The molecular phylogeny of a nematode-specific clade of heterotrimeric G-protein α-subunit genes

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 alpha subunit confers functional specificity to G-proteins. Vertebrate and insect G alpha-subunit genes are divided into four subfamilies based on functional and sequence attributes. The nematode Caenorhabditis elegans contains 21 G alpha 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 G alpha subunits within each olfactory neuron. Here we show that, in addition to having at least one member of each of the four mammalian G alpha gene classes, C. elegans and other nematodes also possess two lineage-specific G alpha gene expansions, homologues of which are not found in any other organisms examined. We hypothesize that these novel nematode-specific G alpha 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.