Tracking, in real time, how insects utilize macronutrients and their role in both reproductive success and survival.

About My Research

An important question in biology is how do organisms allocate the nutrients they eat. With the Picarro CRDS we are able to track, in real time, how insects utilize macronutrients such as carbohydrates, lipids and amino acids. Many animals feed on nectar from flowers that is mainly comprised of sugar water. Using ¹³C-enriched glucose in the nectar we found that in nectar-feeding hawkmoths, unfed females mated to fed males laid more eggs than females mated to unfed males and this labelled glucose was not present either in the female's body or in her eggs. This shows that although fed males provided females direct benefits from the sugar in the nectar, the sugar was not used as a nutrient by the females.

The ability to allocate nutrients, even when limited, is essential for survival and fitness of any organism. By combining both CRDS and CM-CRDS we can distinguish when animals use nutrients such as amino acids, simple sugars, and fats to reproduction (eggs) versus body growth, versus maintenance (metabolism). In addition to sugar and water, flower nectars contain two macronutrients, amino acids and fatty acids. We created artificial nectars spiked with ¹³C-labelled amino acids and fatty acids and fed these to adult female hawkmoths to understand how they allocate these nutrients among the competing sinks (reproduction, growth, and metabolic fuel). We found that both essential and nonessential amino acids were allocated to eggs and flight muscles in the adult female and were still detectable in her young offspring. All amino acids in the nectar were also used as metabolic fuel by the adult, but the non-essential amino acids were oxidized at higher rates than essential amino acids. Surprisingly, the nectar fatty acids were not vertically transferred to offspring, but were readily used as a metabolic fuel by the moth, minimizing losses of endogenous lipid nutrient reserves. This work suggests that the non-carbohydrate components of nectar may play important roles in both reproductive success and survival of these nectar-feeding animals.

How Picarro Analyzers Helped

Our lab was among the first to couple the CRDS with flow through respirometry that measures O2 consumption and CO₂ production in real time. This has enabled novel ways to explore nutrient use in animals. Nectar-feeding animals have among the highest metabolic rates ever recorded. High aerobic performance is tightly linked to the accumulation of oxidative damage in muscles, and antioxidants in nectar are scarce to nonexistent. Many nectarivores, such as hummingbirds, some insects (e.g. monarch butterfly), and bats migrate thousands of miles feeding only on nectar along the way. How is it, that these long-distance migrating nectarivores don’t fall out of the sky from accumulated oxidative damage? By ¹³C labelling of both the first and second carbons of glucose and tracking in real time with a combined CRDS-respirometry setup, we showed that while flying, moths use the glucose sugar to fuel flight. However, within seconds of ceasing flight, they generate antioxidant potential by shunting nectar glucose to the pentose phosphate pathway (PPP), resulting in a reduction in oxidative damage to the flight muscles. In fact, sugar-fed moths that were forced to fly had lower oxidative damage to their flight muscle membranes than unfed moths that did not fly showing how this antioxidant potential can mitigate oxidative damage imposed by highly aerobic flight. This study suggests that nectar feeding, the use of the PPP, and intense exercise, are causally linked and have allowed the evolution of powerful fliers that feed only on nectar.

Coupling the CRDS with respirometry has also enabled us to solve a long standing mystery in metabolic physiology. It is very well established that the quotient of CO₂ produced, over O2 consumed (Respiratory Quotient, or RQ for short), allows us to identify the macronutrient used to fuel metabolism. When all the metabolic fuel is derived only from carbohydrates: for every molecule of O2 consumed, one molecule of CO₂ is produced, so that RQ = 1. When only lipids are oxidized, RQ = 0.7, and for proteins RQ ~ 0.85. In every physiology text book, RQ is upper bound at 1.0 and lower bound at 0.7. However, the scientific literature is replete with examples were RQ > 1.0: 1.3, 1.4, 1.6 and even as high as 1.8. None of these numbers should be theoretically possible. Most studies simply don’t comment on these high numbers. Those that do, attribute it to limitations of the equipment and not to biology. By aligning in real time CRDS with flow-through respirometry we have shown that RQ >1 is a biological phenomenon and not due to equipment limitations. When glucose is shunted to the pentose phosphate pathway, CO₂ molecules are produced with no O2 being consumed, resulting in RQ > 1.