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. 2009 Apr 21:9:79.
doi: 10.1186/1471-2148-9-79.

The chemoreceptor genes of the waterflea Daphnia pulex: many Grs but no Ors

D Carolina Peñalva-Arana  1 , Michael Lynch, Hugh M Robertson
Affiliations

The chemoreceptor genes of the waterflea Daphnia pulex: many Grs but no Ors

D Carolina Peñalva-Arana et al. BMC Evol Biol. .

Abstract

Background: Chemoreception is vitally important for all animals, yet little is known about the genetics of chemoreception in aquatic organisms. The keystone species Daphnia pulex, a well known crustacean, is the first aquatic invertebrate to have its genome sequenced. This has allowed us the initial investigation of chemoreceptor genes in an aquatic invertebrate, and to begin the study of chemoreceptor evolution across the arthropod phylum.

Results: We describe 58 Grs (gustatory receptors), belonging to the insect chemoreceptor superfamily, which were identified bioinformatically in the draft genome of the crustacean waterflea Daphnia pulex. No genes encoding proteins similar to the insect odorant receptors (Ors) were identified. These 58 Grs form 3 distinctive subfamilies of 37, 12, and 5 genes, as well as a highly divergent singleton (Gr58). In addition, Grs55-57 share distinctive amino acid motifs and cluster with the sugar receptors of insects, and may illuminate the origin of this distinctive subfamily. ESTs, tiling array, and PCR amplification results support 34 predicted gene models, and preliminary expression data comparing the sexes indicates potential female-biased expression for some genes.

Conclusion: This repertoire of 58 chemoreceptors presumably mediates the many chemoperception abilities of waterfleas. While it is always possible that the entire Or gene lineage was lost at some point in the history of Daphnia pulex, we think it more likely that the insect Or lineage is indeed a relatively recently expanded gene lineage concomitant with the evolution of terrestriality in the insects or their hexapod ancestors.

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Figures

Figure 1
Figure 1
Phylogenetic relationships of the 58 Daphnia pulex Grs to each other and a representative set of insect Grs. This is a corrected distance tree, with the highly conserved CO2 receptor lineage designated as the outgroup to root the tree. Bootstrap values from 10,000 replications of uncorrected distance analysis are shown on major branches, followed by Bayesian posterior probabilities. DpuGr (D. pulex) lineages are highlighted in red. Major groups of insect Grs whose ligands are known or which are mentioned in the text are highlighted in blue (Ag – Anopheles gambiae, Am – Apis mellifera, Bm-Bombyx mori, Hv- Heliothis virescens, and Tc- Tribolium castaneum).
Figure 2
Figure 2
Daphnia pulex Gr expression. The bars represent tiling array results which where qualitatively analyzed; expression differences were assessed based on average height of signal for each gene between the sexes. We also indicate other types of expression support from ESTs, cDNA amplification, and qRT/PCR. Black bars – female support; gray bars – male support; Black filled diamond – Genes that were successfully amplified using standard PCR techniques; black filled star – genes with EST support; and black filled circle- genes amplified through qRT/PCR.
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