Development and morphology of the clock-gene-expressing lateral neurons of Drosophila melanogaster

The clock-gene-expressing lateral neurons are essential for the locomotor activity rhythm of Drosophila melanogaster. Traditionally, these neurons are divided into three groups: the dorsal lateral neurons (LNd), the large ventral lateral neurons (1-LNv), and the small ventral lateral neurons (s-LNv), whereby the latter group consists of four neurons that express the neuropeptide pigment-dispersing factor (PDF) and a fifth PDF-negative neuron. So far, only the 1-LNv and the PDF-positive s-LNv have been shown to project into the accessory medulla, a small neuropil that contains the circadian pacemaker center in several insects. We show here that the other lateral neurons also arborize in the accessory medulla, predominantly forming postsynaptic sites. Both the 1-LNv and LNd are anatomically well suited to connect the accessory medullae. Whereas the 1-LNv may receive ipsilateral photic input from the Hofbauer-Buchner eyelet, the LNd invade mainly the contralateral accessory medulla and thus may receive photic input from the contralateral side. Both the LNd and the 1-LNv differentiate during midmetamorphosis. They do so in close proximity to one another and the fifth PDF-negative s-LNv, suggesting that these cell groups may derive from common precursors.