Agenda

Ciliary locomotion relies on local cellular components to generate coordinated movement, a strategy conserved from unicellular eukaryotes to metazoans. In most animals, the coordination and direction of ciliary locomotion are governed by the spatial organisation of basal bodies anchoring cilia at the apical cell surface. During development, body axes and tissue architecture drive the establishment of basal body orientation. However, Placozoa, an early-diverging lineage of animals, lacking both defined body axes and fixed shape, exhibit efficient ciliary gliding through the collective beating of cilia covering their lower epithelium. This raises questions about the establishment of basal body polarity and ciliary coordination in the absence of directional cues. In my thesis, using Trichoplax sp. H2 as a placozoan model, I combined co-immunostaining and quantitative analyses to characterise the organisation of basal body polarity. My results revealed that basal body polarity in the lower epithelium is coordinated with the global direction of movement, during locomotion and body shape changes. Furthermore, bisection of live animals causes each half to move away from each other in a stereotyped manner, and the analyses of their basal body polarity showed a fast reorganisation of polarity in opposite directions in each half. Finally, pharmacological inhibition of calcium signalling, known to mediate rapid cellular responses, indicated that the proper mechanoresponsive reorientation of basal bodies is dependent on calcium activity. In conclusion, these results demonstrate that basal body polarity and coordinated ciliary locomotion can emerge and persist independently of fixed body axes, relying instead on mechanochemical cues, and provide new insights into basal body polarity properties and the fundamental mechanisms underlying collective motility in early-diverging animal lineages.