Title: Mobility in Context: Thermotaxis in a Non-Neuronal Animal and Long-Range Endosymbiont Mobility in an Animal-Algal Symbiosis

Abstract: 

Model systems are tremendously useful in advancing our understanding of life on our planet, yet much of the existing biodiversity is not represented in this scientific understanding. My thesis leverages non-model organisms to answer curiosity-driven questions in the spaces of “neuroscience without neurons” and symbiosis, with a focus on mobility. 

In the first part of my thesis, we use an apolar, non-neuronal animal - Trichoplax adhaerens - to ask how an animal without a known nervous system or muscles coordinate millions of cells in responding to external stimuli. ​ Creating fine linear thermal gradients on custom instruments, we discover robust thermotaxis (sensitive to at least 0.1°C cm−1) that occurs over the timescale of hours. This is a surprising example of an apolar organism demonstrating thermotaxis, where the animal does not maintain a fixed orientation while performing thermotaxis. Using natural diversity in size of adult organisms (100 µm to a few millimetres), we find no apparent size-dependence in thermotaxis behaviour across an order of magnitude of organism size. Several transient receptor potential (TRP) family homologues have been previously reported to be conserved in metazoans, including in T. adhaerens. We discover naringenin, a known TRPM3 antagonist, inhibits thermotaxis in T. adhaerens. The discovery of robust thermotaxis in T. adhaerens provides a tractable handle to interrogate information processing in a brainless animal. 

In the second part of my thesis, I explore the role of mobility in animal-microbe symbiosis. Established symbionts are often thought to be confined to a niche and generally lose their motile appendages such as flagella. Whether this loss implies immobility within the animal host remains an open conundrum. We discover long-range, host-mediated 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 entire body of an acoel worm host (Waminoa sp.) at high velocities (around 1 𝝁𝒎/𝒔) while under high confinement inside a network of ultra-thin host parenchymal cells (∼200 𝒏𝒎 thin). We unveil the molecular mechanism of this mobility to be host actin machinery. We further reveal spatiotemporal regulation of symbiont distribution associated with diel rhythms and during regeneration. We establish the presence of host-mediated mobility in animal-microbe symbioses, which suggests the existence of previously overlooked regulatory processes in holobionts’ maintenance of dynamic homeostasis.

Zoom Link: Please contact Leyre Caracuel for the zoom link