Abstract
Chemoreception of environmental stimuli is a major sensory system in small soil nematodes like C. elegans. As in other animals, chemoreception is mediated in C. elegans by members of the seven-transmembrane G-protein-coupled receptor class (7TM GPCRs). We summarize the many large putative chemoreceptor gene families, including the str family (which includes odr-10, the only receptor with an identified ligand), and the sra, srab, srb, srbc, srd, sre, srg, srh, sri, srj, srm, srr, srsx, srt, sru, srv, srw, srx, srxa, and srz families. Together these comprise +/-1280 apparently intact genes and +/-420 apparent pseudogenes, about 7% of the total gene count of C. elegans. These genes are unusually clustered on chromosomes, both within and between families, and are enigmatically concentrated on the large chromosome V. Comparative studies with C. briggsae have revealed extraordinary divergence of the chemoreceptor repertoire between the two species, including frequent amplifications of subfamilies in C. elegans and positive selection in the srz family. The size and complexity of the chemoreceptor gene families also facilitate studies of promoter elements using paralogous and orthologous comparisons, as well as other aspects of gene family and genome evolution.
Highlights
Chemoperception is a central sense of soil nematodes like Caenorhabditis elegans, as it is for most other animals
A decade after their first recognition, we describe here all the remaining families and relate them in a series of superfamilies that comprise a remarkable array of roughly 1300 potentially functional candidate chemoreceptor genes, along with about 400 apparent pseudogenes
Annotation of the genes is not nearly as straightforward as for mammalian olfactory receptors, because the C. elegans genes have complex gene structures with 1–8 introns each, and the intron locations are commonly different between gene families
Summary
Chemoperception is a central sense of soil nematodes like Caenorhabditis elegans, as it is for most other animals (reviewed in Mori, 1999; Sengupta, 1997; Bargmann, 1997). A decade after their first recognition, we describe here all the remaining families and relate them in a series of superfamilies that comprise a remarkable array of roughly 1300 potentially functional candidate chemoreceptor genes, along with about 400 apparent pseudogenes. Together they comprise roughly 7% of the gene complement of this tiny animal, more than any other sequenced genome. On the basis of genetic complexity, this system probably constitutes the bulk of signal transduction that occurs in C. elegans We first discuss these families below in superfamily groupings, loosely defined as consisting of readily recognizable groupings based on protein sequence similarity, as well as sometimes shared intron locations in their genes. It seems likely that some of the genes have been subsequently recruited to other roles such as monitoring of internal chemistry, so the function of any specific gene should be interpreted with caution
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