ITS as an environmental DNA barcode for fungi: an in silico approach reveals potential PCR biases - PubMed (original) (raw)

ITS as an environmental DNA barcode for fungi: an in silico approach reveals potential PCR biases

Eva Bellemain et al. BMC Microbiol. 2010.

Abstract

Background: During the last 15 years the internal transcribed spacer (ITS) of nuclear DNA has been used as a target for analyzing fungal diversity in environmental samples, and has recently been selected as the standard marker for fungal DNA barcoding. In this study we explored the potential amplification biases that various commonly utilized ITS primers might introduce during amplification of different parts of the ITS region in samples containing mixed templates ('environmental barcoding'). We performed in silico PCR analyses with commonly used primer combinations using various ITS datasets obtained from public databases as templates.

Results: Some of the ITS primers, such as ITS1-F, were hampered with a high proportion of mismatches relative to the target sequences, and most of them appeared to introduce taxonomic biases during PCR. Some primers, e.g. ITS1-F, ITS1 and ITS5, were biased towards amplification of basidiomycetes, whereas others, e.g. ITS2, ITS3 and ITS4, were biased towards ascomycetes. The assumed basidiomycete-specific primer ITS4-B only amplified a minor proportion of basidiomycete ITS sequences, even under relaxed PCR conditions. Due to systematic length differences in the ITS2 region as well as the entire ITS, we found that ascomycetes will more easily amplify than basidiomycetes using these regions as targets. This bias can be avoided by using primers amplifying ITS1 only, but this would imply preferential amplification of 'non-dikarya' fungi.

Conclusions: We conclude that ITS primers have to be selected carefully, especially when used for high-throughput sequencing of environmental samples. We suggest that different primer combinations or different parts of the ITS region should be analyzed in parallel, or that alternative ITS primers should be searched for.

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Figures

Figure 1

Figure 1

Commonly used primers for amplifying parts or the entirety of the ITS region. a) Relative position of the primers, design of the subsets and number of sequences in each subset. b) Primer sequences, references and position of the primer sequence according to a reference sequence of Serpula himantioides (AM946630) stretching the entire nrDNA repeat.

Figure 2

Figure 2

Percentage of sequences amplified from each subset using different primer pairs allowing a maximum of 0, 1, 2, or 3 mismatches.

Figure 3

Figure 3

Box plots illustrating length differences between the amplicons obtained using different primer combinations for each of the three subsets. The plot in each subset represents the primer pair used to create the subset (*).

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