Thursday, 12 August 2010

Slime Moulds (also, hello)

This is something I learnt about back in first year, and I thought was pretty awesome, so, having written this up previously on a personal journal, I'm reposting it in edited form to hopefully give this new blog a good start :)

Name: Dictyostelium dicoideum

What is it?: Dictyostelium is a unicellular organism... most of the time. It lives in soil, feeding on bacteria which themselves feed on decaying leaf matter (yum). Slime moulds were formerly classed as fungi, since both reproduce by releasing spores. Unlike fungi, however, these single-cell amoebae are able to move around.

Image: Dictyostelium labelled with GFP, by Richard Firtel, University of
California,San Diego. From the website of Eduardo Kac.

So why is it exciting?: Dictyostelium's life cycle is unique (as far as I know?), and very strange compared to what most people think of as a life cycle - say, the life cycle of the frog we all learnt about in primary school!

When the amoebae have exhausted their supply of food, they enter starvation mode - and this is where it gets weird. A founder cell releases a signal in the form of cyclic AMP. cAMP causes receiving cells to do two things: produce cAMP themselves, and move towards the source of the original signal via chemotaxis. The cAMP signalling pathway is negatively regulated: production of cAMP inside a cell blocks the reception of exterior signals for a short time. This means the signal propagates in waves, and is directional. Each cell responds only to external signals, not to those it produces itself.

The cells all move towards the original signal, and begin to aggregate into a single body. Once gathered together, they act as a multicellular organism. The 'slug' formed migrates to a region with available food. There it forms a 'fruiting body' - essentially, the organism changes shape, grows a stalk on top of a which a clump of cells forms, containing prespores. The fruiting body mature and eventually releases the spores, spreading them over a wide area - well, wide for something only 2mm tall...

This video shows movement of cells towards an artificially induced signal:

This one shows the 'slugs' moving and forming fruiting bodies (and is AWESOME):

What is it used for?: The cAMP signalling pathway in Dictyostelium, or something pretty similar, is used in many other organisms. Dictyostelium has a lifecycle that is (fairly) simple, quick, and has very distinct stages, and is reasonably easy to grow in the lab. That makes it a useful model for processes of cell signalling and motility.

But, more interestingly - in my opinion anyway - Dictyostelium has the potential to give insights into how organisms first became multicellular and how cells evolved to cooperate and differentiate into distinguishable parts of an aggregate. Its genome has been sequenced and found to contain genes that are homologous to some in higher eukaryotic organisms, but which are not present in Saccharomyces cerevisiae (aka brewers' yeast). Since S. cerevisiae is a single-celled eukaryotic organism that does not form cooperative aggregates in this way, it is likely that such genes will be very relevant to this process in both Dictyostelium  and high eukaryotes.



  1. Hello :)
    Whats the purpose of the fruiting bodies in Dictyostelium? Are they reproductive or are they an elaborate dispersion mechanism like is seen in biofilms?

  2. Erm... as far as I can tell, I think it's a bit of both. The slugs can migrate to new environments, so it's not purely dispersal (they migrate towards light - I guess this could be because light is necessary for succesful spore germination). Since aggregation and sporulation are triggered by starvation, though, the fruiting bodies are a good way to spread spores that can resist such conditions for a while, and may be spread to regions where food is available.

    This link on Google books might help, as I'm not sure I explained that very well!