Dictyostelium discoideum is normally a single-celled amoeba but, under starvation conditions, it aggregates to form a fruiting body.
About 80% of the cells become spores but the remaining 20% die to form a stalk to aid in the spores' dispersal. Thus, some cells
sacrifice themselves to aid others, just as do social insect workers. Different clones can aggregate together and cheat each other by
avoiding the stalk and making more spores (Strassmann et al. 2000). Knockout mutants can be made and we are studying a number of these
mutants that cheat, with particular attention to how cheating is controlled. CsaA- is a cell adhesion mutant that cheats by sliding
back to the pre-spore region at the back of the slug. However, it is controlled on soil substrates by a greenbeard effect; it lacks the
homophilic cell adhesion protein needed to join others to get into aggregates (Queller et al. 2003). DimA- is a social defector mutant
that ignores the signal to become a prestalk cell. However, this defection does not result in successful cheating because of
deleterious pleiotropic effects, illustrating what may be a common form of cheater control (Foster et al. 2004). FbxA- is a cheater
that also has a deleterious effect, but this effect is minimal at low relatedness. This mutant can spread at low relatedness but suffers
at high relatedness. We show that relatedness in the field is high enough to prevent the spread of this kind of mutant. We have been
conducting selections that are revealing numerous additional cheater mutants for further studies of cheater control.

Strassmann J. M. et al. 2000. Nature 408: 965-967
Queller D. C. et al. 2003. Science 299:105-106
Foster, K. R. et al. 2004. Nature 431:693-696