Temperature mediated transgenerational plasticity influences movement behaviour in the green algae Chlamydomonas reinhardtii

Background/Question/Methods

To survive and reproduce organisms must be able to sense and respond to their environment across space and time. Movement is a key mechanism by which organisms are able to do this amidst directional environmental warming. However, how behavioural plasticity acts as a buffer in novel thermal environments, especially across multiple generations remains underexplored. Our research focuses on how transgenerational plasticity (TGP) affects the movement behaviour of the motile green algae Chlamydomonas reinhardtii in response to thermal perturbations. We first acclimated populations in thermal environments well below, near and above temperatures that maximize fitness (12.5OC, 25OC, 37.5OC). Then performing a pairwise cross and assaying micro-scale movement behaviour across two weeks to determine how many generations are affected by historic environments in novel acute environments. Next, we conducted a macro-scale experiment to explore whether micro-scale responses affect the ability of populations to navigate thermally heterogeneous environments. We established a horizontal water gradient ranging from ~32 to ~24.5℃. Populations were acclimated to 12.5OC, 25OC or 37.5OC and were inoculated into the warmest end of the gradient on days 0, 1 and 4 after populations were transferred to 37.5℃, and dispersal was measured for each acclimation history.

Results/Conclusions

Our results indicate that micro-scale movement behaviour is affected by the original acclimation environment for up to 10 generations following a thermal perturbation. Populations acclimated to 37.5OC initially had the lowest mean speed (28.81 μm·s-1), the highest proportion of moving particles (51.4%), and greatest absolute turning angle (indicating highly non-linear movement; 1.77 radians), while 25OC acclimated populations initially had the highest mean speed (48.49 μm·s-1). Populations acclimated to 12.5 and 37.5OC moved into the 25OC environment exhibited the most sustained behavioural TGP as it took six days (~10-15 generations) for the traits to converge on the 25OC acclimated values. A community trajectory analysis also indicated that the trajectory by which these phenotypes converged on acclimated values can be highly non-linear. Populations that were fully acclimated exhibited stationary behaviours throughout the experiment, except for the proportion of moving particles measured in from the 37.5OC acclimated population, which decreased over time. Results from the macro-scale dispersal assay indicated that acclimation history can have an effect on movement patterns at ecologically meaningful spatial scales for phytoplankton. Collectively, these findings show that transgenerational behavioural plasticity can be a large determinant of future ecological processes in thermally dynamic environments.