Chromera velia is a recently discovered, photosynthetic, marine alveolate closely related to apicomplexan parasites, and more distantly to perkinsids and dinoflagellates. To date, there are no published studies on the sterols of C. velia. Because apicomplexans and perkinsids are not known to synthesize sterols de novo, but rather obtain them from their host organisms, our objective was to examine the composition of the sterols of C. velia to assess whether or not there is any commonality with dinoflagellates as the closest taxonomic group capable of synthesizing sterols de novo. Furthermore, knowledge of the sterols of C. velia may provide insight into the sterol biosynthetic capabilities of apicomplexans prior to loss of sterol biosynthesis. We have found that C. velia possesses two primary sterols, 24-ethylcholesta-5,22E-dien-3β-ol, and 24-ethylcholest-5-en-3β-ol, not common to dinoflagellates, but rather commonly found in other classes of algae and plants. In addition, we have identified computationally three genes, SMT1 (sterol-24C-methyltransferase), FDFT1 (farnesyl diphosphate farnesyl transferase, squalene synthase), and IDI1 (isopentenyl diphosphate Δ-isomerase), predicted to be involved in sterol biosynthesis by their similarity to analogous genes in other sterol-producing eukaryotes, including a number of algae.

The heterotrophic genus Oxyrrhis contains two species, O. marina and O. maritima, which occupy positions basal to the dinoflagellate lineage. Oxyrrhis is also related to apicomplexan parasites, which have recently been shown to have a nonphotosynthetic, relic plastid referred to as the apicoplast. A recent study by Slamovits & Keeling (2008) demonstrated the presence of plastid-targeted proteins within O. marina. We hypothesized that if O. marina does indeed have plastids, then monoand digalactosyldiacylglycerol (MGDG and DGDG, respectively), which are the two most prominent plastidial membrane lipids, would be present. Therefore, we examined three isolates of O. marina to determine if they can produce MGDG and DGDG. We observed that O. marina, when fed the chlorophyte Dunaliella tertiolecta, possessed forms of MGDG and DGDG containing a C 18:3 fatty acid at the sn-1 position and most containing a C 16:3-4 fatty acid at the sn-2 position; these were derived solely from the prey itself. Examination of published expressed sequence tag (EST) and transcriptome databases of O. marina for the genes encoding MGDG and DGDG synthases, two enzymes integral to the incorporation of galactose in the final forms of these lipids, failed to reveal their presence. Taken together, these results indicate that O. marina does not produce a non-photosynthetic, relictual plastid. However, the presence of plastid-targeted proteins may indicate that O. marina maintains, however briefly, plastids acquired from its prey in a form of kleptoplasty that has been observed previously in dinoflagellates.

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