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[momsalive1]

Slime molds, as you may suspect, are very cool. Sometimes, they are typical little one-celled blobs living in the soil. Sometimes, they are massive blobs streaming over your mulch and rotting tree stumps, looking for food. This behavior is not always amusing:

(From http://www.mwrop.org/W_Needham/ManyHeadedSlime_060127.htm

“In 1973, large yellow growths were noticed on some suburban lawns in Dallas, Texas, causing some residents to become alarmed. Fearing that an alien species was encroaching on the city, they summoned the fire department to eradicate it. Unfortunately, the high pressure water served only to spread and accentuate the growth of the yellow substance. As the demand for National Guard action against the invasion reached the governor's desk, a scientist was called in who identified the alien blob as slime mold..."

There are two types of slime molds that are well studied. Cellular slime molds are not nearly as photogenic as plasmodial slime molds, but there are a lot of fun videos of them, and I have collected a few here. Later I'll do the plasmodial ones.



People interested in coordinated cell movements (during processes like embryo development and formation of nets of new blood vessels) are big fans of cellular slime molds, and so there is a fair amount know about at least one species, Dictyostelium discoideum (how do you do italics on this?). The videos here are from either dictybase.org. or http://www.zi.biologie.uni-muenchen.de/zoologie/dicty. There are also a bunch at http://cosmos.bot.kyoto-u.ac.jp/csm/movies.html, but I couldn't figure out how to link them.

Here's the life cycle - click for a bigger version.



The amoeba wander around in leaf litter and other moist places eating bacteria or other interesting bits. Like most amoeba, they are microscopic. When food begins to be scare, they start doing things no other amoeba do. They release a chemical that attracts other amoeba towards them, and the gathering cells start to form one huge aggregate.

Here they are streaming toward a source of this chemical, on the left. This video is composed of frames taken every 6 minutes (image from P. Devreotes, Johns Hopkins Medical Institutions). I’m not sure what is going on in the last few frames:

http://dictybase.org/Multimedia/development/Dzl4243.mpg

There are distinct patterns to the aggregation, and this is the subject of much research I don't understand. It has something to do with chemicals going out in waves: I think a group will get the signal, respond, then send out its own signal.

Here’s a still of the pattern....from http://www.scienzaesperienza.it/controluce/mic.php?id=026.php)



...and a video of it.

http://dictybase.org/Multimedia/morphogenesis/spirw.avi

(Legend: “Dark field waves of D. discoideum cells on caffeine agar. Time between images is 36 seconds. From F. Siegert and C. J. Weijer, J. Cell Sci. 93, 325-335 (1989).”)

Tens of thousands of these guys gather, and the pattern is now a swirl. A movie made from frames taken every 15 seconds (cites Siegert F. & C. J. Weijer (1995), Current Biology Vol 5 No. 8, 937-943 in the legend)

http://www.zi.biologie.uni-muenchen.de/zoologie/dicty/mpg/multa.mpg

So now they are a mound which moves around as a "slug"

http://www.zi.biologie.uni-muenchen.de/zoologie/dicty/mpg/perslug.mpg

(Time between frames is 20 seconds, scale bar is 100 µm, and the cite in the legend is Dormann D., Siegert F. & C.J.Weijer (1996), Development, 122, 761-769 (Abstract).)

Eventually they somehow decide its time to reproduce. They make a stalk, they make spores in the top of the stalk, and these dry up and blow away. They hatch into amoeba.

Two videos. Keep in mind that this was, a day or so ago, thousands of individual amoeba who had never met.

http://www.zi.biologie.uni-muenchen.de/zoologie/dicty/mpg/kulm1_1.mpg

(Time between successive images 5 seconds, scale bar is 50 µm, and the legend cite is the same as in the previous one.)

http://dictybase.org/Multimedia/development/culm.avi

(from R. Chisholm, Northwestern University).

A still of a fruiting body: I don't know where I got this.



Some cells get to be stalk cells, others get to be spore cells. This is one of the traits of slime molds that make an evolutionary biologist’s toes curl with delight because it isn't easy to explain. From the point of view of natural selection, this is just wrong. Why contribute to a structure that may not have anything to do with reproducing your genes? Why be a stalk when someone else gets to be the spore? Unless there is some benefit to you, taking such an altruistic stand is not thought to be a very long lasting strategy. Mutants that are cheaters appear: always the spore, never the stalk. Future generations quickly become all spore producers and everyone loses.

If the amoeba were all related, this would make a lot more sense. Bee hives are the classic example of related individuals acting together as a sort of “super” individual: you may not reproduce, but your sister does and therefore the genes you share with her make it to the next generation.

It is more difficult to explain different individuals making up a multi-cellular whole, as in cellular slime molds, since the assumption is that they are unrelated. Who decides whose genes get passed on?

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[nutmeg3.livejournal.com]

Quick! Get Mulder! (Too bad The X-Files is gone or we could have an Invasion of the Slime Mold ep.)