SPATIAL AND TEMPORAL EXPRESSION OF THE PERIOD AND TIMELESS GENES IN THE DEVELOPING NERVOUS-SYSTEM OF DROSOPHILA - NEWLY IDENTIFIED PACEMAKER CANDIDATES AND NOVEL FEATURES OF CLOCK GENE-PRODUCT CYCLING

The circadian timekeeping system of Drosophila functions from the firs t larval instar(L1) onward but is not known to require the expression of clock genes in larvae. We show that period (per) and timeless (tim) are rhythmically expressed in several groups of neurons in the larval CNS both in light/dark cycles and in constant dark conditions. Among the clock gene-expressing cells there is a subset of the putative pace maker neurons, the ''lateral neurons'' (LNs), that have been analyzed mainly in adult flies. Like the adult LNs, the larval ones are also im munoreactive to a peptide called pigment-dispersing hormone. Their put ative dendritic trees were found to be in close proximity to the termi nals of the larval optic nerve Bolwig's nerve, possibly receiving phot ic input from the larval eyes. The LNs are the only larval cells that maintain a strong cycling in PER from L1 onward, throughout metamorpho sis and into adulthood. Therefore, they are the best candidates for be ing pacemaker neurons responsible for the larval ''time memory'' (infe rred from previous experiments). In addition to the LNs, a subset of t he larval dorsal neurons (DN(L)s) expresses per and tim. Intriguingly, two neurons of this DNL group cycle in PER and TIM immunoreactivity a lmost in antiphase to the other DN(L)s and to the LNs. Thus, the tempo ral expression of per and tim are regulated differentially in differen t cells. Furthermore, the light sensitivity associated with levels of the TIM protein is different from that in the heads of adult Drosophil a.