Genome-wide identification and expression profiling of odorant receptor genes in the malaria vector Anopheles sinensis

Abstract

Background: The olfactory system plays a crucial role in regulating insect behaviors. The detection of odorants is mainly mediated by various odorant receptors (ORs) that are expressed in the dendrites of olfactory neurons of chemosensilla. Anopheles sinensis is a major malaria vector in Eastern Asia and its genome has recently been successfully sequenced and annotated. In this study, we present genome-wide identification and expression profiling of OR genes in different chemosensory tissues of An. sinensis.

Methods: The OR genes were identified using the available genome sequences of An. sinensis. A series of bioinformatics analyses were conducted to investigate the structure, genome distribution, selective pressure and phylogenetic relationships of OR genes, the conserved domains and specific functional sites in the OR amino acid sequences. The expression levels of OR genes were analyzed from transcriptomic data from An. sinensis antennae, proboscis and maxillary palps of both sexes.

Results: A total of 59 putative OR genes have been identified and characterized in An. sinensis. This number is significantly less than that in An. gambiae. Whether this difference is caused by the contraction or expansion of OR genes after divergence of the two species remains unknown. The RNA-seq analysis showed that AsORs have obvious tissue- and sex-specific expression patterns. Most AsORs are highly expressed in the antennae and the expression pattern and number of AsORs expressed in antennae are similar in males and females. However, the relative levels of AsOR transcripts are much higher in female antennae than in male antennae, which indicates that the odor sensitivity is likely to be increased in female mosquitoes. Based on the expression patterns and previous studies, we have speculated on the functions of some OR genes but this needs to be validated by further behavioral, molecular and electrophysiological studies. Further studies are necessary to compare the olfactory-driven behaviors and identify receptors that respond strongly to components of human odors that may act in the process of human recognition.

Conclusions: This is the first genome-wide analysis of the entire repertoire of OR genes in An. sinensis. Characterized features and profiled expression patterns of ORs suggest their involvement in the odorous reception of this species. Our findings provide a basis for further research on the functions of OR genes and additional genetic and behavioral targets for more sustainable management of An. sinensis in the future.

Keywords: Anopheles sinensis; Characteristics; Expression pattern; Genome; Odorant receptor.

Fig. 1

Comparison of chromosomal or scaffold distribution of An. sinensis (in blue) and An. gambiae (in red) OR genes. The position of each gene is mapped on the chromosomal or scaffold in Mb scale. Orthologous gene pairs are linked with lines and the unigenes are marked in red

Fig. 2

Venn diagram of orthologous groups of OR genes in four mosquito genomes and the phylogenetic relationship of six important mosquito species. A Ortholog categories were analyzed by Blast searches against Diptera orthologs. The numbers displayed in the Venn diagram correspond to the number of ortholog groups. Overlapping regions show the number of ortholog groups shared by two, three or four species. Areas that do not overlap between circles indicate unique genes for each species. B The phylogenetic tree was constructed through the taxonomy browser in GenBank. The divergent time was cited from Hao et al. [78]. The numbers in brackets indicate the number of OR genes

Fig. 3

Phylogenetic tree of OR proteins in An. sinensis, An. gambiae, Ae. aegypti and Cx. quinquefasciatus. Conserved domains of AsOR proteins were aligned using ClustalX, and the phylogenetic tree was constructed by RAxMLv8.2.0, using the maximum-likelihood method (1000 bootstrap) based on the best fit model of JTT + I + G

Fig. 4

Ka/Ks ratios of AsORs

Fig. 5

Expression profiles of the OR genes in different tissues, based on RNA-seq. Expression levels of the OR genes in the six transcriptomes represented as heat plots, based on log-transformed FPKM values. Zero expression is represented by the lightest blue color

Fig. 6

Expression profiling of selected preferentially expressed AsOR genes in different tissues, based on RT-qPCR. The relative expression levels of nine genes in different tissues of adult mosquitoes of both sexes. Three biological and three technical replications were tested using RT-qPCRs. The expression ratios of each gene, normalized against the Rps7 reference gene, are represented in bars, and the standard deviation is shown on the top of the bar, with different letters indicating significant differences determined by a one-way ANOVA (P < 0.05)