Living with other people can be tough at times, particularly if you’ve gotten used to living on your own before. For example, someone can be driven slowly potty when living with a ‘rollophobic’ by his or her pathological inability to change the loo roll. As such, sharing your living space harmoniously requires both patience and cooperation, but at least it makes the rent cheaper and you are not stuck with the people you live with 24/7. Other creatures take ‘living together’ to the extreme—for example, can you imagine if you actually became physically fused to your housemates to form one living organism?
This is exactly what slime moulds, such as Dictyostelium discoideum, will do when the going gets tough. Slime moulds are bizarre living things—distinct from plants, animals and fungi—and I have blogged their problem solving abilities before, in a guest post on the Plant Scientist’s blog.
In the wild, Dictyostelium spends most of its time as single amoeba-like cells that forage in soil and damp leaf litter. These cells will multiply as they feed on bacteria in the decaying plant material; but when this food starts to run out, these unicellular organisms will gather together. Previously free-living cells aggregate and literally ‘glue’ themselves together with sugar-coated proteins. Doing so forms a multicellular organism resembling a tiny slug (about 2 – 4 mm long), which will then crawl towards warmth and light, though layer of its own slime.
Once it has found a suitable spot, the cells in the slug undergo one of two changes: some form a stalk-structure that lifts up the other cells, which become spores. This ‘fruiting body’—a bit like a miniature mushroom—will then release these spores, which can hatch into new free-living cells and the lifecycle can start over again.
One question that has attracted much attention of people interested in this slime mould’s lifecycle: why would a cell in the slug ‘decide’ to become part of the stalk rather than a spore? Even though the cells that come together to form the slug could be unrelated; those that form the stalk will give-up their chance to reproduce, and instead choose to lift up the spore cells, helping them to disperse. Further, aggregated cells appeared to be randomly assigned either a stalk or a spore fate very early on. At first ‘pre-stalk’ and ‘pre-spore’ cells appear in a jumbled manner, and later sort themselves out, such that pre-stalk cells move to the front of the slug, and pre-spore cells move to its back or tail-end. But what causes the cells to make these decisions in the first place?
Interestingly, Dictyostelium diet during its free-living stage can have a massive influence on each cell’s role later on in life. Cells that were well fed before gathering in the slug are more likely to become spores; whilst free-living cells that grew under limited nutrients are more likely to form the stalk. Using ‘nutritional history’ to influence which cells will become spores, so-called ‘survival of the fattest’, would mean that the healthiest cells are bequeathed to the next generation.
Recent discoveries, published in eLife, have uncovered the genes that seem to allow Dictyostelium cells that are choosing their development fate, to remember how well they were fed during their early days; however, as with most things the story is more complex. For example, it is still not entirely clear what maintains cooperation and stops Dictyostelium ‘cheaters’—that would always become spores—from taking over. And just possibly, uncovering the slime mould’s secrets may provide us with the answers regarding how we can live more harmoniously with others.
RESEARCH ARTICLE : Chattwood et al. (2013), Developmental lineage priming in Dictyostelium by heterogeneous Ras activation, eLife DOI: http://dx.doi.org/10.7554/eLife.01067
INSIGHT : Morgani and Brickman (2013), Cell fate choice: Survival of the fattest, eLife, DOI: http://dx.doi.org/10.7554/eLife.01760
STUNNING PHOTOS : Dictyostelium discoideum by Alex Wild
PS If any of my current housemates are reading this: you guys are great! And I really don't mind about the loo-roll thing - honest!