The kakapo mutation affects terminal arborization and central dendritic sprouting of Drosophila motorneurons

The lethal mutation l(2)CA4 causes specific defects in local growth of neur onal processes. We uncovered four alleles of l(2)CA4 and mapped it to bands 50A-C on the polytene chromosomes and found it to be allelic to kakapo (Pr out et al. 1997. Genetics. 146:275-285). In embryos carrying our kakapo mut ant alleles, motorneurons form correct nerve branches, showing that long di stance growth of neuronal processes is unaffected. However, neuromuscular j unctions (NMJs) fail to form normal local arbors on their target muscles an d are significantly reduced in size. In agreement with this finding, antibo dies against kakapo (Gregory and Brown. 1998. J. Cell Biol. 143:1271-1282) detect a specific epitope at all or most Drosophila NMJs. Within the centra l nervous system of kakapo mutant embryos, neuronal dendrites of the RP3 mo torneuron form at correct positions, but are significantly reduced in size. At the subcellular level we demonstrate two phenotypes potentially respons ible for the defects in neuronal branching: first, transmembrane proteins, which can play important roles in neuronal growth regulation, are incorrect ly localized along neuronal processes. Second, microtubules play an importa nt role in neuronal growth, and kakapo appears to be required for their org anization in certain ectodermal cells: On the one hand, kakapo mutant embry os exhibit impaired microtubule organization within epidermal cells leading to detachment of muscles from the cuticle. On the other, a specific type o f sensory neuron (scolopidial neurons) shows defects in microtubule organiz ation and detaches from its support cells.