Acknowledgements

Special thanks to Steve Salser, who coded hundreds of mating assay plates. Thanks are also due to many other members of the Kenyon lab for discussions, suggestions, strains and coding of mating assays. Thanks to David Bentley, Bob Horvitz, and Gian Garriga for their comments on this manuscript. Thanks to Gian Garriga for providing the strain containing the mutation bas-1(ad446) and for suggestions regarding individual worm anti-serotonin staining. Thanks to Cori Bargmann for strains and suggestions. We thank the following persons for sharing their initial observations that led us to do these experiments and for other unpublished information that aided us: G. Garriga, M. Finney, C. Desai, S. McIntire, H. R. Horvitz, J. Lewis, A. Ruiz-Morales, J. Mancillas, J. Sulston, K. Liu and P. Sternberg. Some of the strains used were obtained from the Caenorhabditis Genetics Center, which is funded by the NIH National Centers for Research Resources. C.M.L. was supported by an NIH Postdoctoral Fellowship and Traineeship. Funding for this work also came from a Searle Scholarship and a Packard Foundation Fellowship to C.J.K. Thanks also to James McKerrow, Department of Pathology, UCSF, in whose laboratory the manuscript was completed.