The emergence of patterned movement during late embryogenesis of Drosophila.

Larval behavioral patterns arise in a gradual fashion during late embryogenesis as the innervation of the somatic musculature and connectivity within the central nervous system develops. In this paper, we describe in a quantitative manner the maturation of behavioral patterns. Early movements are locally restricted "twitches" of the body wall, involving single segments or parts of segments. These twitches occur at a low frequency and have low amplitude, reflecting weak muscle contractions. Towards later stages twitches increase in frequency and amplitude and become integrated into coordinated movements of multiple segments. Most noticeable among these is the peristaltic wave of longitudinal segmental contractions by which the larva moves forward or backward. Besides becoming more complex as development proceeds, embryonic movements also acquire a pronounced rhythm. Thus, late embryonic movements occur in bursts, with phases of frequent movement separated by phases of no movement at all; early movements show no such periodicity. These data will serve as a baseline for future studies that address the function of embryonic lethal genes controlling neuronal connectivity and larval behavior. We have analyzed behavioral abnormalities in two embryonic lethal mutations with severe neural defects, tailless (tll), which lacks the protocerebrum, and glial cells missing (gcm), in which glial cells are absent. Our results reveal prominent alterations in embryonic motility for both of these mutations, indicating that the protocerebrum and glial cells play a crucial role in the neural mechanism controlling larval movement in Drosophila.