Figure 8.

Model of the dynamic control of body posture by peristaltic waves of muscle contractions in Drosophila embryos. A, Effect of contraction waves on body torsion. Type 1 and type 2 waves have opposite effects on posture, inducing right-handed and left-handed torsion, respectively. These effects result from differential embryo rolling within the eggshell. Upon wave propagation, embryos roll in a chiral manner, while displacement of individual segments depends on the strength of muscle contractions. Type 1 waves induce right-handed torsion because these contractions are on average stronger at their point of initiation (solid vertical blue and green arrows of different thickness represent propagating contractions of varied strength). This causes an accumulation of torsion over multiple waves, regardless of direction of wave propagation (empty horizontal blue and green arrows represent distance rolled, red boxes show overall segment displacement). Type 2 waves include two phases that correspond to contractions propagating in opposite directions. Type 2 waves induce left-handed torsion because first-phase and second-phase contractions cancel the effect of each other in the posterior but not the anterior part of the embryo, where the second phase induces stronger rolling. B, Control of body posture by sensory feedback. To maintain straight posture, sensory neurons send feedback to the CNS to control the pattern of alternating type 1–type 2 waves. In POMT mutants, sensory axon termini are abnormal, which affects sensory circuit assembly, causing aberrant feedback from sensory neurons. This compromises the control of the wave pattern and results in an excessive proportion of type 2 waves, which leads to body torsion.