Molecular characterization of the Aedes aegypti odorant receptor gene family

Abstract

The olfactory-driven blood-feeding behaviour of female Aedes aegypti mosquitoes is the primary transmission mechanism by which the arboviruses causing dengue and yellow fevers affect over 40 million individuals worldwide. Bioinformatics analysis has been used to identify 131 putative odourant receptors from the A. aegypti genome that are likely to function in chemosensory perception in this mosquito. Comparison with the Anopheles gambiae olfactory subgenome demonstrates significant divergence of the odourant receptors that reflects a high degree of evolutionary activity potentially resulting from their critical roles during the mosquito life cycle. Expression analyses in the larval and adult olfactory chemosensory organs reveal that the ratio of odourant receptors to antennal glomeruli is not necessarily one to one in mosquitoes.

Figure 1

Phylogenetic relationships of the Ae. aegypti and An. gambiae odourant receptor (Or) families. Corrected distance tree generated using paup* v4 using distances corrected in TreePuzzle v5 using a maximum-likelihood model and the BLOSUM62 amino acid matrix [(Hill et al., 2002; Robertson et al., 2003; Robertson & Wanner, 2006) for methods]. The tree is rooted using the AaGPRor7and AgGPRor7 pair based on the basal position of their highly conserved ortholog DmOr83b in the Orfamily in a phylogenetic tree of the entire superfamily (Robertson et al., 2003). Bootstrap support from 1000 replications of neighbour-joining with uncorrected distances is shown on the relevant branch points. Ae. aegypti (bold branches) and An. gambiae (thin branches) gene expansions are indicated by vertical black bars. Brackets indicate apparent orthology between Ae. aegypti and An. gambiae OR proteins. Protein names are abbreviated to AaOr and AgOr, with suffixes: (P) – pseudogene; (C) (N) – lacking sequence at C- or N-terminus, respectively. Introns are named according to Fig. S3, and their presence within a lineage is indicated by bold letters. Inferred intron losses are indicated by italicized letters on the relevant branches.

Figure 2

Analysis of microsynteny between orthologous and paralogous Ae. aegypti and An. gambiae Or genes. Or genes (black arrows) are abbreviated to AaOr and AgOr. Interspecific neighbouring homologs that share at least 50% amino acid identity are shaded in grey. The Anopheles neighbouring genes are labelled by the last five digits of their ENSANG number preceded by an underscore character (e.g. ENSANG00000008672 abbreviated to _08672). Gene orientation is indicated by arrows. Ae. aegypti supercontig number, An. gambiae scaffold number, and chromosome number are on the right side. Centromeres are indicated by a dot.

Figure 3

Intron analysis of Ae. aegypti and An. gambiae Or genes. (A) The intron positions and phases of Ae. aegypti and An. gambiae (AaOrs and AgOrs) are shown relative to a scale of the average receptor size in amino acids. The ancestral intron locations o, p, q and s (arrowheads) are shared by the two mosquito subfamilies, while idiosyncratic introns (small letters with asterisk and prime symbols) are species specific. The total number of Ors harbouring each conserved intron is indicated in (B).

Figure 4

Peripheral topography of the Or transcriptome in adult and larval olfactory organs of Ae. aegypti. Schematic representation of a larval and chimeric adult Ae. aegypti heads. Left and right sides represent male and female olfactory appendages, respectively. Non-antennal expression is indicated with an asterisk. AaOr8 was detected only in the maxillary palp. Female-specific and larval-specific Ors are indicated by an arrowhead and ‘L’, respectively. Olfactory sensilla bearing segments are in shaded in grey.