Comparative molecular and morphological phylogenetic analyses of taxa in the Chaetocerotaceae (Bacillariophyta)

The diatom family Chaetocerotaceae (Mediophyceae) includes two exclusively phytoplanktonic genera: Chaetoceros and Bacteriastrum. Its hallmark feature constitutes setae: hollow, spine-like appendages protruding from the valves. Chaetoceros is morphologically diverse, includes c. 400 described species and is common worldwide; whereas, Bacteriastrum includes only 11, is less common and occurs mainly in temperate and tropical seas. In the present study we gathered morphological information and/or sequence data from 86 strains belonging to 17 morphologically defined species in Chaetoceros and one in Bacteriastrum (B. cf. hyalinum). The Chaetoceros species included in this study belong to 14 of the 22 sections and two of the three subgenera: Chaetoceros (Phaeoceros) and Hyalochaete. A consensus cladogram reconstructed from states associated with morphological characters gathered from strains belonging to these 18 morphological taxa in the Chaetocerotaceae and Hemiauhus hauckii as outgroup resolved Bacteriastrum inside Chaetoceros and demonstrated monophyly for the subgenus Chaetoceros; whereas, the subgenus Hyalochaete was found to be paraphyletic. Molecular phylogenies inferred from the hypervariable region (D1-D4) of the LSU rRNA gene of all strains included in this study and from a subset of these strains corroborated the findings in the cladogram and showed evidence for cryptic or pseudocryptic diversity in C curvisetus, debilis, C. diadema, C. lorenzianus, C pet-talcums and C socialis. The molecular trees differed topologically from the morphological ones, but characters exhibiting only a few state changes in the molecular tree showed also only a few changes in the morphological one; whereas, characters showing multiple changes in the molecular tree revealed many changes in the morphological tree as well.