Survey of the Efficacy of a Short Fragment of the rbc L Gene as a Supplemental DNA Barcode for Diatoms

DNA barcoding is a tool that uses a short, standard segment of DNA to identify organisms. In diatoms, a consensus on an appropriate DNA barcode has not been reached, but several markers show promise. These include the 5.8S gene plus a fragment of the internal transcribed spacer 2 ( ITS ‐2) of nuclear‐encoded ribosomal RNA , a 420‐bp segment of the 18S rRNA gene, and a 748‐bp fragment at the 3′‐end of the ribulose bisophosphate carboxylase large subunit ( rbc L) gene. Here, we tested a 540‐bp fragment 417‐bp downstream of the start codon of the rbc L gene for its efficacy in distinguishing diatom species in a wide range of taxa. Overall, 381 sequences representing 66 genera and 245 species from the classes M ediophyceae and B acillariophyceae were examined. Intra/interspecific thresholds were set at p  = 0.01 differences per site (diff./site) for M ediophyceae and p  = 0.02 diff./site for B acillariophyceae and correctly segregated 96% and 93% of morphological congeners, respectively. When testing reproductively isolated or biological species, which are only available from B acillariophyceae, 80% of species were discriminated. Therefore, we concluded that, alone, the rbc L region tested herein as potential a DNA barcode was not a sufficient discriminator of all diatoms. We suggest that this fragment could be used in a dual‐locus barcode with the more variable 5.8S+ ITS ‐2 to discriminate species without sufficient interspecific divergences in the tested rbc L region and to provide insight into species identity from a separately evolved genome.