.

Conclusions: We provide here the first detailed transcriptome of the main Ae. albopictus sensory appendages, i.e. antennae and maxillary palps. A deeper knowledge of the olfactory repertoire of the tiger mosquito will help to better understand its biology and may pave the way to design new attractants/repellents.

Keywords: Aedes albopictus; Antennae; Chemosensory genes; Maxillary palps; Mosquito; Olfaction; mRNA-sequencing.

Fig. 1

Tissues, RNA-seq method and cluster analysis. a. Schematic representation of female head with highlighted in red antennae and palps (upper part of the panel), male head and whole female mosquito (lower part of the panel). b. Workflow chart of procedures used for the assembly and the annotation of the Ae. albopictus olfactory transcriptome. c. Multidimensional plot of RNA-seq duplicates used in this study

Fig. 2

Pairwise sample comparisons. Proportional Venn diagram showing pairwise comparisons between female antennae, female palps and male heads. Gene subsets enhanced in each sample versus female body according to the edgeR threshold at P < 0.001 (see Table 2) were compared to each other. Overlaps (and relative numbers) represent the subsets of genes that are differentially expressed in more than one tissue

Fig. 3

Differential expression (DE) of chemosensory genes. Volcano plots show the relative expression of contigs in pairwise comparisons. The x-axis represents the logFC (fold change) between tissues. The y-axis represents the negative log10 of the p-value (false discovery rate) as calculated by the Fisher’s Exact test. a. Female bodies vs Antennae. b. Female bodies vs Palps. c. Female bodies vs Male Heads. Only differentially expressed contigs (P < 0.05, logFC <−2 and >2) are shown in the plot (grey dots) with OBP indicated in black, OR in red, GR in green and IR in blue

Fig. 4

Odorant binding proteins (OBPs) in Ae. albopictus transcriptome. Left panel, abundance profile map: intensity scale (color gradient from blue to yellow indicates levels from high TPM to low) as indicated at the bottom. FA, female antennae; FP, female palps; MH male heads and FB, female body. Assigned OBP names, ID in Ae. albopictus (VectorBase ID AALFxxxxxx when available or contig ID in the transcriptome), number of cysteines, OBP subfamily, name and ID of the presumed Ae. aegypti ortholog and percentage of identity are reported

Fig. 5

Odorant receptors (OR) in Ae. albopictus transcriptome. Left panel, abundance profile map: intensity scale (color gradient from blue to yellow indicates levels from high TPM to low) as indicated at the bottom. FA, female antennae; FP, female palps; MH male heads and FB, female body. OR name and Ae. albopictus ID columns indicate respectively the name assigned to OR in Ae. albopictus and the ID (VectorBase ID AALFxxxxxx when available or contig ID in the transcriptome). Ae. aegypti OR name ID columns list the OR name in Ae. aegypti and the VectorBase code (AAELxxxxxx) of Ae. aegypti orthologs, respectively, with the percent (%) of identity reported in the last column

Fig. 6

Ionotropic receptors (IR) in Ae. albopictus transcriptome. Left panel, abundance profile map: intensity scale (color gradient from blue to yellow indicates levels from high TPM to low) as indicated at the bottom. FA, female antennae; FP, female palps; MH male heads and FB, female body. IR name and Ae. albopictus ID columns indicate respectively the name assigned to IR in Ae. albopictus and the ID (VectorBase ID AALFxxxxxx when available or contig ID in the transcriptome). Ae. aegypti IR name and ID columns list the IR name in Ae. aegypti and the VectorBase code (AAELxxxxxx) of Ae. aegypti orthologs, respectively, with the percent (%) of identity reported in the last column

Fig. 7

Gustatory receptors (GR) in Ae. albopictus transcriptome. Left panel, abundance profile map: intensity scale (color gradient from blue to yellow indicates levels from high TPM to low) as indicated at the bottom. FA, female antennae; FP, female palps; MH male heads and FB, female body. GR name and Ae. albopictus ID columns indicate respectively the name assigned to GR in Ae. albopictus and the ID (VectorBase ID AALFxxxxxx when available or contig ID in the transcriptome). Ae. aegypti GR name and ID columns list the GR name in Ae. aegypti and the VectorBase code (AAELxxxxxx) of Ae. aegypti orthologs, respectively, with the percent (%) of identity reported in the last column

Fig. 8

Abundance profiles of sensory genes in Ae. albopictus. Abundance profile map: intensity scale (color gradient from blue to yellow indicates levels from high TPM to low) as indicated at the bottom. Gene name and contig ID (VectorBase codes when available) are also reported

Fig. 9

qPCR validation. Correlation between transcriptional abundance of 11 genes in both antennae (a) and maxillary palps (b) revealed by qPCR and RNA-seq. Level of abundance is defined as the ratio between each sample value over the group median (fold change, FC) in both qPCR and RNA-seq approaches. For both techniques, statistical evaluation throughout Spearman and Pearson tests was performed and results are reported in the figure insets