Fat Flies: Investigating Obesity in Dragonflies
a) 12-spotted skimmer (Libellula pulchella) male, b-c) gregarine parasite spores and reproductive stages, d) heavily infected dragonfly intestine (abdomen pinned open and intestine sliced open to reveal contents).
“I’m hoping for a really bad winter.”
It’s not what most of us are wishing for as we head into the latter half of November, but for Ruud Schilder, assistant professor of entomology and biology, a snowy winter means good conditions for dragonflies the following summer. That’s because winter snow provides much of the water in the warm-season ponds where dragonflies live, he says.
Schilder uses dragonflies (Libellula pulchella) and other insects to broadly investigate the effects of environmental conditions on an organism’s fitness. His previous work has shown that dragonflies that are reared in acidic water—which could be influenced by factors such as soil type and the presence of fertilizers—are more likely to become infected with protozoans than dragonflies that are reared in neutral or basic water. In turn, the protozoans cause the dragonflies to become obese, which affects their flight performance and fuel use.
Schilder notes that an infectious origin of obesity has been suggested for several species of vertebrates, but until now the phenomenon has not been examined in invertebrates.
“People are starting to realize there is more involved in the development of obesity and metabolic diseases then just diet and exercise,” says Schilder. “I think there may be an environmental factor that we’re missing. Maybe it’s the type of water or types of environmental pollutants that we are exposed to, maybe that changes our susceptibility to these types of things.”
Schilder conducts his research both in the field—in ponds in State College—and in the laboratory.
“We examine dragonfly larvae that have been growing in different water pH conditions, and then we sequence their microbiomes to see what changes may have occurred,” he says. “We then expose the adult dragonflies to parasites to see whether or not they become infected more or less depending on the pH of the water in which they grew up as larvae.”
In other words, Schilder is investigating whether or not water pH influences the composition of the microbiome and, in turn, whether or not the composition of the microbiome affects the susceptibility to infection by protozoans.
In addition to examining an environmental trigger for obesity in dragonflies and the poor flight performance that results, Schilder also is investigating a genetic influence. He recently finished a large-scale genomics/transcriptomics project on Libellula pulchella in which he searched for genetic variation associated with high and low flight performance.
“We sequenced the genomes and the flight muscle transcriptomes of about 65 individuals across the population to see if we could tease out any kind of genetic variation associated with being a really high performer,” says Schilder. “We’re finding that the genes involved with the tracheal system, which is basically the breathing system, are important for being a high performer. Also genes involved in fatty acid or lipid mobilization, lipid stores, and fatty acid metabolism are important. These two things together make sense for a high-powered flight machine; you need proper delivery of oxygen and removal of CO2, and you need proper fuels. We are seeing this at the genomic and transcriptomic levels.”
Schilder says he is looking forward to putting the field and laboratory data together with the genomic/transcriptomic data to get a more complete picture of the causes of obesity in dragonflies. In fact, he is so interested in the combined study of ecology and genetics that he and his colleagues recently wrote a review paper on the topic, in which they synthesize studies on the evolution, ecology and physiology of odonates (dragonflies and damselflies), highlighting those areas in which the integration of ecology with genomics would yield significant insights into evolutionary processes. The paper appears in the October 10, 2016, issue of Frontiers in Zoology.
“Determining what environmental factors cause disease, for example, in some natural populations,” he says, “could one day yield important insights that could help prevent or treat related human diseases.”