Ecological variation within traditional diatom morphospecies: diversity of Frustulia rhomboides sensu lato (Bacillariophyceae) in European freshwater habitats

More than 300 strains of Frustulia rhomboides sensu lato were isolated from populations across Europe to estimate its molecular diversity and geographic distribution in oligotrophic freshwater habitats. Phylogenetic analyses, based on partial large subunit (LSU) rDNA sequences, revealed six lineages that did not fully correspond with the separation of F. rhomboides into traditional varieties. Although four closely related lineages showed substantial overlaps in morphology, the characteristic morphological features could be determined using morphometric techniques. The phylogenetic lineages of F. rhomboides showed different patterns of distribution among regions. Sequence frequencies within samples from different habitats indicated that distribution of the common lineages most likely reflected their different ecological preferences rather than dispersal constraints.