Larvae of the leaf beetle Chrysomela lapponica attack two different tree species: willow and birch. To fend off predator attacks, the beetle larvae produce toxic butyric acid esters or salicylaldehyde, whose precursors they ingest with their leafy food. Scientists of the Max Planck Institute for Chemical Ecology in Jena, Germany, and colleagues have now found that a fundamental change in the genome has emerged in beetles that have specialized on birch: The activity of the salicylaldehyde-producing enzyme salicyl alcohol oxidase (SAO) is missing in these populations, whereas it is present in willow feeders. For birch beetles, the loss of this enzyme and thereby the loss of salicylaldehyde is advantagous: the enzyme is not needed anymore because its substrate salicyl alcohol is only present in willow leaves, but not in birch. Birch beetles can therefore save resources instead of producing the enzyme at a cost. First and foremost, however, the loss of salicylaldehyde also means that birch-feeding populations do not betray themselves to their own enemies anymore, that can trace them because of the odorous substance. These new findings were reported online on March 7, 2011, in PNAS. Beetle larvae are part of a food chain. They are attacked by predatory insects and parasites, such as hover flies and bugs, as well as infested by bacteria and fungi. To protect themselves, some leaf beetle larvae have developed interesting defense mechanisms, which function externally and metabolically: In case of danger, they emit substances from their defensive glands in form of vesicles (a short video is available at http://www.ice.mpg.de/ext/735.html). These defensive secretions contain toxins that the larvae sequester from chemical precursors they have ingested with their plant food.
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