The Molecular Phylogeny of a Nematode-Specific Clade of Heterotrimeric G-Protein α-Subunit Genes
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In animal olfactory systems, odorant molecules are detected by olfactory receptors (ORs). ORs are part of the G-protein-coupled receptor (GPCR) superfamily. Heterotrimeric guanine nucleotide binding G-proteins (G-proteins) relay signals from GPCRs to intracellular effectors. G-proteins are comprised of three peptides. The G-protein α subunit confers functional specificity to G-proteins. Vertebrate and insect Gα-subunit genes are divided into four subfamilies based on functional and sequence attributes. The nematode Caenorhabditis elegans contains 21 Gα genes, 14 of which are exclusively expressed in sensory neurons. Most individual mammalian cells express multiple distinct GPCR gene products, however, individual mammalian and insect olfactory neurons express only one functional odorant OR. By contrast C. elegans expresses multiple ORs and multiple Gα subunits within each olfactory neuron. Here we show that, in addition to having at least one member of each of the four mammalian Gα gene classes, C. elegans and other nematodes also possess two lineage-specific Gα gene expansions, homologues of which are not found in any other organisms examined. We hypothesize that these novel nematode-specific Gα genes increase the functional complexity of individual chemosensory neurons, enabling them to integrate odor signals from the multiple distinct ORs expressed on their membranes. This neuronal gene expansion most likely occurred in nematodes to enable them to compensate for the small number of chemosensory cells and the limited emphasis on cephalization during nematode evolution.
KeywordsHeterotrimeric G-protein α subunit Olfaction Nematode Caenorhabditis elegans Chemoreception
This work was funded by the Irish Higher Education Authority Programme for Research in Third Level. Many thanks go to Dr. Ralf Schmid for sending us the clustered version of NEMBASE. The authors wish to acknowledge the SFI/HEA Irish Centre for High-End Computing (ICHEC) for the provision of computational facilities and support. Supplementary material can be found at http://www.biology.nuim.ie/staff/JMESupp.shtml.
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