Pulver, S. | Search bioRxiv

bioRxiv2026

Promotion of Structured Motor Program Diversity Through Since-last-state Memory in Drosophila Larvae

Motor systems controlling locomotion must generate repetitive rhythmic activity, while also still retaining the ability to generate a diverse range of outputs. How motor systems monitor, regulate, and promote diversity of their own outputs is not well understood. Here, we perform single-step, variable-order and hidden-state Markov modelling (HSMM) on spontaneous fictive locomotor activity in the isolated Drosophila larval nervous system to examine how a motor system balances constraint and promotion of diversity amongst competing motor programs. We show that spontaneous fictive activity is structured by interacting mechanisms operating at multiple levels of sequence organisation. Analysis of one-step transition rules revealed a bias in activity towards activity states underlying exploration that in turn, promote transition to diverse outputs. In contrast, higher-order Markov, N-gram, and HSMM analysis indicated a memory biased towards revisiting recently executed motor programs. These mechanisms together suggest that the Drosophila larval locomotor system maintains a dynamic repertoire of possible motor outputs by monitoring recent activity and biasing future transitions accordingly. In this sense, fictive rhythmogenesis reflects a diversity-generating process: the larval locomotor network does not simply repeat a fixed motor programme or randomly transition from one state to another, but rather continually regulates access to rhythmic states based on recent experience. Together, these findings suggest that fictive locomotor dynamics are consistent with adaptive winner-takes-all competition between central pattern generating (CPG) modules that balance constraint and promotion of motor program diversity.

neuroscience↗

bioRxiv2026

Optogenetic Control of Activity in Descending Tdc2+ Neurons Modulates Motor Program Bias in the Drosophila Larval Locomotor System

Motor systems must flexibly select between competing outputs while preserving stability of rhythmic outputs. In Drosophila larvae, the isolated central nervous system is capable of maintaining rhythmicity by generating multiple different fictive motor programs. The biogenic amines octopamine and tyramine are known to regulate larval locomotion, however, how the tdc2+ octopaminergic/tyraminergic system regulates motor program competition is not well understood. Here, we combine dual-colour calcium imaging and optogenetic manipulation to explore how tdc2+ neurons track, permit, and bias fictive motor activity in 3rd instar Drosophila larvae. We find that tdc2+ activity in the larval ventral nerve cord is strongly coupled to motor neuron activity across multiple fictive behaviours, indicating that the system is recruited broadly across the motor repertoire. Optogenetic depolarisation of tdc2+ neurons increases motor root bursting and induces a robust fictive forward bias, whereas optogenetic hyperpolarisation suppresses or abolishes fictive rhythms and generates a short-lasting, post-inhibitory rebound in fictive activity. Spatially-restricted stimulation reveals that posterior abdominal activation is especially effective at promoting fictive forward activity. Separating VNC-residing from brain-residing tdc2+ populations further shows that activation of descending brain-residing tdc2+ projections is sufficient to recapitulate this forward bias. Finally, tdc2+ activation induces short-lived post-stimulation changes in motor programme probability, including transient elevation of competing fictive backward instantaneous frequency. Together, these findings suggest that tdc2+ neurons act as a permissive and biasing modulatory layer within larval motor circuits, linking adrenergic-like signalling to motor programme competition.

neuroscience↗