High concentrations of dauer pheromone trigger entry of C. elegans larvae into the alternate dauer developmental stage. Dauer pheromone has been shown to be a complex mixture of structurally related ascarosides, and regulates both dauer formation and adult behaviors (Butcher, et. al., 2007, 2008;Srinivasan et. al., 2008;). Pheromone signals are sensed by chemosensory neurons, and are transduced to downregulate TGF-b and IGF/Insulin signaling, leading to inhibition of reproductive growth and ultimately, dauer arrest. We recently showed that the SRBC-64 and SRBC-66 serpentine receptors mediate dauer formation in response to a subset of ascarosides (see abtract by Kim et al). Both receptors are expressed in, and localized to, the sensory endings of the ASK chemosensory neuron type, and genetic ablation of the ASK neurons abolishes dauer formation in response to some, but not all identified ascarosides. The srbc subfamily is predicted to encode a total of 73 chemoreceptors (Thomas & Robertson, 2008), suggesting that one or more of these receptors may play a role in the responses to additional ascarosides. We are currently determining the expression patterns of all srbc chemoreceptor genes, and genetically ablating amphid neurons in order to identify additonal neurons and receptors required for pheromone sensation. Pheromone signals sensed by the ASK neurons must be transduced to the ASI neurons to downregulate TGF-b expression and promote dauer formation. This interneuronal communication is hypothesized to be mediated via neuroendocrine/peptide signaling. Previously, we and others had shown that pheromone also downregulates expression of the str-3
chemoreceptor gene in the ASI neurons, and that this downregulation requires srbc-64
and -66 (Nolan, et. al., 2002; Peckol, et. al., 1999; Kim, et al submitted). To identify genes required to transduce the pheromone signal within the ASK neurons, and from the ASK to the ASI neurons, we carried out a screen for mutants which fail to exhibit pheromone-mediated downregulation of str-3p
::gfp expression. This screen identified several promising mutants, which also exhibit defects in pheromone-regulated dauer formation. We are currently further characterizing these mutants, and identifying the affected genes via whole genome re-sequencing (Sarin, et al 2008). We expect that identification of the intra- and inter-neuronal signaling mechanisms required for pheromone signal transduction will provide a better understanding of how animals respond to complex environmental signals, and how these signals are integrated to result in the appropriate developmental and behavioral responses.