Comparative study of galactolipid composition and biosynthetic genes for galactolipid synthases in Vitrella brassicaformis and Chromera velia, two recently identified chromerids with red algal-derived plastids
Khadka, Manoj; Dahmen, Jeremy L.; Salem, Mohamed; Leblond, Jeffrey D.
The phylum Chromerida consists of two species Chromera velia and Vitrella brassicaformis that are close photosynthetic relatives of apicomplexans. To date, no studies have been published regarding the characterization of the plastid lipids of V. brassicaformis. Such studies could be useful for making a chemotaxonomic inference between V. brassicaformis and C. velia, and to the presumed plastid ancestor of chromerids in general. We used a combination of mass spectrometry techniques and RNA-seq data to elucidate the fatty acid regiochemical composition of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and identify the MGDG and DGDG synthase genes in Vitrella brassicaformis, respectively. V. brassicaformis primarily contained C20/C14, C20/C16 and C20/C18 (sn-1/sn-2 regiochemistry) fatty acid components attached to the glycerol moieties of MGDG and DGDG; these forms of MGDG and DGDG are different and structurally more diverse than the previously identified C20/C20 forms that compose nearly the entirety of C. velia's MGDG and DGDG. This biochemical difference is consistent with previously observed ultrastructural and pigmentation differences between V. brassicaformis and C. velia. However, phylogenetic analyses of the MGDG and DGDG synthase genes showed that both were closely related, sharing a red algal origin. Though the plastids of C. velia and V. brassicaformis seem to be originated from a red algal ancestry, the galactolipid diversity indicates possible differences in galactolipid biosynthesis. Thus, we present hypotheses for how other aspects of the galactolipid biosynthesis pathways, namely fatty acid elongation and desaturation, in the two organisms must differ prior to the addition of galactose to form MGDG and DGDG.