Long-range actin-driven endosymbiont mobility in an animal-algal symbiosis

In animal-microbe symbioses, established symbionts are often thought to be confined to a niche and to lose motile appendages such as flagella. Whether this loss implies immobility within the host remains poorly understood. We report long-range, host-driven symbiont mobility in a dinoflagellate-acoel worm symbiosis. Using long-term tracking, fluorescence, and electron microscopy, we find that hundreds of dinoflagellate symbionts (Amphidinium sp., 10-20 μm in size) travel across the body of an acoel worm host (Waminoa sp.) at around 1 μm/s while confined inside a network of ultra-thin (∼200 nm) host parenchymal cells. We show this mobility is driven by host actin machinery, with symbionts moving faster than comparable actin-based transport systems despite confinement. We further show spatiotemporal regulation of symbiont distribution during diel rhythms and regeneration. By characterizing an active, host-cytoskeleton-driven mechanism of symbiont transport, our work expands our understanding of the regulatory strategies by which holobionts maintain dynamic homeostasis.