Evolution of the diatoms: V. Morphological and cytological support for the major clades and a taxonomic revision

Phylogenetic reconstructions of the diatoms have been inferred with the 18S and 16S ribosomal RNA genes. Previous studies have shown that the group is divided into two major clades, with support coming initially from the arrangement of the Golgi bodies inside the cells in extant taxa. Features of extinct taxa that also support these clades can be found in the earliest fossil record of the diatoms and include the presence or absence of a central structure in the valve wall and the type of peripheral linking mechanisms between cells. Here we demonstrate that the general pattern of the auxospore expansion and the structure of their walls, the structure of the pyrenoid and the ultrastructure of the spermatozoid further support the molecular clades. Given the combined molecular and morphological support, we propose two new subdivisions (Coscinod-iscophytina and Bacillariophytina), emend the classes Coscinodiscophyceae and Bacillariophyceae and propose a new class, the Mediophyceae for the bipolar centrics and the Thalassiosirales.