Figure 2. Coulson plot indicating the taxonomic distribution of the different ascorbate pathways.

40 eukaryote genomes were analysed for the presence of genes in the ascorbate biosynthetic pathways. The two potential terminal enzymes in the pathway are boxed. GLDH is common to both the ‘plant’ and ‘euglenid’ type pathways. A schematic tree depicts the currently accepted phylogenetic relationships between organisms. The predicted route of ascorbate biosynthesis in each organism is shown. Note that ‘euglenid’ and ‘rhodophyte’ type pathways cannot currently be distinguished from sequence analysis alone and the predictions are based on biochemical evidence. Asterisk denotes a genome assembly was not available for Euglena gracilis and its transcriptome was analysed (‘Materials and methods’). Grey circles in VTC4 represent the presence of a highly similar enzyme, myo-inositol-1-phosphate phosphatase that exhibits l-galactose-1-phosphatase activity. GULO in trypanosomes and yeasts acts to oxidise the alternative substrates l-galactonolactone or D-arabinonolactone respectively. VTC3 is not a biosynthetic enzyme, but represents a dual function Ser/Thr protein kinase/protein phosphatase 2C that may play a regulatory role in the plant pathway (Conklin et al., 2013). Black cross represents a pseudogene encoding a non-functional enzyme. Organisms with sequenced genomes that were found to lack both of the terminal enzymes in the known pathways (GULO and GLDH) are likely to be ascorbate auxotrophs and were not included in the plot. These include Giardia intestinalis, Trichomonas vaginalis, Entamoeba invadens, Plasmodium falciparum and Perkinsus marinus.

DOI: http://dx.doi.org/10.7554/eLife.06369.004

Figure 2—figure supplement 1. Distribution of GULO and GLDH in the Archaeplastida.

A schematic tree demonstrating the currently accepted phylogenetic positions of the major lineages in the Archaeplastida (Yoon et al., 2006; Leliaert et al., 2012). The presence of either GULO or GLDH in representatives of each lineage is shown. The boxes denote the Viridiplantae (green), Rhodophyta (red) and Glaucophyta.

DOI: http://dx.doi.org/10.7554/eLife.06369.005

Figure 2—figure supplement 2. Distribution of the different ascorbate pathways.

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.

DOI: http://dx.doi.org/10.7554/eLife.06369.006