Physiological responses to transient conditions may result in costly responses with little fitness benefits, and therefore, a trade-off must exist between the speed of response and the duration of exposure to new conditions. Here, using the puparia of an important insect disease vector, Glossina pallidipes, we examine this potential trade-off using a novel combination of an experimental approach and a population dynamics model. Specifically, we explore and dissect the interactions between plastic physiological responses, treatment-duration and -intensity using an experimental approach. We then integrate these experimental results from organismal water-balance data and their plastic responses into a population dynamics model to examine the potential relative fitness effects of simulated transient weather conditions on population growth rates. The results show evidence for the predicted trade-off for plasticity of water loss rate (WLR) and the duration of new environmental conditions. When altered environmental conditions lasted for longer durations, physiological responses could match the new environmental conditions, and this resulted in a lower WLR and lower rates of population decline. At shorter time-scales however, a mismatch between acclimation duration and physiological responses was reflected by reduced overall population growth rates. This may indicate a potential fitness cost due to insufficient time for physiological adjustments to take place. The outcomes of this work therefore suggest plastic water balance responses have both costs and benefits, and these depend on the time-scale and magnitude of variation in environmental conditions. These results are significant for understanding the evolution of plastic physiological responses and changes in population abundance in the context of environmental variability.
Environmental variability effects on Glossina
Glossina pallidipes (Diptera, Glossinidae) puparia were subjected to simulated weather front scenarios (also referred to as acclimations), involving varying humidity and temperature for different durations. We determined water loss rates (WLR, in µg H2O h-1) individually using conventional gravimetric methods by recording body mass on an electronic microbalance at two experimental time points after a 3-day acclimation or after a 5-day acclimation. Individual puparia were placed on cotton wool in separate, open, numbered 0.6 ml micro centrifuge (eppendorf) tubes randomly assigned into replicated plastic 100ml airtight vials, (volume = 166 cm3). Each vial contained six tubes (N = 6 puparia per vial), and each vial was replicated five times to give a total sample size of N = 30 puparia per treatment. Care was taken to ensure that all treatment groups were handled for the same duration during transfer from the climate chamber to the vials (~ 7 min per group), and spent a similar amount of time outside of the vials whilst being weighed (~ 15 min per group). Relative humidity and temperature were recorded during treatments using Thermochron iButtons (DS1402D-DR8, Philippines; sampling rate = 10 min, ± 1 °C temperature accuracy, ± 0.5 % r.h. accuracy). Hygrostatic solutions were used to control the different humidities. Saturated solutions of magnesium chloride (MgCl2.6H2O), magnesium nitrate (Mg(NO3)2.6H2O) and sodium chloride (NaCl) were employed to correspond to humidities of 33 %, 55 % and 76 % r.h., respectively, and the same batch of each solution was used across all 5 vials to ensure similar conditions between replicates. Filtered, doubly-distilled water was always used as the solvent for production of saturated salt solutions. Silica gel was used for completely desiccating conditions (<5 %, referred to as 0 % r.h.) and double-distilled water for fully hydrated air: 95 – 100 % (referred to as 99 % r.h.). To test the effect of acclimation duration on WLR, we measured WLR over a five-day period at 25 °C, 76 % r.h. after exposure to acclimations (combinations of temperature and r.h.) lasting for different durations. The first experimental group was subjected to acclimation conditions for three days (15 temperature and r.h. combinations, N = 30 individuals per combination), while the second experimental group was subjected to acclimation conditions for five days (15 temperature and r.h. combinations, N = 30 individuals per combination). Acclimations included a combination of three temperatures (21, 25 and 29 °C) and five relative humidities (0, 33, 55, 76 and 99 % r.h.) resulting in a total N = 15 acclimations per three and five day exposure. After we measured WLR, we scored the time to eclosion (in days) and survival (%) at 25 °C, 76 % r.h. Given that the conditions for optimal population growth rates are already well established for constant stable conditions we compare all our results to this baseline condition (referred to as optimal rearing conditions throughout) as a relative improvement or decline for each treatment. The simulated experimental changes in thermal and hygric conditions allowed us to explore the impacts of plastic responses on population growth rates and relative changes thereof. Population growth rates are calculated as a function of the duration of the puparial life-stage (pupal eclosion rate) and pupal survival. The Euler-Lotka equation has been used in several studies of population dynamics, relating the growth rate of a population (r) to age-dependent fecundity and mortality. The puparial life-stage lasts for a duration τa (in days) during which time the daily survivorship is described by σa. In our treatments we estimated τa under a range of environmental conditions varying in both temperature and humidity and use the estimated fraction survival minus the 1 % constant predation rate for tsetse puparia as an estimate of σa.
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