A multigene phylogeny to infer the evolutionary history of Chaetocerotaceae (Bacillariophyta)

The diatom family Chaetocerotaceae (Bacillariophyta) is common in the marine plankton worldwide, especially in coastal areas and upwelling zones. Its defining character constitutes hollow processes, called setae, which emerge from the valves of the vegetative cells. The family comprises two extant genera: Bacteriastrum and Chaetoceros. Current systematics is based on morphological features of vegetative cells and resting spores and is summarised in a classification scheme subdividing Bacteriastrum in two sections, Isomorpha and Sagittata, and Chaetoceros in three subgenera: Hyalochaete, Chaetoceros (Phaeoceros) and Bacteriastroidea, and further into 22 sections. Phylogenies inferred from single molecular markers (18S and partial 28S rDNA) show only partial topological agreement and many poorly or unresolved basal ramifications. Since classification should not only satisfy practical needs but also reflect well-supported evolutionary relationships of the taxa under investigation, we inferred a multigene phylogeny of the family Chaetocerotaceae amplifying five genes of 100 strains encompassing six Bacteriastrum and 60 Chaetoceros species. We also compared the phylogenetic signal of nuclear, plastid and mitochondrial compartments to ascertain if the inferred tree topologies were congruent. Our results provided a robust multigene phylogeny of the family Chaetocerotaceae, offering a solid framework to test the validity of the traditional taxonomical classification. The genera Bacteriastrum and Chaetoceros were resolved as sister clades, whilst the subgenus Hyalochaete was found to be paraphyletic. Consequently, we rejected the subdivision in subgenera and only considered sections. Most of the already recognised sections were found to be monophyletic. We emended one section, rejected seven and erected three new ones. As a consequence of our proposed changes, all the sections investigated are supported by morphological and molecular characters alike. Thus, a natural classification is feasible for this important and very diverse marine planktonic family.