Phenotypic plasticity influences the effects of UV-B and its interaction with other environmental factors

Abstract

UV-B can damage DNA, proteins and membrane directly and by reactive oxygen species (ROS) generation. Cells protect themselves from the negative effects of excess ROS by antioxidant defenses. However, both antioxidant enzyme activities and ROS generation are sensitive to ecological factors such as temperature and increased physical activity. So the effects of UV-B may vary in different environments and depend on the balance between ROS concentration and antioxidant activities. On the other hand, phenotype can be plastic and buffer physiology from environmental changes. Phenotype plasticity can be induced during early development or can reversibly occur within adult individuals, referred to as developmental plasticity and acclimation respectively. The results of this thesis are important to understand the effects of UV-B on physiology and performance in interaction with other factors, and the role of phenotypic plasticity in increasing the ability of individuals to cope with changes in environment. I hypothesized that increased antioxidant enzyme activities at higher temperatures or resulting from regular exercise/flowing water alleviate the negative effects of UV-B, and that antioxidant defenses can thermally acclimate, so the effects of UV-B would not be temperature sensitive. My results showed that regular prolonged exercise and higher temperature alleviated the negative effects of UV-B which was paralleled with increased antioxidant enzyme activities. However, the interaction between UV-B and temperature was more complicated and affected by acclimation temperatures and cohorts. I showed that both high (32oC) and low (18oC) acclimation temperatures worsened the effects of UV-B paralleled with increased ROS damage. The negative effects of UV-B were alleviated at 28oC acclimation temperature but only in summer fish. It shows different acclimation capacities of the cohorts of mosquitofish, and these results need to be considered to better predict the effects of climate change on populations and ecosystem. My results also showed that UV-B impaired locomotor performance without affecting metabolic scope which is an indicator of ATP supply. I therefore designed an experiment to determine if UV-B impaired locomotor performance by damage to mitochondria and ATP production or damage to muscle proteins and impaired contractile functions. Contrary to the paradigm of mitochondrial vulnerability to ROS, mitochondria and ATP production were not affected by UV-B-induced ROS. Instead, UV-B reduced tail beat frequency which is an indicator of muscle contraction and relaxation functions. I measured sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) to test if UV-B reduced locomotor performance by impairing muscle relaxation function. However, it was not the case and UV-B more likely reduced the swimming performance by damage to proteins involved with muscle contraction. The effects of UV-B and its interaction with factors such as temperature and water flow are important considering temporary and spatial environmental changes and on a larger scale, climate change. However, developmental plasticity can modify the effects of environmental factors on physiology and performance. I hypothesized that parental exposure to UV-B increases the resilience of guppies offspring to later life exposure to UV-B by enhancing their defense mechanisms, which was confirmed by our results. However, exposure to UV-B was detrimental for offspring whose parents were not exposed to UV-B (control) because they were unable to acclimate their antioxidant defenses to the environment.