Fig 1. Sterol-auxotrophic oomycetes lack most canonical sterol biosynthesis enzymes.

The sequential order of enzymes in the sterol biosynthesis pathway (left) and presence of the encoding genes in representative species from different eukaryotic lineages (in green). The simplified tree (bottom) shows the Stramenopiles in the right branch. It is anticipated that organisms showing de novo sterol biosynthesis have the complete set of enzymes. Most sterol autotrophs indeed have one (+) or multiple (++) copies of the encoding genes. However, not all genes that are predicted to be present have been detected (?). Possibly, there are alternative enzymes that take over, as is the case for squalene oxidase in some diatoms and brown algae (*). The sterol auxotrophs lack all or nearly all genes (▯). Data are from literature or obtained by mining databases.