Drosophila hb9 specifies subsets of somato-motor neurons and interneurons and is required for the development of their projections. In Caenorhabditis and Drosophila, the brn3 orthologs unc86 and acj6 are essential for specification and axonal targeting of the primary touch and olfactory somatic-sensory neurons, respectively. Previous studies indicated that the brn3-hb9- phox2-based differential specification of the somatic and visceral neurons is conserved outside vertebrates, at least to some degree. Finally, in almost all sensory and motor neurons of the vertebrate viscero-reflex-circuits the paired-like homeodomain transcription factor phox2b is present, likewise controlling specification, differentiation and axonal connectivity. In somato-motor neurons the EGH homeodomain transcription factor hb9/mnx is active, controlling specification, migration and axonal projection. Subsets of somato-sensory neurons are specified by the POU domain factors brn3a/b/c, contributing to their development, axonal projection and survival. The vertebrate somatic-visceral ‘duality’ is reflected by the differential usage of specific homeodomain transcription factors that drive cell-type specification.
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