Dendritic remodeling and growth of motoneurons during metamorphosis of Drosophila melanogaster.

Insect motoneurons display dramatic dendritic plasticity during metamorphosis. Many larval motoneurons survive to adulthood but undergo dendritic regression and outgrowth as they are incorporated into developing circuits. This study explores the dendritic remodeling and development of Drosophila motoneurons MN1-MN5, which innervate indirect flight muscles of the adult. MN1-MN5 are persistent larval neurons exhibiting two distinct metamorphic histories. MN1-MN4 are born in the embryo, innervate larval muscles, and undergo dendritic regression and regrowth during metamorphosis. MN5, which was identified through a combination of intracellular dye injection and retrograde staining at all stages, is also born embryonically but remains developmentally arrested until the onset of metamorphosis. In the larva, MN5 lacks dendrites, and its axon stops in the mesothoracic nerve without innervating a target muscle. It is dye coupled to the peripherally synapsing interneuron, which will become part of the giant fiber escape circuit of the adult fly. During pupal development, MN5 undergoes de novo dendritic growth and extension of its axon to innervate the developing target muscle. Its unique developmental history and identifiability make MN5 well suited for the study of dendritic growth using genetic and neurophysiological approaches.