Slime molds are interesting organisms that receive surprisingly little attention. Take the case of Dictyostelium discoideum, a single-celled amoeba that, when starved, will aggregate with other D. discoideum amoeba cells in the neighborhood to create a motile, multicellular structure known as a slug. Eventually the slug differentiates into a reproductive structure, with some individuals making a long stalk and others producing spores. In other words, some individuals help other reproduce but do not reproduce themselves.
But why form a slug? Why would a single celled organism decide to cooperate with other, genetically different individuals, particularly when it may provide no direct passage of its genes? The evolutionary benefits of kin relationships aside, previous work has shown that slugs do provide multiple benefits to the population as a whole.
Kessin found in 1996 that a secreted protein sheath provides protection to the individuals within the slug from nematode predators. Kessin also found in 2001 that slugs are photo- and thermo-taxic, providing the population the ability to find an unobstructed area to release its spores into the wind.
A recent analysis by Kuzdzal-Fick et al demonstrates an additional benefit of becoming multicellular in D. discoideum [Kuzdal-Fick 2007]. As the slug migrates, it leaves behind a slime sheath (similar to a banana slug) that is made of amoeba cells that fall off the tail of the slug. This study found that these cells can dedifferentiate from a slug cell back into an amoeba and scavenge for new food sources. The authors found that while the slug itself cannot eat food sources it encounters as it moves, cells in this slime trail can. Hence, those cells likely to be destined to become the stalk in the reproductive organ are provided an opportunity to be locally dispersed along the migration trail (though this is not to say that only stalk cells form the slime trail) and, when the time comes, form a new slug.
The implications this holds for sociobiology and the study of multicellularity are interesting. It suggests that, in sociobiological systems, individuals can leave the social structure and start a new colony. In regards to the origins of multicellularity, it may be that multicellular interaction is a two way street, and that individuals can enter and leave a biological social contract. Time and research will determine if this is the exception or the rule; evaluating these questions in other systems will either identify a pattern or suggest that D. discoideum is simply a unique organism capable of some curious things.