Emergent collective behaviour is observed in dynein-driven microtubules and modelled by taking into account only local interactions and the reptation-like motion of individual microtubules. Spontaneous collective motion is of interest in many scientific fields, from animal behaviour to cell dynamics. Motility assays provide a system in which the underlying physical principles can be studied, using protein filaments driven by molecular motors grafted to a substrate in the presence of ATP. Here, Sumino et al. report on experiments in which microtubules are propelled by surface-bound dyneins, and are seen to self-organize into large-scale vortices. This process can be explained by a surprisingly simple mathematical model, based on only the smooth motion of single microtubules and their local interaction (they align with high probability on collision). As well as potentially being relevant to biological systems, the study hints at the existence of a new universality class of collective-motion or active-matter phenomena. Spontaneous collective motion, as in some flocks of bird and schools of fish, is an example of an emergent phenomenon. Such phenomena are at present of great interest1,2,3,4,5 and physicists have put forward a number of theoretical results that so far lack experimental verification6,7,8. In animal behaviour studies, large-scale data collection is now technologically possible, but data are still scarce and arise from observations rather than controlled experiments. Multicellular biological systems, such as bacterial colonies or tissues9,10, allow more control, but may have many hidden variables and interactions, hindering proper tests of theoretical ideas. However, in systems on the subcellular scale such tests may be possible, particularly in in vitro experiments with only few purified components11,12,13. Motility assays, in which protein filaments are driven by molecular motors grafted to a substrate in the presence of ATP, can show collective motion for high densities of motors and attached filaments. This was demonstrated recently for the actomyosin system14,15, but a complete understanding of the mechanisms at work is still lacking. Here we report experiments in which microtubules are propelled by surface-bound dyneins. In this system it is possible to study the local interaction: we find that colliding microtubules align with each other with high probability. At high densities, this alignment results in self-organization of the microtubules, which are on average 15 µm long, into vortices with diameters of around 400 µm. Inside the vortices, the microtubules circulate both clockwise and anticlockwise. On longer timescales, the vortices form a lattice structure. The emergence of these structures, as verified by a mathematical model, is the result of the smooth, reptation-like motion of single microtubules in combination with local interactions (the nematic alignment due to collisions)—there is no need for long-range interactions. Apart from its potential relevance to cortical arrays in plant cells16,17 and other biological situations, our study provides evidence for the existence of previously unsuspected universality classes of collective motion phenomena.

Large-scale vortex lattice emerging from collectively moving microtubules