The cryptic origins of environment-indicating phantom midges (Chaoborus)

Fragile freshwater ecosystems have acted as important sentinels for global environmental change. A detailed understanding of the implications of global change in aquatic ecosystems relies on indicator species whose body parts are preserved in aquatic sediments for millennia. The phantom midge, Chaoborus flavicans (Meigen, 1830), for example, has well-preserved mandibles in aquatic sediments and may act as an indicator of boreal zone temperatures, lake depth, and the presence of fish. Nevertheless, we report that Chaoborus with C. flavicans-type mandibles are common in fishless tundra ponds above the tree line in western Alaska. We carried out genetic comparisons of nuclear and mitochondrial deoxyribonucleic acid (mtDNA) sequences in Chaoborus at a Holarctic scale to determine the origins of the Alaskan "C. flavicans". Nuclear DNA, mtDNA, and morphological results indicated that the Alaskan C. flavicans represents a unique lineage of Chaoborus. The average mtDNA divergence (Kimura's two-parameter) between the Palearctic C. flavicans and the Alaskan C. flavicans was >17%. mtDNA strongly supported Chaoborus crystallinus (De Geer, 1776) as the sister species to the Alaskan C. flavicans. Our present understanding of larval mandible morphology disagrees with the major ecological properties and evolutionary lineages of Chaoborus. We identify a new mandible character and highlight the value of validating the diagnostic morphologies of environmental indicator species with genetic studies.