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Johnston´s organ has been shown to act as an antennal auditory organ across a spectrum of insect species. In the hemimetabolous desert locust Schistocerca gregaria, Johnston´s organ must be functional on hatching and so develops in the pedicellar segment of the antenna during embryogenesis. Here we employ the epithelial cell marker Lachesin to identify the pedicellar domain of the early embryonic antenna, and then triple-label against Lachesin, the mitosis marker phosphohistone-3, and neuron-specific horseradish peroxidase to reveal the sense-organ precursors for Johnston´s organ and their lineages. Beginning with a single progenitor at approximately a third of embryogenesis, additional precursors subsequently appear in both the ventral and dorsal pedicellar domains, each generating a lineage or clone. Lineage locations are remarkably conserved across preparations and ages, consistent with the epithelium possessing an underlying topographic coordinate system which determines the cellular organization of Johnston´s organ. By mid-embryogenesis twelve lineages are arranged circumferentially in the pedicel as in the adult structure. Each sense-organ precursor is associated with a smaller mitotically active cell from which the neuronal complement of each clone may derive. Neuron numbers within a clone increase in discrete steps with age, are invariant between clones and across preparations of a given age. At mid-embryogenesis each clone comprises five cells consolidated into a tightly bound cartridge. A long scolopale extends apically from each cartridge to an insertion point in the epithelium, and bundled axons project basally towards the brain. We review data from Drosophila to suggest mechanisms which might regulate the developmental program of Johnston´s organ in the locust. |
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