The metabolisms of many creatures have to adapt in the face of changing energetic challenges, and Drosophila melanogaster is no exception. Having originated in the tropics of Africa, the insect has colonised a wide range of temperate environments as human populations moved across the globe. Focusing on natural selection in the insect's energy-producing glycolytic pathway, Walter Eanes from Stony Brook University, USA looked for genetic polymorphisms in populations that could indicate adaptation to new environments. Analysing gene polymorphisms in the context of geographical location (p. 165), Eanes found that enzyme genes at the top of the pathway, around glucose-6-phosphate, may be adapting to the insect's altered circumstances. He suggests that these genes may control flux through the pathway to enable survival in the temperate environments that the insects have occupied recently.
Switching from insects to mammals, Raul Suarez explains that few vertebrates have modified their exercise energy metabolism to the extent of hovering nectarivorous bats and hummingbirds. Suarez and his colleagues Gerardo Herrera and Kenneth Welch explain that, instead of relying primarily on stored carbohydrates or fats to fuel exercise, hovering bats and hummingbirds essentially refuel on the wing, rapidly transporting oxygen and nectar sugars to directly fuel the fast twitch muscles that power flight (p. 172). Outlining elements of the sugar transport and oxidation cascades that consume sugar and oxygen to produce ATP, Suarez and his colleagues describe how hummingbirds sustain the highest vertebrate metabolic rates ever measured. Pointing out that fuelling exercise with sugar yields 15% more ATP per oxygen atom than fuelling exercise with fat, the team suggests that ‘ingested sugar serves as a premium fuel for hummingbird flight’.
While Suarez and his colleagues have detailed the uniquely adapted metabolism of hovering flight in vertebrates, Anthony Zera, from the University of Nebraska, USA, discusses the evolutionary lessons that can be learned from analysing metabolic pathways (p. 179). Describing various metabolic pathways in organisms ranging from Escherichia coli to D. melanogaster and the cricket Gryllus firmus, Zera discusses how changes in an organism's resource allocation lead to trade-offs, where an increase in resource allocation to one life history trait leads to a decrease in the resources available to another. Reviewing Ronald Burton's work on the mitochondrial electron transport chain in copepods, Zera explains how this has shed light on fundamental evolutionary processes such as the mechanisms of reproductive isolation. Zera also describes how analysing the kinetics of allozymes (enzymes encoded by different alleles of a gene) such as alcohol dehydrogenase in D. melanogaster and looking at their effect on lipid metabolism has shown how different metabolic allozymes can confer fitness advantages on the insects carrying them.
A group of animals that routinely experiences metabolic challenge is fish exposed to hypoxia. Starved of oxygen, they either have to improve oxygen uptake from the environment or reduce energy expenditure to survive. A group of fish that routinely experience hypoxia are the tide-pool-dwelling sculpins. Intrigued by the evolution of hypoxia tolerance, Jeffrey Richards from the University of British Columbia, Canada outlines his work on the behavioural and metabolic adaptations of sculpin species exposed to different levels of hypoxia across the tidal range (p. 191). Richards describes variations in mass-specific gill surface area, red blood cell haemoglobin oxygen-binding affinity and oxygen consumption rates that allow hypoxia-tolerant species to survive conditions that could prove lethal for less tolerant species further down the tidal range. He also outlines the metabolic reorganisation that is required to reduce ATP consumption rates as oxygen levels decline. Speculating that hypoxia tolerance will have selected for higher levels of metabolic rate suppression and fuel storage, Richards adds ’[but] we are still far from a unified concept of the important adaptations underlying hypoxia tolerance’.