Many factors affect membrane composition in ectotherms, including allometry, temperature, toxins such as PCB-153, and osmotic stress. This thesis seeks to describe the relationship between membrane composition, size, and phylogeny in twelve species of cypriniform fish; to describe interactions between the homeoviscous responses to temperature and to PCB-153 in goldfish and rainbow trout; and to describe the membrane response to hypoosmotic stress in goldfish. Commonalities in these patterns provide insight into shared mechanisms of phospholipid modulation. In particular, such similarities indicate whether the membrane pacemaker theory of metabolism, which connects allometric relationships between body size, membrane phospholipids, and metabolic rate, can serve as a general framework for understanding membrane composition. Chapter 2 investigates how cypriniform membrane unsaturation decreases with mass through different fatty acid substitutions than in endotherms, but these fatty acids are in turn shown to be due to the species’ relatedness to one another rather than to purely physiological causes. In Chapter 3, PCB-153 is shown to increase cholesterol in liver and brain, while high temperature primarily reduces phospholipid unsaturation. In Chapter 4, these patterns are further explored in trout. As in goldfish, cholesterol modulation is the primary response to PCB-153, whereas temperature primarily reduces phospholipid unsaturation. Trout show more pervasive fatty acid changes than goldfish in all tissues except the liver, which does not respond to PCB exposure, suggesting that PCB-153 pushes trout’s homeoviscous response to a limit that similarly-exposed goldfish do not face. Chapter 5 shows that goldfish intestines decrease membrane saturation; kidneys decrease membrane cholesterol; gills decrease neither; and muscles decrease both in response to long-term exposure to hypoosmotic conditions. The intestine and kidney are both involved in recovering ions from body fluids, but gills suppress ion loss and muscle concentrates ions from the bloodstream. Temperature, osmotic stress, PCB-153, and increasing body size are all addressed via a similar set of membrane responses in fish, which fits with the membrane pacemaker theory’s predictions regarding membrane composition, metabolic rate, and size.

Membrane diversity associated with variation in body mass and body temperature of hymenopterans was investigated. Body mass of organisms affects most aspects of their biology, including physiological traits. One trait influenced by mass is metabolic rate, but the basis for its variation among organisms remains unclear. Recent work revealed that membrane phospholipid composition varies systematically with body mass: smaller vertebrates with higher mass-specific metabolic rates have more fluid membranes containing more long chains of polyunsaturated fatty acids (PUFA) and less monounsaturated fatty acids (MUFA). This “membrane pacemaker” theory of metabolism has recently been challenged by a lack of link between metabolic rate and membrane composition. To test this relationship, we have quantified the membrane lipid composition of 22 species of Panamanian orchid bees with a 22-fold range in mass. Results incorporating phylogenetic analysis show significant relationships for 18:1 (MUFA) and 18:3 (PUFA) with body mass that are consistent with the “pacemaker” theory, and unexpected relationships with saturated fatty acids (SFA). Moreover, changes in membrane fatty acid composition with temperature are a strategy in ectothermic poïkilotherms that is part of the “homeoviscous adaptation” model. Here, we report systematic variations in fatty acid composition linked with thoracic temperature excess in North American Hymenopterans, a novel discovery in these heterotherms. These findings are discussed in the context of diet, metabolism and lifespan.

Membrane diversity associated with variation in body mass and body temperature of hymenopterans was investigated. Body mass of organisms affects most aspects of their biology, including physiological traits. One trait influenced by mass is metabolic rate, but the basis for its variation among organisms remains unclear. Recent work revealed that membrane phospholipid composition varies systematically with body mass: smaller vertebrates with higher mass-specific metabolic rates have more fluid membranes containing more long chains of polyunsaturated fatty acids (PUFA) and less monounsaturated fatty acids (MUFA). This “membrane pacemaker” theory of metabolism has recently been challenged by a lack of link between metabolic rate and membrane composition. To test this relationship, we have quantified the membrane lipid composition of 22 species of Panamanian orchid bees with a 22-fold range in mass. Results incorporating phylogenetic analysis show significant relationships for 18:1 (MUFA) and 18:3 (PUFA) with body mass that are consistent with the “pacemaker” theory, and unexpected relationships with saturated fatty acids (SFA). Moreover, changes in membrane fatty acid composition with temperature are a strategy in ectothermic poïkilotherms that is part of the “homeoviscous adaptation” model. Here, we report systematic variations in fatty acid composition linked with thoracic temperature excess in North American Hymenopterans, a novel discovery in these heterotherms. These findings are discussed in the context of diet, metabolism and lifespan.

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