The Case for a Nuclear Barcode: Using the CAD CPS Region for Species and Genus Level Discrimination in Beetles

Abstract

DNA barcoding has revolutionized how we discover, identify, and detect species. A substantial foundation has been established with millions of mitochondrial cytochrome c oxidase I sequences freely available for eukaryotes. However, issues with COI ranging from uniparental inheritance and small genetic population sizes to nuclear and asymmetric introgression can impede its use. We propose using CAD as the “nuclear barcode” to complement the COI barcode and ameliorate these concerns. We focused on beetles from taxonomically diverse species-level studies that used COI and CAD. An ambiguous barcode gap was present between intra- and interspecific genetic distances in CAD and COI; this led to difficulty with automated gap detection methods. We found pseudogenes, problematic population structure, introgression, and incomplete lineage sorting represented in the COI data. A CAD gene tree illuminated these cryptic problems. Placement tests of species and outgroups using distance-based tree building were largely successful for CAD, demonstrating its phylogenetic signal at the species and genus levels. Species placement issues were typically unique to one locus, allowing for recognition of misdiagnosis. We conclude that a CAD barcode is a valuable tool for beetle diagnostics, metabarcoding, and faunistic surveys.