Genetic analysis of rhythmic behavior in C. elegans

Abstract

How do genetic networks control the behavior of an organism? To approach this problem, I chose the model organism C. elegans, a simple metazoan that displays several easily observed behavioral programs such as locomotion, egg-laying, and defecation. C. elegans defecation is a rhythmic behavior regulated by the intestine, and consists of three muscle contractions occurring at regular intervals. My results suggest that chemosensation and external mechanosensation play no role in regulating the defecation cycle. However, conditions affecting the metabolism of the animal (e.g. starvation, poor food quality, defects in genes important for basic cellular functions) lead to alterations in the cycle period. In addition, I present evidence suggesting that the amount of food consumed is sensed by internal mechanosensors.Several mutants had been previously isolated for their effects on the defecation rhythm, but in most cases their molecular identity was unknown. One of these mutants is dec-2(sa89), which causes a long defecation period. My work reveals that dec-2 encodes a novel secreted protein expressed exclusively in the hypodermis, in contrast to other Dec genes, all of which function in the intestine.dec-2(sa89) is allelic to osm-7(n1515), and both mutations cause resistance to osmotic stress. I show that adaptation to osmotic stress alters the defecation behavior of wild-type C. elegans to the same extent observed in osm-7/dec-2 mutants. C. elegans adapts to osmotic stress by increasing glycerol production, and I find that osm-7/dec-2 mutants have high basal levels of glycerol in the absence of osmotic stress. I have also identified several other genes with mutant phenotypes similar to that of osm-7/dec-2 . These include osm-11, a gene I identified based on homology to osm-7/dec-2, and three collagen genes required for the formation of a substructure in the cuticle.These results lead me to propose a model in which osm-7 and osm-11 are secreted from the hypodermis, interact with the cuticle, and function as negative regulators of the response to osmotic stress. This hypothesis reveals interesting parallels with osmotic stress response in yeast, and future work on these mutants should provide insight into general mechanisms of stress resistance.