The physiological and behavioral activities of many animals are restricted to specific times of the day. The daily fluctuation in the mating activity of some insects is controlled by an endogenous clock, but the genetic mechanism that controls it remains unknown. Here we demonstrate that wild-type Drosophila melanogaster display a robust circadian rhythm in the mating activity, and that these rhythms are abolished in period-or timeless-null mutant flies (per 01 and tim 01 ). Circadian rhythms were lost when rhythm mutant females were paired with wild-type males, demonstrating that female mating activity is governed by clock genes. Furthermore, we detected an antiphasic relationship in the circadian rhythms of mating activity between D. melanogaster and its sibling species Drosophila simulans. Female-and species-specific circadian rhythms in the mating activity of Drosophila seem to cause reproductive isolation.M ost organisms show circadian 24-h rhythmicity in their behavior and physiology. Various behavioral phenomena in insects are controlled by an endogenous clock (1). A core oscillator mechanism of circadian rhythm and feedback loops, involving several clock genes including period (per) and timeless (tim), control locomotor activity and eclosion of the fruit fly, Drosophila melanogaster (2-6). Mutants of D. melanogaster with defective feedback loops (7-10) provide the means of studying the relationship between behavioral rhythms and circadian clock genes.Mating by animals is the most important and fundamental process to select the best partner and to produce progeny. Insects in particular show daily rhythms in mating activity (11)(12)(13)(14)(15)(16)(17), and these are controlled in some by an endogenous clock (14,15). Mating activity of the fruit fly Drosophila mercatorum shows the daily rhythms of the mating activity under 12-h͞12-h light͞dark (LD) cycles (16), and several Drosophila species show the daily rhythms of male courtship under LD cycles (17). However, the genetic mechanism(s) that modulates mating rhythm in these insects remains unknown.The present study shows that wild-type D. melanogaster display a robust circadian rhythm in mating activity that is governed by clock genes and that females are responsible for generating the mating rhythms. Furthermore, we found that the mating activities of disconnected (disco) mutants, which have a severe defect in the optic lobe and are missing lateral neurons, are arrhythmic. We also identified an antiphasic relationship in the circadian rhythms of the mating activity between D. melanogaster and its sibling species, Drosophila simulans. Materials and MethodsFly Strains. D. melanogaster wild-type strains (Canton-S and OGS-4, Ogasawara, Japan), rhythm mutants [period 01 (per 01 ), timeless 01 (tim 01 ), and disconnected 3 forked (disco 3 f )], transgenic flies (hsp-per s13) carrying heat shock (HS)-inducible per coding sequences (18), forked ( f ) mutant, and D. simulans wild-type strains (Og, Ogasawara, Japan; and Ots, Otsuki, Japan) were grown on glucose͞yea...

The cAMP-responsive transcription factor, CREB, is required for formation of long-term memory (LTM) in Drosophila melanogaster and regulates transcription of a circadian clock gene, period (per). Involvement of CREB both in LTM and circadian rhythm raises the possibility that per also plays a role in LTM. Assaying the experience-dependent courtship inhibition in male flies as a measure for LTM, we show here that per mutants are defective in LTM formation. This defect was rescued by induction of a wild-type per transgene in a per-null mutant, and overexpression of per enhanced LTM formation in the wild-type background. Furthermore, we found that synaptic transmission through per-expressing cells is most likely to be required during retrieval of LTM. In contrast, mutations in other clock genes (timeless, dClock, and cycle) did not affect LTM formation. Thus, independent of the core oscillator of circadian clock, per plays a key role in LTM formation.