Still on my honeymoon, far away from any form of internet, so this is another old post from my previous blog. The post itself is not one of the best I've written, but the subject matter was so fascinating I feel it needed reposting!
This post came to light due to Captain Skellet (whose been around SciAm lately!) alerting me to Hatena. I've heard of several organisms containing proto-plasmids; symbiotic chloroplasts which haven't completely been endosymbiosed, but the strange life-cycle of the protist Hatena was a new one so I went to look it up. And I'm very glad I did, because it's pretty amazing.
The picture above shows a micrograph of Hatena. The green blob is the symbiont living inside it and the scale bar is 10um. As a quick point of background information chloroplasts are the little membrane enclosed vesicles in plants which carry out photosynthesis. The current theory for how they developed is that they were once free-living bacterial type organisms (cyanobacteria) which were engulfed by a larger cell and over time lost their own identity to become photosynthesising factories inside the larger cell.
Hatena arenicola doesn't have a chloroplast, but it does have a symbiotic relationship with another organism; nephroselmis. The nephroselmis is always found in the same place in the Hatena, and carries out photosynthesis to provide energy for both of them. Unlike regular chloroplasts, nephroselmis has its own proper nucleus and even its own mitochondria although most of the internal cellular organisation and any kind of motile apparatus (such as flagella) has been lost. Nephroselmis is a sort of half-symbiont, with enough of it's own machinery to be a clearly distinct organism, but once it gets inside its host organism, it's happy to stay there and mutually benefit the both of them.
The weirdest thing about these two organisms though, is their replication cycles. When Hatena replicates, the nephroselmis doesn't, and as a result only one of the offspring gets the photosynthesising symbiont. The other organism remains colourless and develops a complex feeding apparatus at the apex of the cell, presumably as it can no longer rely on the symbiont for food. This wierd 'half plant, half predator' lifecycle is shown below. (Picture taken from the reference, scale bar 10um):
That's just weird. Seriously odd. The Hatena is able to move seemingly freely between being a predator consuming other cells for food, and being a plant-like organism once it settles down with its symbiotic partner. The grey non-symbiont organisms can be induced to take up free-moving nephroselmis and (in the words of the paper) "tentitavely" maintain a symbiotic relationship with them but it also seems perfectly happy to survive on its own.
The paper suggests that Hatena cycles between these two modes of living, depending on circumstance. Thus the 'predator' grey cell shown above will continue eating fellow cells until it consumes a nephroselmis, at which point it degrades its complex feeding apparatus, accepts energy from the symbiont until it's ready to divide. One of the daughter cells will then go through the whole cycle again while the other remains as a non-predating plant. The authors freely admit that there is little evidence for much of these stages, but it seems a reasonable way to explain what is going on.
As this is clearly a very early stage in symbiotic capture it has important implications for the endosymbiotic theory of chloroplast evolution. Along with various other 'intermediate' symbionts (such as Karenia mikimotoi and Lepidodinium viride) the Hatena helps to show how chloroplasts might have first formed in the cellular ancestor of plants. Hatena and its symbiont have already acquired an intimate structural association, only the coordination of their cell cycles would be required to turn the nephroselmis into an internally replicating plastid.
Ref: OKAMOTO, N., & INOUYE, I. (2006). Hatena arenicola gen. et sp. nov., a Katablepharid Undergoing Probable Plastid Acquisition Protist, 157 (4), 401-419 DOI: 10.1016/j.protis.2006.05.011