Local interneurons play essential roles in information processing by neural circuits. Here we present a comprehensive genetic, anatomical, and electrophysiological analysis of local interneurons (LNs) in the Drosophila antennal lobe, the first olfactory processing center in the brain. We find that LNs are diverse in their neurotransmitter profiles, connectivity, and physiological properties. Analysis of >1500 individual LNs reveals major morphological classes characterized by coarsely stereotyped glomerular innervation patterns. Some of these morphological classes exhibit distinct physiological properties. However, the finer-scale connectivity of an individual LN varies considerably across brains and there is notable physiological variability within each morphological or genetic class. Finally, we show that LN innervation requires interaction with olfactory receptor neurons during development, and some individual variability also likely reflects LN-LN interactions. Our results reveal an unexpected degree of complexity and individual variation in an invertebrate neural circuit, a result that creates challenges for solving the Drosophila connectome.

Muscles organise pseudo-crystalline arrays of actin, myosin and titin filaments to build force-producing sarcomeres. To study sarcomerogenesis, we have generated a transcriptomics resource of developing Drosophila flight muscles and identified 40 distinct expression profile clusters. Strikingly, most sarcomeric components group in two clusters, which are strongly induced after all myofibrils have been assembled, indicating a transcriptional transition during myofibrillogenesis. Following myofibril assembly, many short sarcomeres are added to each myofibril. Subsequently, all sarcomeres mature, reaching 1.5 µm diameter and 3.2 µm length and acquiring stretch-sensitivity. The efficient induction of the transcriptional transition during myofibrillogenesis, including the transcriptional boost of sarcomeric components, requires in part the transcriptional regulator Spalt major. As a consequence of Spalt knock-down, sarcomere maturation is defective and fibers fail to gain stretch-sensitivity. Together, this defines an ordered sarcomere morphogenesis process under precise transcriptional control – a concept that may also apply to vertebrate muscle or heart development.

In Drosophila, fibrillar flight muscles (IFMs) enable flight, while tubular muscles mediate other body movements. Here, we use RNA-sequencing and isoform-specific reporters to show that spalt major (salm) determines fibrillar muscle physiology by regulating transcription and alternative splicing of a large set of sarcomeric proteins. We identify the RNA-binding protein Arrest (Aret, Bruno) as downstream of salm. Aret shuttles between the cytoplasm and nuclei and is essential for myofibril maturation and sarcomere growth of IFMs. Molecularly, Aret regulates IFM-specific splicing of various salm-dependent sarcomeric targets, including Stretchin and wupA (TnI), and thus maintains muscle fiber integrity. As Aret and its sarcomeric targets are evolutionarily conserved, similar principles may regulate mammalian muscle morphogenesis.