Motility Symbiosis - A Clue to Cellular Evolution?

Swimming inside the gut of the drywood termite Cryptotermes cavifrons is a remarkable microorganism called Caduceia versatilis¹. C. cavifrons and C. versatilis share a symbiotic relationship – that is, a partnership between two species from which both parties benefit in some way. The tiny protists inside the intestines of the termite help it to digest wood², allowing it to absorb the products it needs, whilst it provides a food source and habitat for the protists³.

What is so special about C. versatilis? The answer lies in the way in which it moves through the gut of the drywood termite. Looking through a microscope, it would appear at first that the microbe propels itself forward by the wavelike motion of tail-like structures called flagella, as is the case in many species of bacteria⁴.

In fact, the surface of the protist cell is lined with thousands of even tinier rod-shaped bacteria called spirochetes. These bacteria possess their own flagella, which rotate in perfect synchrony to create a wave of movement that propels C. versatilis forward. In other words, the protist is involved in another symbiotic relationship – the rod-like bacteria on its surface actually allow it to move through the gut of C. Cavifrons⁴. This type of symbiosis, ­­in which bacteria confer the ability to move onto their partners, is called motility symbiosis⁵.

Motility symbiosis is a rarely documented phenomenon; another of the few known examples of it is in a protist called Mixotricha paradoxa¹. Yet, believe it or not, it could hold a clue to a question upon which biologists today are still not agreed. Let’s take a look at the cells of eukaryotes (animals, plants, fungi and protists).

Many eukaryotic cells have their own flagellum-like tails, sometimes referred to as undulipodia as they differ somewhat in structure from the flagella found on bacteria⁶. Undulipodia also include cilia, small hair-like organelles which are found, for instance, in the windpipe of humans where they beat back and forth to keep our airways clear of dirt and mucus⁷. The whipping motion is driven by the action of special proteins inside undulipodia called motor proteins⁸.

The question, then, is: how did undulipodia arise in the first place? Most biologists believe that they evolved gradually from the flagella of prokaryotic cells, their structures changing as they became more specialised and adapted to the needs of eukaryotes. For example, undulipodia are considerably thicker than bacterial flagella and move in a whipping fashion rather than a rotating one⁶, which may be evolutionary adaptations to the bulkier size of eukaryotic cells: undulipodia which propel cells forward, such as the ones found on sperm cells, must be able to create a larger driving force in order to move a heavier cell.

This idea, known as autogeny⁶, is just one hypothesis, and in 1970 a biologist called Lynn Margulis proposed a more radical one: that somehow, undulipodia could have evolved from the symbiotic relationship of an immotile cell with a type of flagellum-possessing bacterium called a spirochete. This is where we come back to the idea of motility symbiosis: from the examples we have discussed above, we already know that it is possible for a symbiotic relationship to exist in which a bacterium confers motility onto another organism by effectively lodging on the membrane and rotating its own flagella to drive both cells forward. Margulis proposes that, at some point in evolutionary history, a spirochete and a eukaryote somehow fused together to form a single cell with the first undulipodium. This eukaryote, now having a competitive advantage over its peers in that in can move independently, would have been favoured by natural selection and hence undulipodia became widespread in eukaryotes.

This idea of endosymbiosis is vastly different to autogeny, which proposes gradual evolutionary changes, but Margulis argues her case strongly, claiming that undulipodia are more closely related in structure to spirochetes than to ordinary flagella⁶. Most biologists are sceptical about the origin of undulipodia by endosymbiosis due to a lack of supporting evidence – for example, if undulipodia were once free-living bacteria, why do they not contain their own DNA? – but Margulis’ hypothesis has not been rejected completely. Her appeal to existing cases of motility symbiosis in today’s biosphere, such as that in C. Versatilis, as evidence for endosymbiosis is perhaps a strong one – but, on the other hand, it could be a mere coincidence. The origin of undulipodia could be one of those great mysterious questions in science which will never be answered for definite.

References

1 Biology of Termites: a Modern Synthesis – D. E. Bignell, Y. Roisin, N. Lo (2011) – 15.7.3: Motility Symbiosis

2 Featured Creatures: Cryptotermes cavifron – A. S. Brammer, R. H. Scheffrahn, University of Florida (2002)
4 The Motility Symbiont of the Termite Gut Flagellate Caduceia versatilis Is a Member of the “Synergistes” Group – Y. Hongoh, T. Sato, M. F. Dolan, S. Noda, S. Ui, T. Kudo, M. Ohkuma (2007)

3 Termite Gut Symbionts – W. v Egmond (2004)

5 A Dictionary of Genetics – R. C. King, W. D. Stanfield, P. K. Mulligan (2007) – Motility Symbiosis

6 Doing Biology – J. Hagen, D. Allchin, F. Singer (1996) – Chapter 3: Lynn Margulis and the Question of How Cells Evolved

7 Structure and Function of Cilia – Ciliopathy Alliance

8 Life: The Science of Biology – B. Purves, D. Sadava, G. Orians, C. Heller (2004) – Chapter 5: Cells: The Basic Units of Life