Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Department

Biological Sciences

First Advisor

Scott McWilliams

Abstract

Migratory songbirds are among the best high-performance endurance athletes on the planet and during their annual peregrinations they are particularly vulnerable to oxidative damage because they maintain relatively high metabolic rates at rest, they are able to fly for long durations while operating at 9 times their resting metabolic rate, and they rely on fats to fuel this exercise which increases reactive oxygen species (ROS) formation. The concept of evolutionary economic design of physiological systems considers such phenotypic flexibility in metabolic and antioxidant responses adaptive in that it allows birds to match the capacity of key physiological systems to prevailing demands and then modulate this capacity as demands change and so provide adequate but not excessive spare capacity at any given time. In addition to energetic demands associated with flying, this phenotypic flexibility is also modulated by environmental factors including diet. During migration, many songbirds consume a berry-heavy diet that is both rich in certain polyunsaturated fat that are known to provide energy savings at a longer-term oxidative cost, and rich in specific water-soluble antioxidants that protect against these metabolic costs. What remains unknown is how regular flight and diet (i.e. fat and antioxidants) influence molecular pathways involved in fat metabolism and antioxidant protection to modulate upper levels of phenotypic flexibility (e.g. metabolic rates, antioxidant defenses) in migratory songbirds. Our study investigated how these relevant ecological factors affected key metabolic and antioxidant transcription factors and their target genes (Chapters 1 and 2, respectively) to modulate the antioxidant defense system (Chapter 3). Molecular and phenotypic flexibility driven by flight and diet would allow songbirds living in shifting environments to track environmental change, whether natural or anthropogenic.

We employed a rigorous, ecologically-relevant experimental design to determine how metabolic (Chapter 1) and endogenous antioxidant (Chapter 2) pathways and oxidative balance (Chapter 3) in songbirds during the migration period responded to 2 weeks of flight training in a wind tunnel, as well as a factorial combination of dietary polyunsaturated fatty acids (18:2n-6 PUFA) and water-soluble antioxidants (anthocyanins). European starlings (Sturnus vulgaris) were fed diets composed of either a high or low 18:2n-6 PUFA and supplemented with or without anthocyanins, and half of these birds were flight-trained in a wind-tunnel while the rest were untrained. In Chapters 1 and 2, we tested specific hypotheses related to the molecular flexibility in the flight-muscle and liver that occurs in response to flight training, dietary 18:2n-6 PUFA, and dietary anthocyanins. In Chapter 1 we measured the expression of 7-10 key genes involved in fat metabolism in flight-muscle and liver, and in Chapter 2 we measured 8 key antioxidant genes in these same two tissues. In Chapter 3, we tested specific hypotheses related to the phenotypic flexibility of the antioxidant defense system in response to flight training, 18:2n-6 PUFA, and dietary anthocyanins. We measured antioxidant capacity and oxidative damage levels in the flight-muscle, liver, and plasma of flight-trained and untrained birds over the course of the flight training and during and acute flight.

The causal network analyses that we conducted as part of these studies suggest that transcription factors implicated in regulating metabolism (PPARs) and antioxidants (NRF2, PPARs) in mammals also regulate the selected genes involved in fat metabolism (Chapter 1) and antioxidant protection (Chapter 2) in migratory songbirds. Furthermore, flight training modulated the causal network between metabolic genes involved in fat metabolism more than did diet (Chapter 1). In addition, our studies revealed that the energetic challenge posed by daily flights modulated metabolic and antioxidant gene expression profiles and oxidative damage levels in a tissue-dependent manner. For example, we found that flight training increased the expression of half of the metabolic genes measured in the pectoralis but not in the liver (Chapter 1), and that flight training increased 40% of the antioxidant genes measured in the liver but not in the pectoralis (Chapter 2). In addition, after two weeks of flight-training, birds maintained antioxidant capacity and oxidative damage levels similar to untrained birds in the pectoralis and plasma and reduced damage in the liver (Chapter 3). These tissue-specific differences in response to flight training may be related to functional differences between tissues as well as fundamental differences in their turnover rates. Furthermore, we demonstrated in Ch. 3 that the variation between individuals in the rate of energy expenditure (kJ/min) during longer (ca. 3-hr) flights was related to the extent to which birds modulated their circulating non-enzymatic antioxidants (OXY, uric acid) during the flight. Taken together, these results suggest that songbirds modulate their metabolic and antioxidant pathways in a tissue-specific manner when faced with an energetic challenge, and they employ condition-dependent antioxidant strategies that depend on the degree of that challenge.

Nutrient content of the diets (i.e. dietary fat and antioxidants) had a more selective effect on metabolism and oxidative status in our study compared to flight training. In Chapter 1, we found that birds fed more 18:2n-6 PUFA (and thereby less 16:0) increased the expression of only two genes, one in the pectoralis that is involved in the hydrolysis of circulating triglycerides (LPL, dependent on flight training) and the expression of a metabolic transcription factor (PPARα) in the liver. In contrast, expression of a gene involved in fat transport across the muscle membrane was higher in birds fed less 18:2n-6 PUFA. In addition, birds that consumed more dietary anthocyanins increased the expression of two antioxidant enzyme genes in the pectoralis (CAT, SOD1; Chapter 2) and had higher circulating levels of oxidative damage immediately after an acute flight (Chapter 3). Thus, dietary anthocyanins have a tissue-dependent stimulatory or inhibitory effect on the antioxidant system. Considered together, these results suggest that dietary fats have a selective signaling role in muscle and liver metabolism and that dietary anthocyanins have multiple roles that depend largely on the metabolic state of both the organism and their various tissues.

Birds during migration undergo regular, often daily flights interrupted by often longer periods at stopover sites as they travel between breeding and wintering areas. Migrating songbirds are quite selective in what they eat, and these diet choices directly affect their supply of nutrients and energy. Our experiment has revealed that molecular-level metabolism is modulated by flight training and dietary fat quality and that molecular-level antioxidant pathways are modulated by flight training and dietary anthocyanins in a migratory songbird. The implication is that these same environmental factors (i.e., flying, dietary fat quality and antioxidants) will affect the migratory performance of birds in the wild. Flight- and diet-mediated molecular flexibility in the endogenous antioxidant system seems to regulate upper level phenotypic flexibility in the antioxidant defense system, as acute flight stimulates the antioxidant system and protects birds from the accumulation of oxidative damage. Our study demonstrates that metabolic and antioxidant flexibility in songbirds is driven by energetic demands and ecological factors associated with migration, yet how the timing and seasonal availability of these factors initiate and maintain these types of molecular and physiological phenotypes remains unknown. Thus, migratory birds possess the ability to track and respond to environmental change relatively quickly (within 2 weeks), yet it is unknown whether birds can adjust their physiology at rates that match the rapidly and unpredictably shifting environments experienced during the Anthropocene.

Available for download on Thursday, November 11, 2021

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