The homeobox gene lim-6 is required for distinct chemosensory representations in C. elegans (vol 410, pg 694, 2001)

The ability to discriminate between different chemical stimuli is crucial f or food detection, spatial orientation and other adaptive behaviours in ani mals. In the nematode Caenorhabditis elegans, spatial orientation in gradie nts of soluble chemoattractants (chemotaxis) is controlled mainly by a sing le pair of chemosensory neurons(1). These two neurons, ASEL and ASER, are l eft-right homologues in terms of the disposition of their somata and proces ses, morphology of specialized sensory endings, synaptic partners and expre ssion profile of many genes(2,3). However, recent gene-expression studies h ave revealed unexpected asymmetries between ASEL and ASER. ASEL expresses t he putative receptor guanylyl cyclase genes gcy-6 and gcy-7, whereas ASER e xpresses gcy-5 (ref. 4). In addition, only ASEL expresses the homeobox gene lim-6, an orthologue of the human LMX1 subfamily of homeobox genes(5). Her e we show, using laser ablation of neurons and whole-cell patch-clamp elect rophysiology, that the asymmetries between ASEL and ASER extend to the func tional level. ASEL is primarily sensitive to sodium, whereas ASER is primar ily sensitive to chloride and potassium. Furthermore, we find that lim-6 is required for this functional asymmetry and for the ability to distinguish sodium from chloride. Thus, a homeobox gene increases the representational capacity of the nervous system by establishing asymmetric functions in a bi laterally symmetrical neuron pair.