Mitochondrial behavior, morphology, and animal performance
Type of DegreePhD Dissertation
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We have a limited understanding of the proximate mechanisms that are responsible for the development of variation in animal performance and life-history strategies. Many components of an organism’s successful life history–for example, mate competition, gestation, lactation, etc.–require a substantial increase in daily energy expenditure. Mitochondrial behavior (positioning within the cell and communication between mitochondria) and morphology affect variation in energy production at the molecular, cellular, and organismal levels. Chapter 1 reviews the adaptations in mitochondrial behavior and morphology that favor efficient energy production and increased animal performance. Previous work has linked greater proportions of inter-mitochondrial junctions and density of the inner mitochondrial membrane, among other traits, with increased energetic demand. Both endogenous and exogenous factors contribute to organism survival and reproduction. One ecologically relevant factor that influences the life history of aquatic organisms is ultraviolet (UV) radiation. Chapter 2 evaluates the impact of UV‐A/B irradiation on life history characteristics in Tigriopus californicus copepods. After exposing copepods to UV‐A/B irradiation (control, 1‐, and 3‐hr UV treatments at 0.5 W/m2), I measured the impact of exposure on fecundity, reproductive effort, and longevity. UV irradiation increased size of the first clutch among all reproducing females in both the 1‐ and 3‐hr experimental groups and decreased longevity among all females that mated in the 1‐hr treatment. UV irradiation had no effect on the number of clutches females produced. These findings indicate a potential benefit of UV irradiation on reproductive performance early in life, although the same exposure came at a cost to longevity. Chapter 3 tests the hypothesis that mitochondria change their behavior and morphology to meet energetic demands of responding to changes in oxidative stress. Using transmission electron microscopy, I found that both three and six hours of UV-A/B irradiation (0.5 W/m2) increased the proportion of inter-mitochondrial junctions (with increasing mitochondrial aspect ratio) and density of the inner mitochondrial membrane in myocytes of T. californicus copepods. Mitochondrial density increased following both irradiation treatments, but mitochondrial size decreased under the six-hour treatment. Metabolic rate was maintained under three hours of irradiation but decreased following six hours of exposure. These observations demonstrate that the density of inner mitochondrial membrane and proportion of inter-mitochondrial junctions can play formative roles in maintaining whole-animal metabolic rate, and ultimately organismal performance, under exposure to an oxidative stressor. Chapter 4 investigates the effect that increasing temperatures impose on copepod respiration. This study used 32 studies spanning 78 years of research and 50 copepod species (three orders) to quantify percent change in respiration rates per one-unit change in temperature. Copepod respiration rates increased by approximately 7% per °C increase in water temperature across the orders Calanoida, Cyclopoida, and Harpacticoida. Neither food availability nor scaling respiration to copepod dry weight affected the rate of change of respiration rates. Chapter 5 reviews how density and morphology of the inner mitochondrial membrane influence performance of the electron transport system. This review outlines the evidence that inner mitochondrial membrane density, association between ATP synthases, and cristae morphology, impact the efficiency of energy production by mitochondria. Further, we consider possible constraints on the capacity of mitochondria to improve efficiency by increasing inner mitochondrial membrane density. The aforementioned studies accomplish two goals: 1) understanding how exogenous, environmental stressors influence whole-animal performance, including metabolic rate and life history variation, and 2) understanding how mitochondrial behavior and morphology can act as mediators in these relationships by being influenced by environmental stressors and influencing animal performance.