An important concept in ethology is that complex behaviors can be constructed from a set of basic motor patterns. Identifying the set of patterns available to an animal is key to making quantitative descriptions of behavior that reflect the underlying motor system organization. We addressed these questions in zebrafish larvae, which swim in bouts that are naturally segmented in time. We developed a robust and general purpose clustering method (clusterdv) to ensure accurate identification of movement clusters and applied it to a dataset consisting of millions of swim bouts, captured at high temporal resolution from a comprehensive set of behavioral contexts. We identified a set of thirteen basic swimming patterns that are used flexibly in various combinations across different behavioral contexts and show that this classification can be used to dissect the sensorimotor transformations underlying larval social behavior and hunting. Furthermore, using the same approach at different levels in the behavioral hierarchy, we show that the set of swim bouts are themselves constructed from a basic set of tail movements and that bouts are executed in sequences specific to different behaviors.

Highlights d Long-term in vivo imaging of dendrite differentiation with automated quantitation d Anterograde-polymerizing microtubules subdivide the dendrite tip to create branches d Actin motor Myosin6 stabilizes filopodia-derived actin tails to guide microtubules d Transcription factor Knot regulates Myosin6 for dendrite arbor diversification

Background: During segmentation of the zebrafish embryo, inside-out signaling activates Integrin a5, which is necessary for somite border morphogenesis. The direct activator of Integrin a5 during this process is unknown. One candidate is Rap1b, a small monomeric GTPase implicated in Integrin activation in the immune system. Results: Knockdown of rap1b, or overexpression of a dominant negative rap1b, causes a mild axis elongation defect in zebrafish. However, disruption of rap1b function in integrin a5 2/2 mutants results in a strong reduction in Fibronectin (FN) matrix assembly in the paraxial mesoderm and a failure in somite border morphogenesis along the entire anterior-posterior axis. Somite patterning appears unaffected, as her1 oscillations are maintained in single and double morphants/mutants, but somite polarity is gradually lost in itga5 2/2 ; rap1b MO embryos. Conclusions: In itga5 2/2 mutants, rap1b is required for proper somite border morphogenesis in zebrafish. The loss of somite borders is not a result of aberrant segmental patterning. Rather, somite boundary formation initiates but is not completed, due to the failure to assemble FN matrix along the nascent boundary. We propose a model in which Rap1b activates Integrin/Fibronectin receptors as part of an "inside-out" signaling pathway that promotes Integrin binding to FN, FN matrix assembly, and subsequent stabilization of morphological somite boundaries.

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