From bristle to brain: embryonic development of labeled lines in the antennal nervous system of the locust Schistocerca gregaria

Abstract

The antennal flagellum of the locust S. gregaria is an articulated structure bearing a spectrum of sensory sensilla that encode environmental stimuli mediating adaptive behavior. In this study we focus on the basiconic-type bristles as a model for sensory system development in the antenna. At the end of embryogenesis these bristles are found at conserved locations on each apical segment of the flagellum, are innervated by a dendrite from a sensory cell cluster in the underlying epithelium with each cluster directing an axon topographically to an antennal tract and the brain. We employ confocal imaging and immunolabeling to (a) identify mitotically active sense organ precursors for sensory cell clusters in the most apical annuli of the early embryonic antenna; (b) follow the subsequent differentiation of their neuronal progeny; and (c) map the spatial and temporal organization of axonal projections into the antennal tracts. We show that early in embryogenesis proliferative precursors are localized circumferentially within discrete epithelial domains of the flagellum. Differentiation of their clonal progeny begins apically and proceeds in a basal direction along the antenna so that the oldest neuronal populations are near the antennal tip with progressively younger populations located more basally. Autotracing reveals that axon fasciculation with a tract is sequential and reflects the location and age of the cell cluster along the flagellum. Cell cluster location and bristle location on the cuticle are therefore encoded topographically and temporally within the axon profile of the tract and its projection to the brain.