The chemoreceptor superfamily in the honey bee, Apis mellifera: expansion of the odorant, but not gustatory, receptor family

Abstract

The honey bee genome sequence reveals a remarkable expansion of the insect odorant receptor (Or) family relative to the repertoires of the flies Drosophila melanogaster and Anopheles gambiae, which have 62 and 79 Ors respectively. A total of 170 Or genes were annotated in the bee, of which seven are pseudogenes. These constitute five bee-specific subfamilies in an insect Or family tree, one of which has expanded to a total of 157 genes encoding proteins with 15%-99% amino acid identity. Most of the Or genes are in tandem arrays, including one with 60 genes. This bee-specific expansion of the Or repertoire presumably underlies their remarkable olfactory abilities, including perception of several pheromone blends, kin recognition signals, and diverse floral odors. The number of Apis mellifera Ors is approximately equal to the number of glomeruli in the bee antennal lobe (160-170), consistent with a general one-receptor/one-neuron/one-glomerulus relationship. The bee genome encodes just 10 gustatory receptors (Grs) compared with the D. melanogaster and A. gambiae repertoires of 68 and 76 Grs, respectively. A lack of Gr gene family expansion primarily accounts for this difference. A nurturing hive environment and a mutualistic relationship with plants may explain the lack of Gr family expansion. The Or family is the most dramatic example of gene family expansion in the bee genome, and characterizing their caste- and sex-specific gene expression may provide clues to their specific roles in detection of pheromone, kin, and floral odors.

Figure 1.

Phylogenetic relationships of the 170 AmOrs and fly and moth Ors. This corrected distance tree was rooted by declaring the DmOr83b protein and its orthologs in the other species (AgOr7, HvOr2, and AmOr2) as the outgroup, based on the position of DmOr83 at the base of the Or family in phylogenetic analyses of the entire superfamily in Drosophila (Robertson et al. 2003). Relatively closely related fly and moth Ors were removed to facilitate phylogenetic analysis and presentation, hence many of the lineage-specific subfamily expansions in these insects are not obvious (see Hill et al. 2002). Clusters of tandem arrays of AmOrs on particular chromosomes are indicated by vertical lines on the right. Numbers above branches are the percentage of 1000 neighbor-joining bootstrap replicates containing that branch and are shown only for major lineages including bee Ors. Symbols after the AmOr numbers: F indicates the missing genomic sequence was manually assembled to complete the gene model; N, the N terminus of the gene model is missing in a gap or is unidentifiable; C, the C terminus of the gene model is missing in a gap or is unidentifiable; and P, pseudogene with one or more in-frame stop codons, frame-shifting indels, or unacceptable intron splice sites.

Figure 2.

Molecular evolution of a 60 AmOr gene array on chromosome 2. The 60 genes are in a perfect uninterrupted tandem array and were numbered from left to right (AmOr2 is the previously named ortholog of the conserved DmOr83b protein and is on chromosome 1). The phylogenetic relationships of these 60 genes are shown in the corrected-distance cladogram of their encoded Ors below the array (AmOr1 and 3 were declared the outgroup based on their location in Fig. 1; branch lengths in this cladogram are meaningless). The few incompatibilities between the tree and the gene location are indicated by diagonal lines.

Figure 3.

Phylogenetic relationships of the 10 AmGrs and three pseudogenes to fly and moth Grs. This corrected distance tree was rooted at the midpoint in the absence of a suitable outgroup. For other details, see Figure 1.

Figure 4.

(A) Or and (B) Gr gene expression in honey bee olfactory and gustatory organs measured as fold increases relative to levels in the body as a reference tissue using the 2^−ΔΔCT equation (Livak and Schittgen 2001). Gene expression levels were determined by real-time quantitative PCR and normalized to levels of A. mellifera ribosomal protein S8 (GenBank accession NM001011604).