Towards writing the encyclopedia of life: an introduction to DNA barcoding

Abstract

An international consortium of major natural history museums, herbaria and other organizations has launched an ambitious project, the 'Barcode of Life Initiative', to promote a process enabling the rapid and inexpensive identification of the estimated 10 million species on Earth. DNA barcoding is a diagnostic technique in which short DNA sequence(s) can be used for species identification. The first international scientific conference on Barcoding of Life was held at the Natural History Museum in London in February 2005, and here we review the scientific challenges discussed during this conference and in previous publications. Although still controversial, the scientific benefits of DNA barcoding include: (i) enabling species identification, including any life stage or fragment, (ii) facilitating species discoveries based on cluster analyses of gene sequences (e.g. cox1 = CO1, in animals), (iii) promoting development of handheld DNA sequencing technology that can be applied in the field for biodiversity inventories and (iv) providing insight into the diversity of life.

Figure 1

Hypothetical clustering analysis of DNA ‘barcoded’ individuals reveals at least three species. Sequence cluster 1 corresponds to a traditionally recognized species based on morphology (species 1). Clusters 2 and 3 can be two cryptic species revealed by DNA barcoding, which were previously embedded within species 2. Bold numbers indicate six unknown individuals sampled for a biodiversity inventory and assigned to their respective species using DNA barcoding. Individual 21 is unplaced and illustrates potential problems with barcoding: the non-assignment of 21 to clusters 1–3 could be due to problems with the barcode marker, the clustering algorithm used, and/or to biological phenomena such as hybridization and introgression; 21 could also be considered to represent a separate entity sampled only once. Some taxonomists prefer to rely on DNA-based clusters whereas others will preferentially consider morphology-based species recognition; most often both approaches converge to the same solution (as for cluster 1 and species 1). A difficulty in tree-based approaches is that the topology may not be well supported based on short mitochondrial DNA sequences, in particular at deeper nodes (indicated by asterisks). Alternative approaches do not use the tree structure but the character states unique to the individuals from particular sets of populations, e.g. see DeSalle et al. (2005).