Parasegmental Organisation of the Motor System

<div><p>(A) Distribution of motor neuron dendritic arbors relative to the domains of <i>en</i> expression. Neurons expressing the <i>en</i> gene were visualised (blue) using <i>en-GAL4;UAS-CD8-GFP</i>. ISN motor neuron dendrites (red) elaborate in the En domain (blue) of the neuromere, whereas SN motor neuron dendrites (green) form in the anterior half of the next posterior segment (asterisks indicate the segment borders). Thus, the motor system appears to be parasegmental in nature. The diagrams to the right indicate which motor neurons were labelled.</p> <p>(B–H) ISN motor neurons (red) with dorsal internal and SN motor neurons (green) with lateral external muscle targets were retrogradely labelled in 15-h-old wild-type embryos (B) and those mutant for different segment polarity genes (C–H). (C) to (H) should be compared with the wild-type control in (B). As far as could be ascertained, similar sets of motor neurons were labelled in the wild-type (B) and mutants (C–H). The neuropile, visualised with anti-HRP, is shown in blue (except for [F]). In all mutant embryos, with the exception of <i>Df(gsb)</i> (H), ISN and SN motor neurons have separate nerve roots and dendritic fields, as in the wild-type. As (H) shows, in <i>Df(gsb)</i> mutant embryos, ISN and SN nerve roots are frequently fused, yet the respective dendritic fields (arrows) do not appear to intermingle.</p> <p>Anterior is left and dorsal is up. Symbols and abbreviations: triangles, ventral midline; AC, anterior commissure; PC, posterior commissure (RP2 cell bodies, the most posterior of the ISN motor neurons, are indicated in [C] and [E]); asterisks, dorsoventral channels (landmarks for the segment borders). Scale bar (not applicable to diagrams of CNS and muscle field): 10 μm.</p>