Caenorhabditis elegans behavioral genetics: where are the knobs?
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OPINION
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Caenorhabditis elegans behavioral genetics: where are the knobs? Leon Avery*
Abstract Thousands of behavioral mutants of Caenorhabditis elegans have been studied. I suggest a set of criteria by which some genes important in the evolution of behavior might be recognized, and identify neuropeptide signaling pathways as candidates.
Evolution occurs by the accumulation of genetic changes. Behavior is under genetic control and evolves in response to selection. It is thought (at least this seems to be the view of prominent textbooks [1,2]) that behavior evolves in large part by changes in quantitative characteristics such as the intensity or frequency of a behavior, the thresholds for eliciting a particular response, or the relative timing of component actions. This implies the existence of genes whose sequences control the values of numbers that determine behavior. I like to think of an animal as a device with a complicated control panel - the genome - covered with buttons and switches and knobs the genes. Mutation and natural selection turn the knobs to adjust behavior so as to optimize fitness in the environment in which the animal finds itself. For almost 50 years geneticists studying the nematode worm Caenorhabditis elegans have been isolating and studying behavioral mutants. (See Box 1 for an explana tion of what I mean by ‘behavior’.) Does this work tell us anything about how behavior evolves? In particular, can it identify candidates for the knobs - genes whose sequences move the numbers that control behavior?
A brief overview of C. elegans behavioral genetics Behavioral genes have figured prominently in C. elegans genetics since Sydney Brenner began isolating mutants over 40 years ago. Of the 95 genes listed in Table 4 of Brenner’s first paper on the genetics of C. elegans, 57 affect nervous system function and behavior. (The others *Correspondence: [email protected] Department of Molecular Biology, University of Texas Southwestern Medical Center, 6000 Harry Hines Blvd, Dallas, TX 75390-9148, USA
affect morphology (29) or muscle contraction (9); another 5 of Brenner’s 100 genes are no longer thought to be distinct genes.) In part this is because Brenner and his postdocs and students were interested in the function of the nervous system, but it was also a consequence of technical constraints. Worms are morphologically simple. Unlike mice or flies, for instance, which bristle with external spikes, hairs and protuberances in a variety of shapes and colors, there is not much to see on the outside of a worm. The insides are visible and a little more complex, but most of the obvious features are too important to mess with in any serious way. In contrast, viable and visible behavioral mutants are easy to isolate and work with. Many behavioral abnor malities are obvious under dissecting microscope obser vation. And viable mutants are common because under laboratory conditions, hermaphrodites can survive and reproduce even with a largely nonfunctional nervous system. We now hav
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