page_title: DNA barcoding
section_title: Marker selection
hierarchical_section_title: DNA barcoding / Methodology / Marker selection
caption_attribution_description: English: A schematic view of primers and target region for PCR amplification, demonstrated on 16S rRNA gene in Pseudomonas. Primers are short conserved sequences with low variability that can target most or all species in the chosen target group. They are used to amplify highly variable target region in between two primers, which is the basis for species discrimination. Modified from »Variable Copy Number, Intra-Genomic Heterogeneities and Lateral Transfers of the 16S rRNA Gene in Pseudomonas - Scientific Figure on ResearchGate«&#160;by Bodilis, Josselin; Nsigue-Meilo, Sandrine; Besaury, Ludovic; Quillet, Laurent, used under CC BY, available from: https://www.researchgate.net/figure/Hypervariable-regions-within-the-16S-rRNA-gene-in-Pseudomonas-The-plotted-line-reflects_fig2_224832532.
context_page_description: DNA barcoding is a method of species identification using a short section of DNA from a specific gene or genes. The premise of DNA barcoding is that, by comparison with a reference library of such DNA sections, an individual sequence can be used to uniquely identify an organism to species, in the same way that a supermarket scanner uses the familiar black stripes of the UPC barcode to identify an item in its stock against its reference database. These "barcodes" are sometimes used in an effort to identify unknown species, parts of an organism, or simply to catalog as many taxa as possible, or to compare with traditional taxonomy in an effort to determine species boundaries.
  Different gene regions are used to identify the different organismal groups using barcoding. The most commonly used barcode region for animals and some protists is a portion of the cytochrome c oxidase I gene, found in mitochondrial DNA. Other genes suitable for DNA barcoding are the internal transcribed spacer rRNA often used for fungi and RuBisCO used for plants. Microorganisms are detected using different gene regions.
context_section_description: Markers used for DNA barcoding are called barcodes. In order to successfully characterize species based on DNA barcodes, selection of informative DNA regions is crucial. A good DNA barcode should have low intra-specific and high inter-specific variability and possess conserved flanking sites for developing universal PCR primers for wide taxonomic application. The goal is to design primers that will detect and distinguish most or all the species in the studied group of organisms (high taxonomic resolution). The length of the barcode sequence should be short enough to be used with current sampling source, DNA extraction, amplification and sequencing methods.
  Ideally, one gene sequence would be used for all taxonomic groups, from viruses to plants and animals. However, no such gene region has been found yet, so different barcodes are used for different groups of organisms, or depending on the study question.
  For animals, the most widely used barcode is mitochondrial cytochrome C oxidase I (COI) locus. Other mitochondrial genes, such as Cytb, 12S or 18S are also used. Mitochondrial genes are preferred over nuclear genes because of their lack of introns, their haploid mode of inheritance and their limited recombination. Moreover, each cell has various mitochondria (up to several thousand) and each of them contains several circular DNA molecules. Mitochondria can therefore offer abundant source of DNA even when sample tissue is limited.
  In plants, however, mitochondrial genes are not appropriate for DNA barcoding because they exhibit low mutation rates. A few candidate genes have been found in the chloroplast genome, the most promising being maturase K gene (matK) by itself or in association with other genes. Multi-locus markers such as ribosomal internal transcribed spacers (ITS DNA) along with matK, rbcL, trnH or other genes have also been used for species identification. The best discrimination between plant species has been achieved when using two or more chloroplast barcodes.
  For bacteria, the small subunit of ribosomal RNA (16S) gene can be used for different taxa, as it is highly conserved. Some studies suggest COI, type II chaperonin (cpn60) or β subunit of RNA polymerase (rpoB)  also could serve as bacterial DNA barcodes.
  Barcoding fungi is more challenging, and more than one primer combination might be required. The COI marker performs well in certain fungi groups, but not equally well in others. Therefore, additional markers are being used, such as ITS rDNA and the large subunit of nuclear ribosomal RNA (LSU).
  Within the group of protists, various barcodes have been proposed, such as the D1–D2 or D2–D3 regions of 28S rDNA, V4 subregion of 18S rRNA gene, ITS rDNA and COI. Additionally, some specific barcodes can be used for photosynthetic protists, for example the large subunit of ribulose-1,5-bisphosphate carboxylase-oxygenase gene (rbcL) and the chloroplastic 23S rRNA gene.
caption: A schematic view of primers and target region, demonstrated on 16S rRNA gene in Pseudomonas. As primers, one typically selects short conserved sequences with low variability, which can thus amplify most or all species in the chosen target group. The primers are used to amplify a highly variable target region in between the two primers, which is then used for species discrimination. Modified from »Variable Copy Number, Intra-Genomic Heterogeneities and Lateral Transfers of the 16S rRNA Gene in Pseudomonas« by Bodilis, Josselin; Nsigue-Meilo, Sandrine; Besaury, Ludovic; Quillet, Laurent, used under CC BY, available from: https://www.researchgate.net/figure/Hypervariable-regions-within-the-16S-rRNA-gene-in-Pseudomonas-The-plotted-line-reflects_fig2_224832532.
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