Slime is going places

In college I kept a pet in my dorm room, possibly against regulations. However, it didn't make any noise, didn't poop all over the place, and didn't need to be fed every single day. When it got too troublesome to take care of it, I simply cut off a piece of it to keep, and killed the rest. Before you think I'm some kind of cruel monster, here's a photo of my pet.

Slime mold Physarum polycephalum growing outwards to feed on oat flakes.

Slime molds are amazing creatures. They're neither plant nor fungus nor animal, but are protists, the "eukaryotic leftovers". The one I had is called Physarum polycephalum, and is a common organism used for demonstrations in schools and biology classes. It's bright yellow, big, grows fast, and is easy to revive from a dormant culture. Slime molds come in two "flavors": the plasmodial molds like Physarum which are essentially giant single cells, and the cellular ones which are an aggregate of numerous individual amoeboid cells.

The New York Times science section, which has possibly the best science reporting of any mainstream newspaper, recently featured an article by Carl Zimmer on slime molds and why they're exciting to current biology research (via Twitter).

He highlights Physarum and its ability to solve "mazes" where the objective is to connect up different particles of food scattered on a substrate. This research won a team of Japanese researchers the prize in Transportation Planning for the tongue-in-cheek Ig Nobels in 2010 (and before in 2008). What the organism does is to spread its plasmodium outwards (much like the photo above) until it encounters these food particles, and then retract those parts of the plasmodium which are on barren areas leaving behind thicker "veins", connecting the food particles with each other and with the exploring front of the plasmodium. The link to transportation? When the particles are scattered in an arrangement resembling the major centers of the Tokyo metropolitan area, the resulting plasmodial vein network is quite similar to a map of the Tokyo rail network!

It sounds relatively easy to design an optimal transport network but such design actually requires a whole lot of trade-offs and is not straightforward. However I wouldn't go quite as far as hire them as transport engineers. The Japanese team used the mold's behavior as a template to come up with an algorithm for making such networks. The computational model is more easily controlled!

In addition to Physarum the other molds mentioned include the cellular slime mold Dictyostelium, which has a long and distinguished history of use as a lab organism for studying simple multicellularity. It's interesting to evolutionary science because to make its fruiting bodies (which release spores for dispersal), some of the slime mold cells in the colony have to sacrifice themselves to form the stalk, which leaves no descendants. This is an example of a cooperative behavior, which is puzzling from an evolutionary standpoint because it should theoretically collapse in a maelstrom of cheating for individual benefit.

Diachea leucopodia fruiting bodies, from MacRitchie Reservoir area in Singapore.

In the wild the diversity of slime molds has not been given as much attention as other organisms, but they have their own distinctive beauty. They come in many colors and shapes, but require some patience to find, by peering closely at decaying wood and leaf litter. There are slime molds in Singapore too, and I've posted a gallery of beautiful images taken by Serena Lee of the Botanic Gardens to the Protists in Singapore webpage. One particularly striking one is Diachea leucopodia, which has an iridescent black fruiting body borne on a white stalk (above). The diversity of slime molds from the tropics is poorly understood, with many species that look like temperate ones but may or may not be the same thing.

These creatures are often sidelined because of their taxonomic obscurity, and maligned because of their unfortunate name. Let's help to spread a better appreciation for them among the nature-loving public!