A floristic survey of marine tube-forming diatoms reveals unexpected diversity and extensive co-habitation among genetic lines of the Berkeleya rutilans complex (Bacillariophyceae)

Abstract

Limited molecular data for marine tube-forming diatoms are available currently and this study provides the first molecular survey of these taxa. To conduct this survey, we used a molecular-assisted alpha taxonomy (MAAT) approach that utilizes DNA barcode data. We used three DNA barcode markers: the 3´ end of the large subunit of RUBISCO (rbcL-3P); the variable D2/D3 region of the nuclear large subunit ribosomal DNA (LSU D2/D3); and the internal transcribed spacer 2 (ITS2) to assign marine tube-forming diatoms from Canada to genetic species groups. The rbcL-3P analysis uncovered 29 genetic groups including representatives of Haslea crucigera, Navicula bottnica, N. brunelii, N. ramosissima, Nitzschia fontifuga, N. tubicola, Parlibellus berkeleya and P. delognei f. elliptica, as well as a complex of 14 closely related groups morphologically consistent with Berkeleya rutilans. We sequenced ITS2 for representatives of the B. rutilans complex; these data were consistent with the rbcL-3P genetic clusters for 86% of the colonies tested. The remaining 14% were in conflict, possibly indicating that more than a single Berkeleya genetic species was present in each colony. To investigate this hypothesis further, we developed ‘species-specific’ ITS2 primers and confirmed heterogeneity of Berkeleya genetic species in 64% of the colonies tested (N = 91). Therefore, a taxonomic assessment of tube-forming species that were originally described on the basis of colony morphology (e.g. Berkeleya rutilans) can only proceed using clonal cultures or single-cell analysis.