A Review of the Evolution of the Diatoms from the Origin of the Lineage to Their Populations

The diatoms are, without doubt, one of the most successful groups of unicellular algae and contribute significantly to the global carbon cycle. They arose within the heterokont lineage no earlier than 250 Ma. The radiation of the pigmented heterokonts and that of the haptophytes and dinoflagellates was likely a response to the Permian–Triassic (PT) extinction event when host cells with a red algal endosymbiont had an adaptive advantage. There are three major clades of diatoms, which have been formally recognized at the class level, and their monophyly is clearly linked to the type of analysis done, the alignment by the secondary structure of the ribosomal RNA molecule, and the number of out-groups. The auxospore continues to be the defining feature of the deeper clades/classes of the diatoms; the three classes being the radial centrics, the bipolar centrics, which include the radial Thalassiosirales, and the pennate diatoms. Additional important defining features are the position of the cribrum in loculate areolae and the presence or absence of a central structure in the annulus. Sublineages within each class generally follow traditional orders of diatoms based on morphology. The araphid diatoms are shown to comprise two groups: the basal araphids that have both a properizonial auxospore like the bipolar, mediophycean diatoms and a perizonial auxospore like the raphid diatoms and the core araphids that have a perizonial auxospore like the raphid diatoms. Raphid diatoms are monophyletic with the Eunotiales as a basal lineage. Canal raphe diatoms have arisen twice. The Bacillariales are a basal divergence, whereas the Surirellales diverged more recently with the canal raphe evolving from amphoroid diatoms through Entomoneis to Surirella. Nearly all of the cosmopolitan diatom species that have been investigated with molecular techniques have been shown to be composed of cryptic species. Breeding studies help to confirm that the cryptic species conform to a biological species concept and underscore the premise that the diatoms are underclassified as a group at the species level. Genetic diversity studies have shown that the diatoms have strongly structured populations both spatially and temporally.