Fig. 5. Restricted expression of beat Ic mRNA is required for normal ISNb development. (A,B) Embryonic mRNA in situ using beat Ia or beat Ic, double-labeled with mAb BP102, which labels all CNS axons. Anterior is towards the left. At stage embryonic 13, beat Ia is expressed strongly in all motoneurons (A), while beat Ic is expressed in a much more restricted subset of neurons (B). In particular, very weak beat Ic expression can be seen in the aCC motoneuron (diagonal arrow) and no beat Ic expression can be detected in the RP motoneurons (vertical arrow). (C) Two abdominal segments of a wild-type stage 16 embryo, stained with anti-FasII mAb 1D4 to visualize motoneurons. At this stage, the axons that compose the ISNb have separated and spread out to innervate their appropriate muscle targets. In particular, the RP3 axon can be seen innervating the cleft between muscles 6 and 7 (arrows). (D) Two abdominal segments from a similarly-prepared elav-GAL4; UAS-beat Ic embryo. Increasing the levels of Beat Ic causes axons to tangle and stall as they enter their target muscle field (large arrow). Motoneuron RP3 is often seen to be delayed in growing into the cleft between muscles 6 and 7 (small arrow).
