Dynamic analysis of larval locomotion in Drosophila chordotonal organ mutants

Abstract

Rhythmic movements, such as peristaltic contraction, are initiated by output from central pattern generator (CPG) networks in the CNS. These oscillatory networks elicit locomotion in the absence of external sensory or descending inputs, but CPG circuits produce more directed and behaviorally relevant movement via peripheral nervous system (PNS) input. Drosophila melanogaster larval locomotion results from patterned muscle contractions moving stereotypically along the body segments, but without PNS feedback, contraction of body segments is uncoordinated. We have dissected the role of a subset of mechanosensory neurons in the larval PNS, the chordotonal organs (chos), in providing sensory feedback to the locomotor CPG circuit with dias (Dynamic Image Analysis System) software. We analyzed mutants carrying cho mutations including atonal, a cho proneural gene, beethoven, a cho cilia class mutant, smetana and touch-insensitive larva B, two axonemal mutants, and 5D10, a weak cho mutant. All cho mutants have defects in gross path morphology compared to controls. These mutants exhibit increased frequency and duration of turning (decision-making) and reduced duration of linear locomotion. Furthermore, cho mutants affect locomotor parameters, including reduced average speed, direction change, and persistence. Dias analysis of peristaltic waves indicates that mutants exhibit reduced average speed, positive flow and negative flow, and increased stride period. Thus, cho sensilla are major proprioceptive components that underlie touch sensitivity, locomotion, and peristaltic contraction by providing sensory feedback to the locomotor CPG circuit in larvae.

Fig. 1.

Touch-insensitivity of cho mutants. Shown is the mean touch sensitivity histogram with standard deviation bars. *, Significant difference from 40AG13 by Dunnett's test (P < 0.05). n = 50 for all genotypes except 5D10, where n = 65. All cho mutants have strongly reduced touch sensitivity compared to 40AG13, whereas the additional control, yw, has normal touch sensitivity.

Fig. 2.

Morphological defects in cho mutants. Late-stage embryos were stained with α-HRP as detailed in Table 2. Shown are abdominal hemisegments of a control embryo (40AG13) (A) and a btv¹ mutant embryo (B). The btv mutants showed a high frequency of outer dendritic segment defects, especially the lack of a clear ciliary dilation (thin arrow). The inner dendritic segments, from the soma (thick arrow) to the basal bodies (arrowhead), appeared relatively normal in btv mutants compared to the 40AG13 controls. We found no obvious defects in the 5D10 mutants (not shown; see also Table 2), which are the weakest of our cho mutants.

Fig. 3.

Representative larval perimeter stacks for gross path morphology video recordings analyzed in dias. Controls have directional locomotion with infrequent turns and regular displacement of centroids. cho mutants exhibit gross defects in wandering behavior. These mutants have more frequent decision-making bouts (darker, overlapping perimeter stacks) and reduced centroid displacement. Most noticeably, ato and btv have “directionless” paths much more frequently than the axonemal class mutants (tilB and smet) and 5D10. The panels in this figure are set to varying scales to augment the presentation of path defects.