Genetic and Physiological Basis of Carotenoid Accumulation in the Cavefish Astyanax mexicanus
Carotenoids are naturally occurring pigments synthesized by plants, algae, and photosynthetic bacteria. For animals, which cannot synthesize these compounds de novo, carotenoids must be obtained through diet. Once ingested, carotenoids are metabolized and stored, playing pivotal roles in biological processes. Carotenoid functions include contributing to the coloration in many vertebrate animals, serving as precursors to vitamin A (essential for vision, growth, and immune function; Britton, 1995), and providing antioxidant protection against oxidative stress (Krinsky & Johnson, 2005). This thesis examines how cave populations of Astyanax mexicanus have altered carotenoid and vitamin A metabolism and how these changes shape physiology and life history.Chapter 1 establishes the foundation for understanding carotenoid metabolism evolution by presenting what is known about the mechanisms of carotenoid metabolism, the biological roles of carotenoid metabolites, and the evolution of carotenoid metabolism across vertebrates. Chapter 2 provides conceptual grounding by reviewing Astyanax mexicanus as a model for linking genotype, development, and physiology. Chapter 3 presents the primary empirical results. Chapter 3 presents the empirical results, including genetic mapping of carotenoid accumulation in hybrids, identification of beta-carotene oxygenase 2a (bco2a) as the candidate gene underlying carotenoid accumulation in cavefish, functional analysis of cavefish and surface fish Bco2a enzyme activity, and dietary manipulation experiments demonstrating differential metabolic responses between morphs. Chapter 4 synthesizes comparative evidence across vertebrates to interpret these findings in a wider context. Overall, this research aims to understand the genetic architecture of altered carotenoid metabolism in cavefish and its effect in physiology and potential adaptations in nutrient-limited caves.
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