Metabarcoding of soil nematodes: the importance of taxonomic coverage and availability of reference sequences in choosing suitable marker(s)

For many organisms, there is agreement on the specific genomic region used for developing barcode markers. With nematodes, however, it has been found that the COI region designated for most animals lacks the taxonomic coverage (ability to amplify a diverse group of taxa) required of a metabarcoding marker. For that reason, studies on metabarcoding of nematodes thus far have utilized primarily regions within the highly conserved 18S ribosomal DNA. Two popular markers within this region are the ones flanked by the primer pairs NF1-18Sr2b and SSUF04-SSUR22. The NF1-18Sr2b primer pair, especially, has been critiqued as not being specific enough for nematodes leading to suggestions for other candidate markers while the SSUF04-SSUR22 region has hardly been tested on soil nematodes. The current study aimed to evaluate these two markers against other alternative ones within the 28S rDNA and the COI region for their suitability for nematode metabarcoding. The results showed that the NF1-18Sr2b marker could offer wide coverage and good resolution for characterizing soil nematodes. Sufficient availability of reference sequences for this region was found to be a significant factor that resulted in this marker outperforming the other markers, particularly the 18S-based SSUFO4-SSUR22 marker. None of the other tested regions compared with this marker in terms of the proportion of the taxa recovered. The COI-based marker had the lowest number of taxa recovered, and this was due to the poor performance of its primers and the insufficient number of reference sequences in public databases. In summary, this study highlights how dependent the success of metabarcoding is on the availability of a good reference sequence collection for the marker of choice as well as its taxonomic coverage.