Chromera velia

Chromera velia, also known as a "chromerid",^[1][2] is a unicellular photosynthetic organism^[3] in the superphylum Alveolata. It is of interest in the study of apicomplexan parasites, specifically their evolution and accordingly, their unique vulnerabilities to drugs.^[4]

C. velia has helped put the spotlight back on the role of protist research in various clinical and scientific applications regarding both algae and parasites. Strict separation of botanical protists (algae) and zoological protists (protozoa) has been conventional but C. velia may be regarded as a good example of a bridge.

C. velia has typical features of alveolates, being phylogenetically related to Apicomplexa (a subgroup of alveolates), and contains a photosynthetic plastid (chloroplast) while the apicomplexans have a non-photosynthetic plastid called the apicoplast.

C. velia uses metabolites (reduced carbon) from its plastid as its primary energy source. The same is true of the algal cousin of C. velia, another chromerid Vitrella brassicaformis. Together these are phylogenetically the closest known autotrophic organisms to apicomplexans.

Parasites in the apicomplexan genus Plasmodium are the causative agents of malaria. Studies of C. velia and V. brassicaformis are broadly useful for understanding the biochemistry, physiology and evolution of the malaria parasite and other apicomplexan parasites.^[1]

Plastid terminology

"Apicoplast" is a specialised word, derived from the word "plastid". Initially the word plastid was more suitable than "chloroplast" when describing organelles of apparent algal descent in any protist, but that lack any chlorophyll or light absorbing pigment. Apicomplexan parasites are a prominent example. The majority of members of their lineage still contain a genome in the plastid,^[5][6] but no light absorbing pigments or light reaction machinery.

While Chromera velia contains a photosynthetic plastid, the majority of apicomplexan relatives contain a non-photosynthetic plastid, and the remainder contain no plastid. The ancestral photosynthetic plastid of ancestral apicomplexans may have been very similar to the plastid of C. velia or the plastid of V. brassicaformis.

Just as the term "plastid" has become widely adopted for chloroplast-derived organelles of non-photosynthetic protists, the term "apicoplast" has also gained acceptance for the plastid of apicomplexans. In current usage, the term plastid may even be used to describe the chloroplast of any photosynthetic organism, and so has a general non-discriminatory use.

Isolation and phylogeny of C. velia

Chromera velia was first isolated by Dr Bob Moore (then at Carter Lab, University of Sydney) from the stony coral (Scleractinia, Cnidaria) Plesiastrea versipora (Faviidae) of Sydney Harbour, New South Wales, Australia (collectors Thomas Starke-Peterkovic and Les Edwards, December 2001).

It was also cultured by Dr Moore from the stony coral Leptastrea purpurea (Faviidae) of One Tree Island Great Barrier Reef, Queensland, Australia (collectors Karen Miller and Craig Mundy, November 2001).^[1]

With the use of DNA sequencing, a relationship between C. velia, dinoflagellates and apicomplexans was noted.^[1] Genomic DNA of C. velia was extracted to provide PCR templates, and when the sequences of the amplified genes were compared with those of other species, biostatistical methods resulted in placement of C. velia on a phylogenetic branch close to the apicomplexans.^[1] Through a variety of phylogenetic tests on the orthologous genes found in similar organisms, researchers were able to relate C. velia to dinoflagellates and apicomplexans which are alveolates. Both the nucleus and the plastid of C. velia showed alveolate ancestry. A subsequent study of the C.velia and V. brassicaformis plastid genomes has shown in greater detail that the plastids of peridinin dinoflagellates, apicomplexans and chromerids share the same lineage, derived from a red-algal-type plastid.^[7]

Description and availability

After the naming of the organism and description of the immotile form,^[1] several papers have since documented the motile flagellate form.^[2][8]

A structure resembling an apical complex in the flagellate includes a pseudoconoid and long sacculate micronemes,^[2] confirming a distant relationship to apicomplexans that has yet to be formalised. However the function of these organelles, if any, is unknown as yet in the case of Chromera.

Live C. velia is available to purchase from the NCMA culture collection in Maine USA,^[9] and is backed up in other culture collections such as CCAP (UK),^[10] and SCCAP (Scandinavia).^[11]

Preserved material is deposited in the Australian Museum, Sydney, as holotype/hapantotype Z.6967, being a preserved culture embedded in PolyBed 812 (electron micrographs of the holotype/hapantotype are shown in Figs 1a and 1b of ref 1), and is separately deposited also in absolute ethanol.

Special features of the C. velia plastid

The plastid of Chromera velia has 4 surrounding membranes and contains chlorophyll a, while chlorophyll c is missing. Photosynthesis has been examined in C. velia, and its photosynthetic carbon assimilation was shown to be very efficient, in the sense of adaptability to a wide range of light regimes, from high light to low light.^[12] Thus like other algae that contain only chlorophyll a (such as Nannochloropsis, a stramenopile), the lack of chlorophyll c does not appear to debilitate chromerids in any way. Accessory pigments in C. velia include isofucoxanthin.^[1]

Unlike other eukaryotic algae which use only UGG codons to encode the amino acid tryptophan in plastid genomes, the plastid genome of C. velia contains the codon UGA at several positions that encode tryptophan in the psbA gene^[1] and other genes.^[7] The UGA-Trp codon is characteristic of apicoplasts, and the mitochondria of various organisms, but until the discovery of C. velia, was unprecedented in any photosynthetic plastid. Discovery of this organism has thus provided a model to study the evolution of the apicoplast.

Mitochondrion

The mitochondrial genome of C. velia has been sequenced.^[13][14] The genome encodes a single gene - cox1 - and several fragmented rRNA molecules. This mitochondrial genome is one step further devolved than those of peridinin dinoflagellates, which contain three protein-coding genes. However both lineages, C. velia and dinoflagellates, contain functioning mitochondria, the genes having moved to the nucleus.

Most of the Apicomplexan mitochondria that have been previously sequenced also have only three protein encoding genes including cox1 and a number of fragmented rRNA genes. Exceptions to this rule are known: the apicomplexan organism Cryptosporidium appears to lack a mitochondrion entirely. Work on characterization of mitochondrial functions in other apicomplexans is ongoing.

An unexpected finding in Chromera was that a large (1μm diameter) ever-present organelle bounded by two membranes, and originally thought to be the mitochondrion,^[1] is not a mitochondrion but an extrusosome called the "chromerosome".^[2] The actual mitochondria, by contrast, were found to be small and multiple,^[2] just as for other alveolates.

Evolution

The discovery of Chromera velia and its unique plastid which is similar in origin to the apicoplasts, provides an important link in the evolutionary history of the apicomplexans. Previous to the description of C. velia, much speculation surrounded the idea of a photosynthetic ancestral lineage for apicomplexan parasites. For a step by step history of the characterization of the apicomplexan apicoplast organelle, see for example the web review by Vargas Parada (2010).^[6]

It is hypothesized that apicomplexans, with their relic chloroplast, the apicoplast, were once able to synthesize energy via photosynthesis. Ancient apicomplexans or their immediate progenitors may have had a symbiotic relationship with the coral reef around them. To achieve that, these ancient organisms would have possessed a working chloroplast. However, if so, this autotrophic ability was lost and apicomplexans have slowly evolved to become a parasitic species dependent on hosts for survival.^[15]

Although researchers are still discussing why apicomplexans would sacrifice their photosynthetic ability and become parasitic, it is suggested that clues might be gathered by studying the evolution of the C. velia plastid relative to dinoflagellates.

In eukaryotes, the process of Endosymbiotic Gene Transfer results in many genes that function in a plastid becoming located in the nucleus, with the gene product - a protein - being subsequently re-imported to the plastid. The current state of Chromera genomics may be insufficient to trace enough of these events conclusively,^[16][17] and rather awaits Expressed Sequence Tag analysis of uncontaminated cultures, and/or a full nuclear genome sequence, for a clear understanding of the evolution of the C. velia plastid.

One aspect of the genomic structure has been conclusively analysed. The telomere sequences of C. velia are (TTTAGGG) similar to those in dinoflagellates and apicomplexans.^[18]

Pharmacological significance

One potentially important contribution that C. velia has to offer the community besides its role as a missing link between parasitic and algal species, is its potential use in studies aimed at finding new antimalarial drugs. Formerly scientists have targeted many drug treatments to the apicoplast in Plasmodium cells.^[19] The essential biological function of the apicoplast is solely the production of isoprenoids and their derivatives,^[20] without which the parasites cannot live.^[20]

The C. velia plastid could serve as a convenient model target for the development of antimalarial drugs, since it effectively contains the original apicoplast, as it were. In the laboratory setting, working with apicomplexan parasites can be difficult, because they must be infected to live host cells to remain viable. Chromera velia, is more easily maintained, and is related to the parasites, so may potentially provide a laboratory model for the understanding (or even the development) of antimalarial treatments. C. velia is able to live independently of its normal animal hosts and can be grown easily and cheaply in a laboratory setting.

Just as for infections in humans by the apicomplexans Plasmodium and Cryptosporidium, a better understanding of the role and function of anti-apicomplexan drugs could also enable improved control of animal infections that are caused by apicomplexans, including Toxoplasma, Babesia, Neospora, and Eimeria. It is said, anecdotally, that almost every animal on earth has one or more species of apicomplexan parasite that challenge it. The economic burden from apicomplexan parasites is estimated in the billions of dollars,^[21][22][23] not to mention the humanitarian welfare and animal welfare impacts of these organisms.

Ecology

One study has shown that Chromera may have a symbiotic role within corals, being vertically transmitted from parent to offspring Montipora digitata via the coral's egg stage.^[24] The Chromera cells could be cultured from the M.digitata eggs and were subsequently used to transiently colonise Acropora coral larvae. Chromera's known host range therefore includes the corals M. digitata, P. versipora (type host) and L. purpurea (alternate host), and extends through tropical and temperate waters.^[1]

Analysis of environmental metagenomic datasets has revealed that there are other species related to C. velia and V. brassicaformis associated with corals, but yet to be described.^[25] These associations are globally distributed.^[25] Among these is the uncultured undescribed "apicomplexan-related lineage-V"^[26] which was inferred by the authors to be potentially photosynthetic, and appears to be a symbiosis specialist. Cultured chromerids by comparison can be hypothesized to move between the free-living and coral-associated states, as they are found in M. digitata eggs^[24] but are also associated with seaweed, judging from correlations in macroalgal metagenomic datasets.^[26] The range of life strategies and niches adopted by apicomplexan-related algae therefore resembles the spectrum of niches occupied by the coral symbiont Symbiodinium.

Research Community

The first Chromera conference and workshop was held at the Heron Island Research Station, Queensland, Australia from November 21–25, 2011.^[27] Highlights included diving and culturing. Presentations included the announcement of the second isolated chromerid, Vitrella brassicaformis. Professors and students alike participated in the conference and workshop, and a broad range of topics was covered. It was agreed that further meetings will follow, and will be conducted on approximately a biennial basis. Conference facilitation is currently arranged by McFadden and Carter labs via open email list, and the proposed next conference dates are late March or early April 2014.

References