Thursday, October 06, 2011

On a thread of slime

This week's New York Times science section did a cover story on slime molds, with color photographs of nine species of these intriguing creatures. Slime molds have long been a favored organism for biological research, and now the genomists, with their new gene sequencing prowess, are getting in on the act.

The story pushed two buttons for me.

First, it reminded me of the time some years ago when I sent away for a vial full of the slime mold Dictyostelium discoideum and spent several delightful weeks following its life cycle under the dissecting microscope. No particular reason; just idle curiosity.

The cycle begins with individual amoebas grazing on bacteria, like cows in a meadow. A chemical communication keeps them evenly spread out on a surface, so that each single-celled organism has maximum access to nourishment.

As the food supply is depleted, a new signal goes out. The microscopic amoebas start making for a central collection point, coalescing into streams, then visible rivers, then one big pool. The pool shapes itself into a slug-like creature, about the size of this letter i, with a nose and a tail, and crawls away -- on a trail of slime -- to what it decides is an appropriate spot. A part of the slug anchors itself to the surface, another sacrifices itself to make a flowerlike stalk, lifting a globe of newly-contrived spores into the air, which are dispersed on the wind to start new colonies.

Sometimes the spores take along intact bacteria to seed a new farm.

Is D. discoideum a single-celled organism? Multicelled? Both? What one has here is a stunning example of cells coming together and specializing for a greater purpose, perhaps recapitulating that epic moment in Earth's history when multicellularity began. All happening on the stage of my microscope.

The other button was John Tyler Bonner.

Bonner is one of the grand old men of American biology, 91 years old, and, as far as I know, still on the faculty of Princeton. I know him through his books, especially his charming autobiography Lives of a Biologist and his endlessly instructive - and beautifully illustrated -- The Evolution of Culture in Animals. In his autobiography, he tells how he serendipitously came to study slime molds -- especially D. discoideum -- as a student, and ended up engaged with them for the rest of his life. D. discoideum makes it into his books on animal culture, development of embryos, and evolution of multicellularity. So humble a creature, so prolific a teacher.

You can meet Professor Bonner's very own slime molds here.