Tracking Cell Movement in Two‐Dimensional, Fragmented Microcosms Reveals Dispersal Syndromes and Strategies in Tetrahymena thermophila

TL;DR

Researchers studied how Tetrahymena thermophila ciliates move in fragmented environments, finding that they use different movement strategies during dispersal phases.

Contribution

The study introduces a novel method to track dispersal strategies in fragmented microcosms using continuous movement data across dispersal phases.

Findings

Two dispersal strategies were identified based on changes in swimming speed and linearity during transience.

Doubling corridor length affected dispersal timing but not emigration rates.

Immigration generally reduced movement traits, contradicting simulated predictions.

Abstract

A major challenge in dispersal ecology consists of testing whether distinct sets of phenotypic traits are associated with the three main phases of dispersal, requiring direct observations of disperser movements during emigration, transience, and immigration. Although freshwater ciliates have been used as a model in artificial dispersal landscapes for over 15 years, most studies would relate dispersal propensity to phenotypic traits measured at the end of dispersal assays. Using ‘two‐dimensional’ fragmented microcosms, abundance, movement and morphology data of Tetrahymena thermophila were collected at numerous time points throughout 6.5 h‐long dispersal assays. Data were compared across distinct zones (‘Start’ and ‘Target’ patches, connected by a ‘Corridor’) to identify shifts in the mean value and distribution of dispersal‐related traits. Inference on the existence of dispersal decisions was obtained by comparing these results to similar outputs generated by a ‘null’ movement model (without decision rules). Five genotypes were used, among which two strategies were identified: swimming speed and linearity either increased (‘hump’) or decreased (‘slope’) during transience, while both traits generally decreased at immigration. Doubling the length of corridors (10 mm vs. 20 mm) modified dispersal timing, but did not affect emigration rates. Simulated data predicted a shift towards increased velocity at immigration; however, the opposite was found in most strains, suggesting a plastic inducement of typical foraging movements after settling in the ‘Target’ patch. Since a ‘snapshot’ approach was used (capturing sparse movement sequences throughout the dispersal process instead of prolonged tracking), phenotypic plasticity could not be confirmed with certainty; however, the hypothesis of strict spatial sorting was insufficient to explain movement patterns. Overall, our results hint at the plastic and reversible nature of dispersal syndromes displayed by
T. thermophila
across fragmented landscapes, which bears significance in the context of habitat loss and the maintenance of metapopulation stability.

Dispersal strategies of the ciliate Tetrahymena thermophila were investigated using ‘two‐dimensional’ artificial landscapes, allowing for camera tracking of individual cell trajectories during all steps of the dispersal process. Strains showed distinct strategies at emigration, either increasing or decreasing movement traits (swimming speed and linearity) in the corridor; all genotypes decreased those traits at immigration. Doubling the length of corridor modified the timing of dispersal, but did not affect emigration rates.

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