Development and morphology of the clock-gene-expreasing 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 (LN_d), the large ventral lateral neurons (1-LN_v), and the small ventral lateral neurons (s-LN_v), 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-LN_V and the PDF-positive s-LN_v 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-LN_v and LN _d are anatomically well suited to connect the accessory medullae. Whereas the 1-LN_v may receive ipsilateral photic input from the Hofbauer-Buchner eyelet, the LN_d invade mainly the contralateral accessory medulla and thus may receive photic input from the contralateral side. Both the LN_d and the 1-LN_v differentiate during midmetamorphosis. They do so in close proximity to one another and the fifth PDF-negative s-LN_v, suggesting that these cell groups may derive from common precursors.