Research article

Candidate chemoreceptor subfamilies differentially expressed in the chemosensory organs of the mollusc Aplysia

Scott F Cummins¹ , Dirk Erpenbeck² , Zhihua Zou³ , Charles Claudianos⁴ , Leonid L Moroz⁵ , Gregg T Nagle³ and Bernard M Degnan¹

¹School of Biological Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia

²Department of Earth and Environmental Sciences, Ludwig-Maximilians-University, Munich, Germany

³Department of Neuroscience and Cell Biology, University of Texas Medical Branch, Galveston, TX 77555, USA

⁴Queensland Brain Institute, The University of Queensland, St. Lucia, Queensland, 4072, Australia

⁵Whitney Laboratory for Marine Science and Department of Neuroscience, University of Florida, St Augustine, Florida 32080, USA

author email corresponding author email

BMC Biology 2009, 7:28doi:10.1186/1741-7007-7-28

Published:	4 June 2009

Abstract

Background

Marine molluscs, as is the case with most aquatic animals, rely heavily on olfactory cues for survival. In the mollusc Aplysia californica, mate-attraction is mediated by a blend of water-borne protein pheromones that are detected by sensory structures called rhinophores. The expression of G protein and phospholipase C signaling molecules in this organ is consistent with chemosensory detection being via a G-protein-coupled signaling mechanism.

Results

Here we show that novel multi-transmembrane proteins with similarity to rhodopsin G-protein coupled receptors are expressed in sensory epithelia microdissected from the Aplysia rhinophore. Analysis of the A. californica genome reveals that these are part of larger multigene families that possess features found in metazoan chemosensory receptor families (that is, these families chiefly consist of single exon genes that are clustered in the genome). Phylogenetic analyses show that the novel Aplysia G-protein coupled receptor-like proteins represent three distinct monophyletic subfamilies. Representatives of each subfamily are restricted to or differentially expressed in the rhinophore and oral tentacles, suggesting that they encode functional chemoreceptors and that these olfactory organs sense different chemicals. Those expressed in rhinophores may sense water-borne pheromones. Secondary signaling component proteins Gα_q, Gα_i, and Gα_o are also expressed in the rhinophore sensory epithelium.

Conclusion

The novel rhodopsin G-protein coupled receptor-like gene subfamilies identified here do not have closely related identifiable orthologs in other metazoans, suggesting that they arose by a lineage-specific expansion as has been observed in chemosensory receptor families in other bilaterians. These candidate chemosensory receptors are expressed and often restricted to rhinophores and oral tentacles, lending support to the notion that water-borne chemical detection in Aplysia involves species- or lineage-specific families of chemosensory receptors.