02636nas a2200217   4500008004100000020001400041245010500055210006900160260002900229300001000258520193300268653001802201653001802219653001602237653003302253100002402286700002502310700001702335700002202352856004402374       2012                        eng d  a1365-294X00aMitochondrial barcodes are diagnostic of shared refugia but not species in hybridizing oak gallwasps0 aMitochondrial barcodes are diagnostic of shared refugia but not   bBlackwell Publishing Ltd  ano-no3 aMitochondrial DNA barcodes provide a simple taxonomic tool for systematic and ecological research, with particular benefit for poorly studied or species-rich taxa. Barcoding assumes genetic diversity follows species boundaries; however, many processes disrupt species-level monophyly of barcodes leading to incorrect classifications. Spatial population structure, particularly when shared across closely related and potentially hybridizing taxa, can invalidate barcoding approaches yet few data exist to examine its impacts. We test how shared population structure across hybridizing species impacts upon mitochondrial barcodes by sequencing the cytochrome b gene for 518 individuals of four well-delimited Western Palaearctic gallwasp species within the Andricus quercuscalicis species group. Mitochondrial barcodes clustered individuals into mixed-species clades corresponding to refugia, with no difference in within- and between-species divergence. Four nuclear genes were also sequenced from 4 to 11 individuals per refugial population of each species. Multi-locus analyses of these data supported established species, with no support for the refugial clustering across species seen in mitochondrial barcodes. This pattern is consistent with mitochondrial introgression among populations of species sharing the same glacial refugium, such that mitochondrial barcodes identify a shared history of population structure rather than species. Many taxa show phylogeographic structure across glacial refugia, suggesting that mitochondrial barcoding may fail when applied to other sets of co-distributed, closely related species. Robust barcoding approaches must sample extensively across population structure to disentangle spatial from species-level variation. Methods incorporating multiple unlinked loci are also essential to accommodate coalescent variation among genes and provide power to resolve recently diverged species.10aDNA barcoding10aintrogression10amulti-locus10aspatial population structure1 aNicholls, James, A.1 aChallis, Richard, J.1 aMutun, Serap1 aStone, Graham, N.  uhttps://quercus.myspecies.info/node/149