(A) The phylogenetic tree depicts the currently accepted evolutionary relationships between the major lineages of eukaryotes. Black vertical lines indicate non-photosynthetic groups, whilst green lines indicate organisms that became photosynthetic after acquiring an ancestral cyanobacterium (via a so-called ‘primary endosymbiosis event’). Red lines indicate organisms that became photosynthetic after acquiring a red alga or green alga (via so-called ‘secondary endosymbiosis events’). Groups with a copy of the gene for an enzyme called GULO in their genome are marked with a closed blue circle, whilst those with a copy of the gene for an enzyme called GLDH are marked with a closed green circle. Empty circles indicate that the gene is absent from the genome. (B) The schematic model suggests how the primary routes of ascorbate biosynthesis arose in the various eukaryotic lineages and offers potential explanations based on Wheeler et al.'s findings. The presence and absence of GULO and GLDH are indicated as above, and chloroplasts (which are originally derived from the cyanobacterial ancestor) are indicated via green ovals. The wide arrows indicate the evolutionary relationships between groups. The thin arrows indicate the acquisition of a red or green alga by a non-photosynthetic eukaryote and dashed arrows represent events that may explain the gain or loss of genes involved in ascorbate biosynthesis.