Oborník et al., 2012
Vitrella brassicaformis, also known as a 'chromerid', is a species of alveolates isolated from the Great Barrier Reef. Its closest known relative is Chromera velia. Vitrella differs from Chromera in having a more complex lifecycle, for instance involving a range of sizes and morphologies. Also, while Vitrella is greenish coloured, Chromera is brown coloured. The differences are in the types of secondary pigments that characterize each genus. Both genera lack chlorophyll b or c; these absences link the two taxonomically, as algae bearing only chlorophyll a are quite few amid the biodiversity of life. Phylogenetically, both Vitrella and Chromera are relatives of the obligately parasitic phylum Apicomplexa, which includes Plasmodium, the agent of malaria. Both Vitrella brassicaformis and Chromera velia are photosynthetic.
Isolation and identification
Extant cultures of Vitrella were isolated by R. Moore. These are available from the NCMA culture collection in Maine USA (cultures 3156, 3157, 3158) and are also backed up in other collections such as NQAIF (Australia), and CCAP (UK). In 2004 the strains deposited to CCMP (now NCMA) by R. Moore were "re-isolated" (repurified) manually by staff at CCMP. The staff worked under the assumption that the flagellate (motile) stage could be separated permanently from the benthic spherical stages, which is not so, as these are stages of a single lifecycle. The fact that this unusual peer-review process could have happened in the history of the description of the species is an example of the very unusual lifecycle/morphology combination of the organism compared to other photosynthetic eukaryotes that many culture collections were accustomed to dealing with. However it is not an unusual lifecycle for dinoflagellates which are photosynthetic relatives of Vitrella.
Besides its varied somatic lifecycle, V. brassicaformis' putative gametogenesis and recombining stages have been well documented.
The term "mixotrophy" defines this lifestyle which mixes phototrophy (light as energy source) and heterotrophy (predation as energy source). Mixotrophic dinoflagellates are extremely common in the food web, and "Vitrella"-like organisms may have been the ancestors of such, raising the possibility that further families of Chromerida may eventually be found in the environment.
Vitrella brassicaformis was described in 2011 by Obornik et al., from type material RM11 (CCMP3155) originally isolated from host Pocillopora damicornis. Major differences between Vitrella and Chromera were noted by the authors and two distinct families Vitrellaceae and Chromeraceae were set up.
Evolution and taxonomy
Just as for its sister family Chromeraceae, the family Vitrellaceae is a bridge between alternate views of protist evolution: the botanical view versus the zoological view. These views need not be opposed. Apicomplexans (being non-photosynthetic) are generally described using the zoological code, while protistan (often unicellular) algae have often been described using the botanical code. Protistologists have always been free to use whichever code they choose, and these two organisms (Vitrella and Chromera) are prime examples of the need for this freedom. They possess a flagellate stage and a benthic stage.
Researcher Thomas Cavalier-Smith investigating the origins of apicomplexans and dinozoans, suggested a joint category Myzozoa encompassing both of these superphyla, plus related groups the colpodellids and perkinsids. By morphology and lifestyle,V. brassicaformis more closely resembles an ancestral Myzozoan than C. velia does. V. brassicaformis has a circular plastid genome as an ancestral Myzozoan would have had, and is also capable of hunting (myzocytosis) as they were. These two lifestyles, autotrophy and heterotrophy, found in one organism (V. brassicaformis) represent the store of potential that was able to lead to the Myzozoan radiation.
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