Staying together: green beginnings

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Published on: November 5, 2013
This seaweed, Ulva linza, would only exist as undifferentiated cells if not for bacterial signals

The second of our series of blog posts Celebrating Basic Plant Science is written by Juliet Coates, Lecturer in Molecular Genetics at the University of Birmingham.

Living organisms can be categorised in a number of ways, but one very obvious “either/or” distinction is between organisms that are made up of a single cell – unicellular organisms – and those that are many-celled, or multicellular.

The multicellular state arose many times during evolution: animals, plants, algae, amoebae, fungi and bacteria can all be multicellular. Multicellular organisms completely underpin life on Earth as we know it today – and they all must have evolved from single-celled ancestors. We understand a little of why they might have done so, as being multicellular gives a number of competitive advantages: increased size and improved nutrient collection being just two. Yet how multicellular organisms came to be is a key biological problem that is still largely unanswered.

I am a plant scientist, so I am particularly interested in the origins of multicellular green things: plants and algae. Without becoming multicellular, plants would never have colonised the land, and the evolution of multicellular plants and algae was key in shaping our climate, our ecosystems and our oxygen-rich atmosphere. How green multicellularity arose seems to me to be a really fundamental thing to understand, but it is a little-addressed question. Here I’ll give an overview of the important findings to date about the evolution of multicellularity.

 

Take-home message 1: If you are a unicell, you might find it pretty easy to evolve multicellularity. Recent research shows that the yeast we use in bread and beer, which is traditionally thought of as unicellular, can spontaneously start forming multicellular colonies in a lab culture, and over relatively few generations. Colonies can be selected for simply by using gravity, as they sink faster than unicells.

In the green world, there are many species of little algae (microalgae) that usually exist as single cells, but can become multicellular if they detect toxins, predators or competitors, or if they get hungry. Excitingly, this gives us a wonderful system to explore multicellular mechanisms very easily in the lab, as we can switch multicellularity on and off; with the right signals these species make colonies pretty much as you watch.

Microalgae (Scenedesmus sp.) in multicellular (left) and unicellular (right) states.

 

Take-home message 2: Interactions with other organisms can drive multicellularity. As mentioned above, becoming multicellular can be a response to competition or predation by other organisms. Moreover, some multicellular evolution can require interactions with more “benign” organisms.

Choanoflagellates: single-celled (left) and multicellular (right) Salpingoeca rosetta.

Nicole King’s lab at the University of California (Berkeley, USA) studies a weird and wonderful collection of organisms called choanoflagellates, which share a common ancestor with all animals. Choanoflagellates comprise both unicellular and multicellular species and, rather like microalgae, some species can exist in both unicellular and multicellular states during their life cycle (for examples see here and here). The researchers have shown that a signal from a bacterium associated with the choanoflagellate induces the switch to multicellularity.

Similarly in the green kingdom, we find that multicellular algae (macroalgae), otherwise known as seaweeds, simply cannot develop correctly into their familiar-looking multicellular blade structures unless they receive signals from the bacteria with which the seaweeds are naturally associated. 

 

Take home message 3: Genome sequencing does not readily distinguish unicells from multicellular organisms. Recent advances in DNA sequencing technology have enabled scientists to catalogue the entire list of genes in a multitude of unicellular and multicellular species. The initial assumption was that this would show us which genes something needs to become multicellular – one might expect certain genes to be present in multicellular species, and absent in unicellular ones.

In reality, the results suggest that genes necessary for multicellularity are also present in unicells, including genes for cell adhesion (sticking together) and cell signalling (cells talking to one another within a multicellular structure). This suggests that multicellularity may have evolved from the mechanisms unicellular organisms use to regulate their interaction with unrelated organisms or their surroundings, which in fact nicely backs up our take home messages 1 and 2.

Where can we go from here? If the genes required to be many-celled are present in unicells, then the fundamental difference between unicells and multicellular organisms must lie elsewhere. I believe that with the technologies now available to us, understanding the multicellularity problem is an exciting and attainable goal for the future.

 

What impacts could this kind of very basic, ‘blue-sky’ research have (beyond being fascinating, of course)? Well, if we understand multicellularity, we are in a position to manipulate it. From my ‘green’ perspective, I believe firstly that this could have profound effects on ecosystems. For example, the clumping microalgae discussed above are widespread in ponds and lakes. Their unicells live close to the water surface, while colonies sink. So, changing the balance of these two states could have massive knock-on effects up the food chain. Moreover, if we understand multicellularity we will also begin to understand much more about how organisms of different species interact with each other too, and perhaps discover more about the widespread phenomenon of self-non-self recognition.

Another example of note that has enjoyed recent publicity is that of green seaweeds: these can be significant ecological pests as ‘green tides’, but the silver lining of this is that they are also a great source of food and nutrition. We understand almost nothing about seaweed multicellularity at present: new knowledge will help us prevent environmental nuisance whilst maximising their useful potential.

This brings me on to a second point: biomass. We are entirely dependent on multicellular green organisms (traditionally mainly land plants) for food/feedstocks and for both fossil- and bio-fuel. Micro- and macroalgae represent an exciting and relatively under-exploited source of new food and fuel products, and provide a source of biomass that can be grown on water, rather than on land, where resources are already stretched. An example of this is in practice on a small scale is in Scotland, but the lack of understanding of multicellular algal development is a major bottleneck in developing scale-up and mass culture methods.

So, although I am not offering you an immediate ready-made high-yielding or stress-resistant food product, or a fully finished alternative to oil or solution to climate change, I have hopefully made the point that in researching a very fundamental unanswered question about biology and evolution we will almost certainly pave the way for massive (and perhaps as-yet un-thought of!) societal benefit in the longer term.

 

References linked to in this post:

William C. Ratcliff, R. Ford Denison, Mark Borrello and Michael Travisano. 2012. Experimental evolution of multicellularity. PNAS 109:1595-1600; doi:10.1073/pnas.1115323109

Mark J. Dayel, Rosanna A. Alegado, Stephen R. Fairclough, Tera C. Levin, Scott A. Nichols, Kent McDonald and Nicole King. 2011. Cell differentiation and morphogenesis in the colony-forming choanoflagellate Salpingoeca rosetta. Developmental Biology 357:73–82 http://dx.doi.org/10.1016/j.ydbio.2011.06.003

Open access Rosanna A Alegado, Laura W Brown, Shugeng Cao, Renee K Dermenjian, Richard Zuzow, Stephen R Fairclough, Jon Clardy and Nicole King. 2012. A bacterial sulfonolipid triggers multicellular development in the closest living relatives of animals. eLife 2012;1:e00013

Stephen R. Fairclough, Mark J. Dayel and Nicole King. 2010. Multicellular development in a choanoflagellate. Current Biology 20: R875-R876, doi:10.1016/j.cub.2010.09.014

 

Image creditsScenedesmus sp. in unicellular and multicellular states from the Culture Collection of Algae and Protozoa, managed by the Scottish Association of Marine Science: single-celled and multicellular Salpingoeca rosetta images courtesy of Nicole King and Mark Dayel (more can be found in the Choanoflagellate Gallery

 

 

 

 



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