Parcelling plastids

The Table of Contents alert for a bright, shiny new issue of Protist arrived in my e-mail box today. This is a journal I can usually rely on to supply something worthy of note (often some things), and today's was no exception. Right near the beginning of the issue, we receive a new class of heterokont (Horn et al., 2007).

So many new chromist classes have appeared in recent years that they're almost becoming routine (see Filling in the Gaps for another post of mine on one, which also includes an overview of the heterokonts). Today's subject has been dubbed Synchromophyceae, and contains a single new species, Synchroma grande, scraped off marine rocks on the coast of the Canary Islands (perhaps not the most unpleasant place to do field work). Synchroma is a photosynthetic amoeboid that forms a meroplasmodium (a network of individual cells connected by branching and anastomosing [fusing together where they meet] reticulopodia). Meroplasmodial forms are extremely rare in chromists - the only other chromist that exhibits this body form is the haptophyte Reticulosphaera japonensis (Cavalier-Smith et al., 1996). The main cell body is contained in a flattened lorica, which is adpressed to the substrate and is more or less circular on a flat surface. In reproduction, one daughter cell remains sessile in the lorica, while the other is released as a migratory cell. Migratory and floating cells are fusiform, with axopodia extending from the anterior and posterior ends. Sessile cells could also spontaneously convert to migratory cells by hatching out of their lorica. At no stage in the life-cycle were flagella present.

Phylogenetic analysis of Synchroma demonstrated that it is part of the Ochrophyta, the photosynthetic heterokont clade. Both rbcL and 18S rDNA maximum likelihood trees placed Synchroma as sister to the Chrysophyceae + Synurophyceae clade, but support in both cases was relatively low. The absence of a girdle lamella in Synchroma's chloroplasts supports its exclusion from either of those two classes.

The most interesting feature of Synchroma, however, lies in the arrangement of chloroplasts in the cell. As I mentioned previously, the chromist chloroplast is believed to be derived from an endosymbiotic red alga in a secondary endosymbiosis. As a result, the chromist chloroplast is surrounded by four membranes that represent (from the inside out) the original cyanobacterium's external cell membrane, the vacuolating membrane of the primary host, the primary host's external cell membrane, and the vacuolating membrane of the secondary host. There is some debate about whether the chloroplasts of all chromist groups (and those of their putative sister group, the alveolates) derive from a single endosymbiotic event, or have been independently derived from multiple events. The majority of authors currently favour the former option, or at least that the number of events was quite few. However, Synchroma has a unique arrangement of chloroplasts in regard to surrounding membranes. Each individual chloroplast is surrounded by two membranes, and then multiple chloroplasts are clustered in packages contained by the remaining two membranes. Horn et al. suggest in passing that Synchroma may preserve an ancestral stage in the endosymbiotic process, after the loss of the eukaryotic endosymbiont's nucleus but before the separation of the endosymbiont's chloroplasts (after all, there is no reason why the eukaryotic endosymbiont would have necessarily had only one chloroplast per cell). In light of the derived position of Synchroma within the chromists, if this is indeed an ancestral state it makes the idea of a single endosymbiosis event rather unlikely, because this would require that the ancestral state was lost repeatedly in all other chromist groups. However, I feel that it is much more likely that the other explanation suggested by Horn et al. for Synchroma's unusual chloroplasts is correct - that it is a derived state resulting from an abnormal division pattern. The possibility that within chromists cryptophytes, haptophytes and heterokonts have gained their chloroplasts independently is not completely unbelievable, but the idea that multiple hterokont groups have done the same seems to be pushing it a little. Especially as all members of this undoubtedly monophyletic clade possess red algal-derived chloroplasts - if they derived them independently, why shouldn't at least some have green algal chloroplasts?

REFERENCES

Cavalier-Smith, T., M. T. E. P. Allsopp, M. M. Häuber, G. Gothe, E. E. Chao, J. A. Couch & U.-G. Maier. 1996. Chromobiote phylogeny: the enigmatic alga Reticulosphaera japonensis is an aberrant haptophyte, not a heterokont. European Journal of Phycology 31 (3): 255-263.

Horn, S., K. Ehlers, G. Fritzsch, M. C. Gil-Rodríguez, C. Wilhelm & R. Schnetter. 2007. Synchroma grande spec. nov. (Synchromophyceae class. nov., Heterokontophyta): an amoeboid marine alga with unique plastid complexes. Protist 158 (3): 277-293.