Abstract
This chapter discusses rhythm mutants and to a more limited extent clock molecules; including how several of the former led to examples of the latter, and how certain aspects of mutationally altered rhythms connect with studies revolving around the molecularly based control of the fly's biological oscillations. The most salient rhythm variants in Drosophila are the period mutants. The chapter concludes that one way to extend the knowledge of and insights about the circadian processes are to continue efforts in the area of pure chronogenetics: to isolate novel rhythm mutants and determine whether the genes specify suspected parts of the pacemaker machinery or whether some of the newly identified loci encode heretofore unanticipated molecular contributors. This kind of clock gene hunting, by mutations and position cloning, is how molecular chronobiology started is discussed.