The role of intrinsic and extrinsic factors in shaping alternative migratory tactics and metabolic phenotypes in brown trout

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Archer, Louise C.
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University College Cork
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Variation among and within populations accounts for a considerable portion of phenotypic diversity produced in nature, and is instrumental to the structure and function of ecosystems. Understanding how, and why, intraspecific diversity persists is essential for predicting and managing the effects of global change, particularly because intraspecific variation may mediate diverse responses to changes in the environment. Alternative phenotypes – i.e. discrete phenotypic variation – can arise from a combination of proximate and ultimate mechanisms. Proximate mechanisms reflect how environmental factors shape phenotypic variation via intermediate physiological processes, which can themselves vary and be decomposed into genetic versus environmental components. Ultimate mechanisms concern the evolutionary function of a given phenotype. In this thesis, I explore how proximate and ultimate factors contribute to a particularly striking example of intraspecific diversity: alternative migratory tactics in brown trout Salmo trutta. Brown trout are iconic for the variety of migratory life histories they exhibit; yet fundamental knowledge gaps remain regarding how environmental, physiological, and genetic factors integrate to underpin life history decisions among and within populations. In Chapter 2, I assessed how food restriction and population background influences the expression of migratory tactics in offspring from two populations that naturally differ in anadromy (i.e. sea-migration). Food restriction affected traits related to size and condition, and resulted in a higher frequency of anadromy in both populations, though populations varied in their responses according to the timing of food restriction treatments. While anadromy was overall more frequent in offspring from the naturally anadromous population, the expression of anadromous phenotypes in offspring from a non-anadromous population indicated that migratory tactics might emerge in response to unfavourable environmental conditions causing energetic limitation. In Chapter 3, I further considered proximate mechanisms by exploring how multiple environmental factors (food and temperature) influence migration. Antagonistic effects of food restriction and increased temperature on condition and size-related traits were not translated at the level of migration tactics, where effects of food restriction and temperature were additive, but opposing (food restriction increased anadromy, whereas temperature decreased anadromy). I explored how components of metabolic rate – a fundamental determinant of physiological status – varied according to food restriction and population background in Chapter 4. Standard metabolic rate (SMR) was lower in food-deprived fish, while SMR, maximum metabolic rate, and aerobic scope (AS) were higher in offspring from a naturally anadromous population compared to a non-anadromous population. Population-specific effects of food restriction on AS also emerged. I further addressed the causes and consequences of metabolic rate variation in Chapter 5, where I found metabolic traits varied according to both population background and temperature, with important consequences for growth rates (a key fitness-related trait that can shape life histories). Collectively, these results contribute to our knowledge of how environmental and genetic factors underpin life-history diversity in terms of migratory tactics and physiology. Changes in environmental conditions will likely alter patterns of life-history diversity (mediated by changes in individual physiology) in ways that will also depend on population-specific factors. While predicting the impacts of multi-faceted environmental change will be complex, knowledge of the links between physiology, environment, and ultimately, life history, is crucial for conserving important biodiversity within brown trout, a species that is already in widespread decline due to pervasive global change.
Salmo trutta , Climate change , Proximate drivers , Genotype by environment , Partial migration , Aquatic , Brown trout , Multiple stressors , Antagonistic , Metabolic , Plasticity
Archer, L. C. 2019. The role of intrinsic and extrinsic factors in shaping alternative migratory tactics and metabolic phenotypes in brown trout. PhD Thesis, University College Cork.
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