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Aquatic Insect ‘Family Trees’ Provide Clues About Sensitivity to Pollution

A North Carolina State University study published online this week in Proceedings of the National Academy of Sciences shows that examining an insect’s “family tree” might help predict a “cousin” insect’s level of tolerance to pollutants, and therefore could be a reliable way to understand why certain insect species thrive or suffer under specific ecological conditions.

Evaluations of the health and well-being of rivers and streams are frequently tied to the presence – or absence – of resident aquatic insects. But these population evaluations are not designed to explain why certain species may be disappearing from specific places, says Dr. David Buchwalter, an NC State assistant professor of environmental and molecular toxicology and the lead author of the paper.

“Our results are exciting because they open up the possibility of predicting species’ tolerance to environmental problems based on their evolutionary histories,” Buchwalter says. This predictive power would give scientists a leg up on understanding insect responses to environmental stressors in the more than 6,500 aquatic insect species in North America.

In the study, Buchwalter and colleagues from the University of California, Riverside, and the U.S. Geological Survey examined how 21 species of insects field-collected from streams in North Carolina, California, Colorado and Oregon tolerated cadmium, a trace metal cancerous to humans that is used in batteries and found near hard-rock mining and industrial sites.

By exposing the insects to a gamma emitting isotope of cadmium – a technique that allowed the scientists to gauge metallic concentrations in live insects over time – the researchers measured cadmium intake rates; cadmium elimination rates; whether insects “detoxified” metals using proteins; and whether related insects showed similar resistance or tolerance to cadmium.

The study showed a great deal of variation in how these insects internally process cadmium, including a 65-fold difference in uptake and a 25-fold difference in the rate at which different species eliminated it from their tissues.

For the most part, though, insects in the same family were similar when it came to pollution sensitivity.

The study also showed that species could face a trade-off between being able to protect cells from cadmium and being able to eliminate it from their tissues. “This paper helps explain why, in the same water, different species can carry around very different concentrations of metals,” Buchwalter says. “And some species can carry those metal loads better than others.”

“Aquatic insect ecophysiological traits reveal phylogenetically based differences in dissolved cadmium susceptibility”
Authors: David Buchwalter, Lingtian Xie, Caitrin Martin, North Carolina State University; Theodore Garland, University of California, Riverside; S.N. Luoma and D.J. Cain, U.S. Geological Survey

Published: The week of June 16, 2008, online in Proceedings of the National Academy of Sciences

Abstract: We used a phylogenetically based comparative approach to evaluate the potential for physiological studies to reveal patterns of diversity in traits related to susceptibility to an environmental stressor, the trace metal cadmium (Cd). Physiological traits related to Cd bioaccumulation, compartmentalization, and ultimately susceptibility were measured in 21 aquatic insect species representing the orders Ephemeroptera, Plecoptera, and Trichoptera. We mapped these experimentally derived physiological traits onto a phylogeny and quantified the tendency for related species to be similar (phylogenetic signal). All traits related to Cd bioaccumulation and susceptibility exhibited statistically significant phylogenetic signal, although the signal strength varied among traits. Conventional and phylogenetically based regression models were compared, revealing great variability within orders but consistent, strong differences among insect families. Uptake and elimination rate constants were positively correlated among species, but only when effects of body size and phylogeny were incorporated in the analysis. Together, uptake and elimination rates predicted dramatic Cd bioaccumulation differences among species that agreed with field-based measurements. We discovered a potential tradeoff between the ability to eliminate Cd and the ability to detoxify it across species, particularly mayflies. The best-fit regression models were driven by phylogenetic parameters (especially differences among families) rather than functional traits, suggesting that it may eventually be possible to predict a taxon’s physiological performance based on its phylogenetic position, provided adequate physiological information is available for close relatives. There appears to be great potential for evolutionary physiological approaches to augment our understanding of insect responses to environmental stressors in nature.

Dr. David Buchwalter | newswise
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