A consensus secondary structure of ITS2 for the diatom Order Cymatosirales (Mediophyceae, Bacillariophyta) and reappraisal of the order based on DNA, morphology, and reproduction

Abstract

In 1983, Hasle and colleagues removed cymatosiroid diatoms from the pennates, and erected a new centric diatom family, the Cymatosiraceae, mainly to accommodate for their newly discovered mode of sexual reproduction. The new family consisted of two subfamilies differing in frustule structure. The family was later elevated to the rank of Order Cymatosirales Round and Crawford. We revisited intra-ordinal relationships within Cymatosirales using combined genetic (DNA sequences), morphological (valve and frustule structure), and reproductive (auxospore type) characters. In total, 36 cymatosiroid strains from 19 species representing 13 genera (80% of the total number of extant genera; nine of them represented by their generitypes) were used in this study. Instead of only the commonly used loci (18S rRNA and plastidal genes) to infer diatom phylogeny, we developed a consensus secondary structure model of the Internal Transcribed Spacer 2 (ITS2) for this order and applied it to aid in sequence alignments for ITS2. This improved the alignment and thus the robustness of the phylogenetic framework. The compensatory base changes (CBCs) found in ITS2 secondary structures were mapped onto the multi-gene (18S rRNA + ITS2 + rbcL) phylogenetic tree topology. In all these trees, all species grouped into two morphologically and genetically distinct clades. Each clade was supported by multiple CBCs, as did all the clades representing genera. However, these clades did not correspond to the previously established subfamilies. Consequently, we amend the Order Cymatosirales and family Cymatosiraceae, and propose a new family, the Leyanellaceae. The structure of the auxospore was an additional synapomorphic character for Cymatosirales. Overall, we demonstrate a novel approach to study diatom phylogeny across a broader taxonomic range using ITS2 secondary structural information. Our results suggest that this approach might be useful in establishing higher taxonomic relationships in other groups of diatoms.