Cellular slime molds are single celled amoebae that live in the soil, where they eat bacteria and reproduce by fission like other amoebae. When they run out of food, however, they aggregate into large "slugs" which crawl upwards to the soil surface, where they develop into fruiting bodies with spores raised above the ground on a stalk, for dispersal by animals. In The Social Amoebae, Bonner offers a broad overview of the biology of these fascinating creatures, attempting "to give something closer to equal time to their evolution, their ecology, and their behavior, as well as their development".

Bonner begins with the slime mold life cycle, covering Dictyostelium discoideum but glancing at other species, which vary in features such as the shape of the fruiting bodies. Phylogenetic analysis using molecular data produces a tree which in parts seems to contradict trees based on morphology. And the amoebae that form the stalk effectively sacrifice themselves for the spores, which invites a sociobiological analysis of "cheaters". There is even a species of slime mold that eats the slugs of other species.

In a nice example of self-organisation, slug formation is initiated by founder cells through production of cyclic AMP; these go on to form the slug tip, which continues to control movement. The speed of movement appears to be dependent on overall slug size and mass, not the number of amoebae. And slugs use ammonia and oxygen gradients, light and heat (targeting warmth during the day and cold at night) to guide them to the surface.

Slime mold amoebae follow metabolic gradients in their differentiation into posterior and anterior groups, which become the spores and stalk. Exactly how the proportions of these are determined is still unclear, however. The individual circumstances of amoebae, in such aspects as food supply, also influence their fate.

The Social Amoebae is illustrated with attractive and effective line drawings. A leading authority who has spent his life studying slime molds, Bonner offers something of a historical perspective on his field. He also gives some background on the often simple experiments that have revealed the details of slime mold behaviour and development. The result is an elegant little synthesis that could usefully be read by other biologists or by school students.

May 2009

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