The schematic tree summarises the distribution of the two terminal enzymes in ascorbate biosynthesis, along with VTC2, the first committed enzyme in plant pathway. Blue lines indicate photosynthetic lineages derived by the primary endosymbiosis (of a cyanobacterium). Red or green lines indicate lineages that have become photosynthetic following a secondary endosymbiosis event with either a red or a green alga respectively. It should be noted that the timing and origin of many secondary endosymbioses remain unclear, particularly within the SAR supergroup where several non-photosynthetic lineages within the stramenopiles, alveolates and even rhizaria may potentially have lost an ancestral plastid. GULO is found in basally derived lineages of the Archaeplastida, Excavata, Opisthokonta, Amoebozoa and the CCTH group. In contrast, GLDH is found predominately in photosynthetic eukaryotes, although it is also found in non-photosynthetic stramenopiles and rhizaria and also in some choanoflagellates. Lineages where there is biochemical evidence determining inversion or non-inversion of the carbon chain in the conversion from D-glucose to ascorbate are shown.