Drosophila P[Gal4] lines reveal that motor neurons involved in feeding persist through metamorphosis.

Two P[Gal4] insertion lines in Drosophila melanogaster, MT11 and MT26, express GAL4 specifically in two to three pairs of pharyngeal motor neurons (PMN) in the suboesophageal ganglion. By using various secondary reporters, the architecture of the PMN, including their efferent axons in the pharyngeal nerve, was visualized. This allowed us to identify a pharyngeal dilator muscle as their target. To study the function of these neurons, we crossed line MT11 with a UAS-tetanus toxin gene construct (TNT-C) that inhibits all synaptic transmission. The offspring shows a reduction in food ingestion of 75% compared to the MT11 and TNT-C controls, demonstrating that PMN control food uptake. More important, lines MT11 and MT26 enabled us to follow PMN and their processes through metamorphosis, since labeling appears in the late third larval instar and persists up to adulthood. The motor axons innervate a pharyngeal muscle in the larva as well and extend through the maxillary nerve, proving that this nerve is homologous to the adult pharyngeal nerve. Efferent arborizations persist throughout metamorphosis, even though the larval muscle histolyzes by 20% of pupal life. Yet, some dedifferentiated structures remain, which may serve as a template for the formation of the adult muscle. Labeling of line MT26 with bromodeoxyuridine at embryonic or larval stages suggests that these neurons undergo their terminal mitosis in the mid to late embryo.