Abstract
Stramenopiles is a monophyletic eukaryotic group characterized by the presence of two flagella, of which the immature flagellum bears tripartite hairs. The group comprises 21 classes with >100
000 species. The majority of the described species are diatoms, estimated at ≥100
000, whereas there are approximately 2000 brown algae, 1000 chrysophytes, and 600 xanthophytes. Most classes consist of photosynthetic algae; however, five classes of colorless taxa are also included in this diverse lineage, for example,
Phytophthora, which includes the causative agent of the Great Irish potato famine. The diatoms, which have beautiful siliceous (glass) walls or frustules and the related brown seaweeds are economically and ecologically important. It appears that the ancient ancestor of the stramenopiles acquired its plastid from a red alga via secondary endosymbiosis, and this ancestor then gave rise to the cryptophytes, haptophytes, alveolates (including ciliates, apicomplexans, and dinoflagellates), and possibly the Rhizaria, as well as the stramenopiles. Of these, the alveolates are probably the closest living relatives of the stramenopiles. On the basis of a molecular clock study, we estimate that the stramenopiles have diverged ∼1025–1077 Ma and radiated during the early Neoproterozoic era, consistent with Proterozoic stramenopile fossils. Phylogenetic relationships within the stramenopiles are largely unresolved due to the use of datasets with a single or few genes. Multigene analyses with broad taxon sampling are urgently needed to establish evolutionary relationships among these extremely diverse and economically, ecologically, and evolutionarily important taxa.