Time flies for Drosophila.

Perhaps the greatest current challenge in Drosophila circadian research is linking the central pacemaker to the control of behavior, metabolism, or development. In an effort to understand how the pacemaker actually converts its molecular cycles into rhythmic behavioral output, recent research has focused on the neuropeptide pigment-dispersing factor gene (pdf) (Park and Hall 1998xPark, J.H and Hall, J.C. J. Biol. Rhythms. 1998; 13: 219–228Crossref | PubMedSee all ReferencesPark and Hall 1998). Renn et al. 1999xRenn, S.C.P, Park, J.H, Rosbash, M, Hall, J.C, and Taghert, P.H. Cell. 1999; 99: 791–802Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesRenn et al. 1999 examined the pdf gene for its potential as a circadian transmitter, and asked whether the ventral cluster of lateral neurons (LNvs), which are candidate pacemaker cells, is the location in which the function of the clock interfaces with output factors. Most LNv cells that express per also express neuropeptides related to the crustacean pigment dispersing hormone (β-PDH) which has pharmacological properties consistent with a circadian role in insects.Analysis of the null mutation for pdf, pdf01, demonstrates that pdf is indeed involved in the circadian output regulating rhythmic locomotor behavior in flies. Renn et al. report that flies lacking PDF are well entrained during LD conditions, although the evening peak in activity occurs ∼1 hr earlier than wild-type, and mutant animals do not anticipate the lights-on signal as well (Renn et al. 1999xRenn, S.C.P, Park, J.H, Rosbash, M, Hall, J.C, and Taghert, P.H. Cell. 1999; 99: 791–802Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesRenn et al. 1999). In free-running/DD conditions, 50%–98% of the flies become arrhythmic depending upon the genetic background (Renn et al. 1999xRenn, S.C.P, Park, J.H, Rosbash, M, Hall, J.C, and Taghert, P.H. Cell. 1999; 99: 791–802Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesRenn et al. 1999). Most of the flies maintain rhythmicity for the first 2–3 days of DD, which is lost with continued free-running conditions. Although PDF is expressed in LNvs, these neurons are normal in number and morphology in pdf01 mutant animals and the mutant phenotypes in pdf01 flies can be rescued with wild-type pdf (Renn et al. 1999xRenn, S.C.P, Park, J.H, Rosbash, M, Hall, J.C, and Taghert, P.H. Cell. 1999; 99: 791–802Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesRenn et al. 1999).What happens to circadian rhythms when these LNvs are missing from the animal? In flies lacking many or all PDF-expressing lateral neurons, locomotor activity phenotypes closely mimic the pdf01 mutant effects (Renn et al. 1999xRenn, S.C.P, Park, J.H, Rosbash, M, Hall, J.C, and Taghert, P.H. Cell. 1999; 99: 791–802Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesRenn et al. 1999). Intriguingly, both the ablated as well as pdf01 mutant animals retain some aspects of rhythmicity. This suggests that additional timekeeping mechanisms are present in neurons other than the LNvs.Recent research has shown that some of the clock genes affect pdf expression. Mutations in dClk or Cyc reduce PDF staining in the large LNvs (l-LNv), and disruption of vri oscillations also results in reduced PDF staining (Blau and Young 1999xBlau, J and Young, M.W. Cell. 1999; 99: 661–671Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesBlau and Young 1999). However, the level of pdf mRNA in l-LNvs is not affected in larvae continuously expressing vri, but it is absent in dClkJrk mutants (Blau and Young 1999xBlau, J and Young, M.W. Cell. 1999; 99: 661–671Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesBlau and Young 1999). Although these three clock proteins affect pdf expression, per and tim null mutations do not affect PDF levels (Blau and Young 1999xBlau, J and Young, M.W. Cell. 1999; 99: 661–671Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesBlau and Young 1999). How dCLK, CYC, and VRI regulate pdf expression remains to be elucidated. However, as pdf mRNAs levels do not appear to cycle (Park and Hall 1998xPark, J.H and Hall, J.C. J. Biol. Rhythms. 1998; 13: 219–228Crossref | PubMedSee all ReferencesPark and Hall 1998), PDF protein activity is likely to be cycling at some level in order for PDF to affect regulation of rhythmic locomotor activity.As we begin to unravel how the molecular clock signals to output relays, we must determine what are the physiological outputs of the circadian clock that might provide an adaptive advantage for the organism. Circadian rhythms in locomotor activity and emergence from the pupal case, eclosion, have been known for quite some time. However, identifying other biological processes the clock controls in flies has only just begun. Previous studies have shown that autonomous rhythms in per expression occur in Drosophila antennae as well as other tissues (8xHege, D.M, Stanewsky, R, Hall, J.C, and Giebultowicz, J.M. J. Biol. Rhythms. 1997; 12: 300–308CrossrefSee all References, 16xPlautz, J.D, Straume, M, Stanewsky, R, Jamison, C.F, Brandes, C, Dowse, H.B, Hall, J.C, and Kay, S.A. J. Biol. Rhythms. 1997; 12: 204–217Crossref | PubMedSee all References) and that almost every body part has a clock (Plautz et al. 1997axPlautz, J.D, Kaneko, M, Hall, J.C, and Kay, S.A. Science. 1997; 278: 1632–1635Crossref | PubMed | Scopus (452)See all ReferencesPlautz et al. 1997a). However, until recently there was no evidence that these peripheral oscillators are necessary for any behavioral or physiological response in flies. One group has now identified circadian rhythms in olfactory responses in flies, and has shown that this novel circadian response fails to occur when the antennae lack a functional clock. Krishnan et al. 1999xKrishnan, B, Dryer, S.E, and Hardin, P.E. Nature. 1999; 400: 375–378Crossref | PubMed | Scopus (200)See all ReferencesKrishnan et al. 1999 measured electroantennogram (EAG) responses to odorants in flies and found that LD-entrained wild-type flies gave EAG responses to ethyl acetate which were elevated at night. In per or tim null mutant flies, the EAG responses remained steady state. Similar EAG patterns were seen in DD and LD. Furthermore, this rhythm is not restricted to food odors since benzaldehyde, an avoidance odor, produced similar results (Krishnan et al. 1999xKrishnan, B, Dryer, S.E, and Hardin, P.E. Nature. 1999; 400: 375–378Crossref | PubMed | Scopus (200)See all ReferencesKrishnan et al. 1999). In per transgenic flies that have rhythmic per expression solely in the LNs, EAG responses were robust in DD but did not have a circadian rhythm. This implies that peripheral oscillators are necessary for circadian rhythms in olfactory responses. Olfaction is essential for acquiring food, predator avoidance, and social interactions including mating in many organisms. Therefore, by maintaining olfactory responses, peripheral clocks could have a profound effect on the ability of an animal to survive in the wild.The ubiquity of circadian clocks from cyanobacteria to humans implies that they have some intrinsic adaptive value. The discovery of homologs of Drosophila clock genes in humans provides one of the richest opportunities to explore the genetics of the behavior in man using a candidate gene approach. Understanding the molecular underpinnings of the clock from input to output in the amenable system of Drosophila will shed light on the workings of the clock and its influence on the behavior and physiology of more complex organisms.*To whom correspondence should be addressed (e-mail: stevek@scripps.edu).