Multiple full-length variants of the Mitochondrial COI DNA Barcode Region are prevalent in North European Sawflies

DNA barcoding, the use of standard DNA fragment for species
identification, has emerged as a major field of biodiversity research. The
effectiveness of these approaches rests on the premise that much less
variation exists within species than between them. While exceptions occur,
this has been demonstrated in many animal taxa where the COI gene is
effective in species discrimination. Sawflies are an exception to this
pattern because DNA barcodes often fail to distinguish congeneric species.
Using high-throughput single-molecule DNA sequencing to recover COI
sequences from thousands of sawflies, we found that single individuals
often possess multiple, seemingly functional, full-length DNA barcodes – a
phenomenon not documented at similar prevalence in any animal taxon. While
the evolutionary causes of multiple variants require further
investigation, our observation is remarkable as it violates the
one-barcode-one-specimen assumption. The presence of multiple variants of
barcodes within individuals does not jeopardize the concept, but it
introduces a complexity for species inventories based on metabarcoding.
They will overestimate the species count when barcode-based operational
species units are used as species proxies. Similarly, reference libraries
must consider how best to deal with the high frequency of multiple
intraindividual variants.