Of the amoebozoan [slime moulds], the best known are the cellular slime moulds or dictyostelids. They have been the life work of the distinguished American biologist J. T. Bonner, and what follows is largely drawn from his scientific memoir Life Cycles.- Richard Dawkins, The Ancestor's Tale, p. 504
Cellular slime moulds are social amoebas. They literally blur the distinction between a social group of individuals and a single multicellular individual. In part of their life cycle, separate amoebas creep through the soil, feeding on bacteria and reproducing, as amoebas will, by dividing in two, feeding some more, then dividing again. Then, rather abruptly, the amoebas switch into 'social mode.' They converge on aggregation centres, from which chemical attractants radiate outwards. As more and more amoebas stream in on an attraction centre, the more attractive it becomes, because more of the beacon chemical is released. It is a bit liike the way planets form from aggregating debris. The more debris acumulates in a given attraction centre, the more its gravitational attraction. So after a while, only a few attraction centres remain, and they become planets. Eventually the amoebas in each major attraction centre unite their bodies to form a single multicellular mass, which then elongates into a multicellular 'slug.' About a millimetre long, it even moves like a slug, with a definite front and back end, and is capable of steering in a coherent direction - for example towards light. The amoebas have suppressed their individuality to forge a whole organism.
After crawling around for a while, the slug initiates the final phase of its life cycle, the erection of a mushroom-like 'fruiting body.' It begins the process by standing on its 'head' (the front end as defined by its crawling direction), which becomes the 'stalk' of the miniature mushroom. The inner core of the stalk becomes a hollow tube made of swollen cellulose carcasses of dead cells. Now cells around the top of the tube pour into the tube like, in Bonner's simile, a fountain flowing in reverse. The result is that the tip of the stalk rises into the air, with the originally posterior end of the stalk at the top. Each of the amoebas in the originally posterior end now becomes a spore encased in a thick protective coat. Like the spores of a mushroom, they are now shed, each one bursting out of its coat a free-living, bacteria-devouring amoeba, and the life cycle begins again.
Sunday, May 17, 2009
101 Interesting Things, part fifteen: Slime Moulds
The following is taken from The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution, by Richard Dawkins. Much like Southland Tales, the life cyle of dictyostelids starts off just a little weird, and then gets increasingly crazy. The text speaks for itself, so I'll let the good doctor do his thing: