The gene mod(mdg4) affects synapse specificity and structure in Drosophila

The mechanisms by which synapse assembly and maturation;are orchestrated du ring development are largely unknown. We used P-element mutagenesis and a l arval anatomical screen to isolate mutants in which synapse structure was a ltered. Here, we describe a mutation isolated with this screen, branch poin t disrupted (bpd), in which both synapse specificity and synapse morphology were altered. Synaptic terminals in bpd mutants developed abnormally, form ing multiple branch points, overgrowing to inappropriate neighboring muscle s, and establishing aberrant folding of postsynaptic membranes. Ultrastruct ural characterization of synaptic boutons in bpd demonstrated abnormal laye ring of the postsynaptic specialization or subsynaptic reticulum (SSR). Gen etic and molecular analyses revealed that bpd is an allele of mod(mdg4), a gene coding for a protein with many similarities to transcription factors, which has been implicated in the regulation of chromatin insulation. Our re sults suggest that mod(mdg4) may regulate a gene(s) essential to normal syn apse formation. (C) 1999 John Wiley & Sons, Inc.