Data from: The model marine diatom Thalassiosira pseudonana likely descended from a freshwater ancestor in the genus Cyclotella

BACKGROUND: Publication of the first diatom genome, that of Thalassiosira
pseudonana, established it as a model species for experimental and genomic
studies of diatoms. Virtually every ensuing study has treated T.
pseudonana as a marine diatom, with genomic and experimental data valued
for their insights into the ecology and evolution of diatoms in the
world's oceans. RESULTS: The natural distribution of T. pseudonana
spans both marine and fresh waters, and phylogenetic analyses of
morphological and molecular datasets show that, 1) T. pseudonana marks an
early divergence in a major freshwater radiation by diatoms, and 2) as a
species, T. pseudonana is likely ancestrally freshwater. Marine strains
therefore represent recent recolonizations of higher salinity habitats. In
addition, the combination of a relatively nondescript form and a
convoluted taxonomic history has introduced some confusion about the
identity of T. pseudonana and, by extension, its phylogeny and ecology. We
resolve these issues and use phylogenetic criteria to show that T.
pseudonana is more appropriately classified by its original name,
Cyclotella nana. Cyclotella contains a mix of marine and freshwater
species and so more accurately conveys the complexities of the
phylogenetic and natural histories of T. pseudonana. CONCLUSIONS: The
multitude of physical barriers that likely must be overcome for diatoms to
successfully colonize freshwaters suggests that the physiological traits
of T. pseudonana, and the genes underlying those traits, might differ from
those of strictly marine diatoms. The freshwater ancestry of T. pseudonana
might therefore confound generalizations about the physiological and
metabolic properties of marine diatoms. The freshwater component of T.
pseudonana's history merits careful consideration in the
interpretation of experimental data collected for this important model
species.