The molecular phylogeny and mode of evolution of a nematode specific clade of heterotrimeric G-protein subunit genes.

In C. elegans, and most probably all animal olfactory systems, odorant molecules are detected by G-protein coupled receptors (GPCRs). Guanine nucleotide binding G-proteins relay signals from these GPCRs to intracellular second messenger pathways that trigger the opening of ion channels and local depolarization. The human genome contains 17 G genes which are divided into four subfamilies based on functional and sequence attributes. C. elegans contains 21 G genes, 14 of which are specifically expressed in the amphid sensory neurons1. In addition to having at least one member of each of the four mammalian G gene classes, our analysis shows that nematodes including C. elegans also possess at least one lineage-specific G gene expansion, homologues of which are not found in other organisms. We constructed a data set containing homologues of putative G genes from a variety of metazoans, protistans, fungi and plants. The final alignment contained 146 taxa and 751 aligned amino acid positions. Amino acids were recoded into the six categories corresponding to the PAM matrix. The recoded data was analysed using a Bayesian criterion with an appropriate model. The resultant phylogeny was compared to a second phylogenetic analysis based on a smaller number of exemplar G proteins. This analysis used maximum likelihood estimation to categorise sites into one of eight categories according to their rate of evolution. Overall, both analyses yielded near identical topologies and showed that the majority of nematode unique G genes are the result of a duplication of a putative ancestral Gi/o gene. We will present our phylogenetic analyses and an evaluation of the selective pressures acting on these nematode-unique G genes. A single C. elegans olfactory neuronal membrane contains multiple GPCRs and multiple heterotrimeric G proteins. These novel nematode specific G genes increase the functional complexity of individual chemosensory neurons, enabling them to integrate and adapt to odor signals from the multiple distinct GPCRs expressed on their membranes2.1. Jansen, G. et al. (1999). Nature Genetics, 21, 414-419.2. Lans, H. et al. (2004. Genetics, 167, 1677-1687.