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Adaptive plasticity meets unpredictability: how do organisms cope with changing environmental variability?

Periodic Reporting for period 1 - THERMOPLAST (Adaptive plasticity meets unpredictability: how do organisms cope with changing environmental variability?)

Reporting period: 2016-01-01 to 2017-12-31

Species and populations tolerate a finite range of temperatures, the breadth of which is a critical determinant of their distribution and abundance. Natural populations can respond to changes in temperature in the short-term via non-genetic, ‘plastic’, mechanisms and over longer time periods by genetic adaptation. Both plastic and genetic responses to changes in the mean temperature are well understood. However, organismal responses to the variation and stochasticity in temperature that is inherent in natural environments are less known. Given that patterns of temperature variability are projected to become increasingly unpredictable in the near future, understanding how organisms will adapt to changing patterns of environmental variability is of vital importance because the previously adaptive responses of organisms to temperature change may be rendered ineffective.
Thus, understanding responses to such changes in environmental variability requires detailed knowledge about how organisms can detect levels and predictability of variation, how they can respond to this through non-genetic, plastic mechanisms and if there is potential for such ‘plasticity’ to evolve given its specific costs and benefits. In this project, using the widespread clonally reproducing zooplankter Daphnia magna, I therefore investigated plastic, non-genetic changes in thermal tolerance, the ability to maintain basic body function at high temperature, in response to different patterns of temperature variability. The major aims of this project were:
1.) Investigate if temperature variability can cause a plastic response in thermal tolerance
2.) Are plastic shifts in thermal tolerance fixed or flexible and does this type of plasticity correspond to natural patterns of temperature variability?
3.) Is there genetic variation in thermal tolerance plasticity and does this carry a physiological cost?
During the course of this fellowship, I worked with clonal lineages of Daphnia magna, meaning that all responses to the performed temperature manipulations represented true estimates of ‘plastic’ responses to environmental variation. To measure if temperature variability per se can alter an organism’s ability to tolerance periods of high temperature (Objective 1), we exposed a single clone of D. magna to the following temperature treatments, stable mean temperature, predictably varying temperature and unpredictably varying temperature. In each of these treatments, mean temperature was the same, and in the two variable treatments, the range and variance in temperature was the same, but critically the pattern in variation differed. In the predictable treatment, temperature cycled between the maximum and minimum temperature in a diurnal fashion, whereas in the unpredictable treatment, temperature could shift (or remain the same), between the maximum and minimum values or other intermediate temperatures in a random fashion. We observed that thermal tolerance differed between the stable mean treatment and the variable treatments, being higher in the latter. Thermal tolerance did not however differ between the predictable variable and the unpredictable variable treatments.
To measure if plastic shifts in thermal tolerance are fixed (i.e. permanent) or flexible, we reared replicate populations of a single D. magna clone at either a constant low temperature, or a constant high temperature. Upon reaching sexual maturity, individuals from the low temperature were transferred to the high temperature and vice versa. Control animals were maintained at the initial developmental temperatures. Following the shift in rearing temperature, treatment and control animals were measured for thermal tolerance after 24 hrs and then at set intervals for 13 days. I observed that thermal tolerance was adjusted rapidly, within 24 hours, being increased in animals that experienced an increase in ambient temperature and decreased in animals that experienced a reduction in ambient temperature, demonstrating that thermal tolerance is not fixed but highly and rapidly adjustable.
Due to delays that occurred that were beyond my control, work towards Objective 3 is underway at the time of writing this report. However, additional scientific highlights during the fellowship included the development of a method for automating the measurement of thermal tolerance. The method utilizes video-based tracking software to record the movement of individuals when exposed to high, lethal temperature. I developed an algorithm in the R computing language that can objectively identify the loss of locomotory function from tracking data analyzed from videos. Using independent experimental data I validated this approach by demonstrating the expected response in thermal tolerance to different acclimation temperatures. This method was used to perform all measures of thermal tolerance conducted in this fellowship.
Research conducted during the tenure of this fellowship has advanced current knowledge of how organisms respond to different patterns of variation in the environment as opposed to changes in constant mean values. Moreover it has shown that organismal responses to environmental change can be flexible and not fixed, both of which will aid our future understanding of how animals will cope with the projected changes in environment variability and predictability.