The prisoner was plucked from a free-living existence and plunged, without trial, into a cell from which it will never leave. It will be provided with food but will have to cater to the needs of its jailer, bereft of its own independence. And yet, this apparent injustice will go unchallenged. No liberal hackles will be raised and no bags of angry letters from Amnesty International will flood the oppressor's mailbox.
For this event did not occur in the world of autocrats and tyrants, but in that of a microscope slide. The captor measures less than a thirtieth of a millimetre across and its captive is smaller still. Even so, their relationship is a snapshot of one of the most important events in the planet's history.
This story begins with scientists Noriko Okamoto and Isao Inouye from the University of Tsukuba, Japan. During a routine sampling of sand from a local beach, they discovered a unique microscopic creature, so puzzling that they named it Hatena, the Japanese word for 'enigmatic'. Hatena is a made up of just a single cell, like the famous Amoeba.
When Okamoto and Inouye looked at Hatena under the microscope, they noticed that it contained a striking green mass in its tip. To their surprise, the mass was not an integral part of Hatena's body; when the microbe reproduced by splitting in two, only one of the daughter cells inherited it. Using some DNA-based detective work, the scientists identified the green structure as Nephroselmis, a type of algae.
The relationship between Hatena and Nephroselmis is called an 'endosymbiosis'- an arrangement where one creature lives within the body of another. It's likely that both microbes benefit from their partnership.
Nephroselmis contains a primitive 'eyespot' that, when engulfed, is always oriented to Hatena's tip. So by taking in this algal lodger, Hatena gains the useful ability to sense light sources and move towards them.In light, Hatena can use its green tenant's ability to photosynthesise - to combine carbon dioxide and water into sugars using solar energy - to make food for itself. And while Nephroselmis seems to have been unwittingly imprisoned by Hatena, it probably receives half-digested food particles in payment.
A snapshot of evolution
Millions of years ago, early microbes struck up a similar partnership and this was a pivotal event in the evolution of life as we know it. Modern plants are famed for their ability to photosynthesise but bacteria had discovered this trick far earlier. Scientists now believe that in ages past, a primitive single-celled plant engulfed one of these photosynthesising bacterium. Instead of digesting it, the prehistoric plant formed an endosymbiotic alliance with the bacterium, in the same way that Hatena and Nephroselmis co-exist today. Gradually, both microbes began to depend on each other more and more, and the symbiont became an integral part of its host.
Today, the descendants of these pioneering symbionts exist inside every single plant cell as chloroplasts, the structures that allow plants themselves to photosynthesise. Other bacteria that took up endosymbiotic residence inside primitive cells have become one of the most important parts of our cells' structures - the mitochondria. These small factories provide the very energy that every animal and plant needs to survive. The fact that chloroplasts and mitochondria have their own membranes and DNA provide clues to their bacterial ancestry.
Nephroselmis and Hatena provide a tantalising glimpse into the early steps of this process. When Hatena divides, one of its daughters inherits the parent's green captive. The other, colourless daughter starts life as an active, animal-like predator. Armed with a feeding tube, it seeks out and engulfs a Nephroselmis of its own. Once this happens, both microbes undergo drastic changes. Hatena loses its now unnecessary feeding tube, and ekes out a more easy-going plant-like existence. Nephroselmis grows bigger, loses some internal structure, discards the tail-like flagellum it once used to move around and becomes a fully-functioning part of Hatena.
Okamoto and Inouye found that Hatena is very picky about its choice of tenant, physically changing only after it has engulfed a specific Nephroselmis strain. Other strains weren't digested but nor were they integrated into Hatena's body. This specific interaction is the first step to true endosymbiosis. After this, the next move for both partners would be to seal their contact by exchanging genes, and the Japanese group is now trying to see if they have begun to do this.
Whatever the answer, this enigmatic partnership does show us how the first steps of endosymbiosis may have occurred. And without this process of two organisms becoming one, life on earth would be devoid of plants and animals as we know them. This truly is a case of the whole being greater than the sum of the parts.
Reference:Okamoto, N. (2005). A Secondary Symbiosis in Progress?. Science, 310(5746), 287-287. DOI: 10.1126/science.1116125
It can photosynthesise and move? It's the early stages of evolution of the triffid, I tell you!
However, a major difference between this "new" endosymbiotic process and that of mitochondria and chloroplasts is that engulfment/phagocytosis is an energetically active process and therefore a feature usually associated with eukaryotes (with their mitochondrial powerhouses). I am not sure how the original endosymbiotes and hosts got around this barrier. Perhaps someone else knows.
Che, so are you saying that having mitochondria makes it easy for a cell to absorb even more endosymbionts?
Nope not necessarily. I would expect that there must be some sort of advantage for the both the host and the endosymbiont for successful partnership to ensue.
Mitochondria give eukaryotic cells sufficient energy to allow for a process like phagocytosis. I would expect that any disruption of the membranes of bacteria would affect their capacity to generate ATP and hence bacteria (as a norm) do not phagocytose, despite the advantages it may offer.
My point was that this fusion event between Hatena and Nephroselmis is therefore different in terms of energetics to the original fusion event between the mitochondrial precursor and its host.
I read a book "Power, Sex, Suicide: Mitochondria and the meaning of Life" by Nick Lane which discusses these issues. I just don't remember the details.
Does Nephroselmis really benefit? Mitochondria and chloroplasts reproduce inside host cells, so they may be much more numerous than their free-living ancestors. But Nephroselmis, which might have been able to reproduce as a free-living cell, appears to lose this ability as a symbiont.
I wonder whether "growing bigger" has anything to do with losing the ability to divide. Those rhizobium cells that get bigger inside legume root nodules lose the ability to divide and basically become slaves to the plant, while their nonswollen sisters retain the ability to reproduce.