Chemoreception in nematodes appears to involve the participation of numerous chemoreceptor proteins from several major families. They include the sra, srb, srd, sre, srg, srh, sri, srj, srm, srn, srr, srt, sru, srv, srw, srx, srz, str, and several more unnamed families. The str family contains the only receptor whose ligand is known, ODR-10 and diacetyl, and is part of the
odr-10 superfamily of about 900 genes and pseudogenes (str, srd, srh, sri, srj, srm, and srn families). The srg, sru, and srv families form another superfamily of about 150 genes and pseudogenes. The srw family of about 150 genes and pseudogenes is distantly related to about 50 candidate FMRFamide receptors. The srt, srx, srz, and several more families are currently being worked up. These nematode-specific chemoreceptor families appear to have evolved from other 7TM-GPCR families, with enormous gene family expansion through both tandem and segmental duplications and transpositions, commonly generating large numbers of genes on chromosome V. Large clusters of unrelated receptors are found on this chromosome, suggesting possible chromatin domains of coordinated gene expression, perhaps in particular chemosensory neurons. Comparison with C. briggsae reveals both highly conserved proteins, with some families having largely orthologous relationships, while others are highly divergent, with lineage-specific gene-subfamily formation. These large gene families offer numerous possibilities for studying gene family evolution, such as the rampant evolution of introns through numerous independent gains and losses. Finally, in addition to these approximately 1500 nematode-specific chemoreceptor genes and pseudogenes, there are five gur genes which are distantly related to the insect chemoreceptor superfamily and may play highly conserved roles in chemoreception. In particular,
gur-3 appears to be expressed in the I2 neurons that might have a role in gustation.