What have we figured out about social amoebas?

  • Altruism in a single Dictyostelium discoideum clone can be lost in 30 experimental evolution rounds at very low relatedness, but is not easily lost with even occasional single-cell bottlenecks (Kuzdzal-Fick et al. 2011).
  • Slug movement across a plate has different costs and benefits if there is a dead stalk during movement (Jack et al. 2011).
  • There are likely to be cryptic species basal to D. discoideum in the neotropics (Douglas et al. 2011).
  • Dictyostelium is an excellent model system for social evolution (review) (Strassmann et al. 2011).
  • Weaker cells are more likely to become stalk (Castillo et al. 2011).
  • The newly sequenced genome of D. purpureum indicates that social genes evolve rapidly (Sucgang et al. 2011).
  • Some clones of D. discoideum and other species have a symbiotic relationship with bacteria – farming – in which they carry bacteria though the spore stage and seed them out after dispersal (Brock et al. 2011).
  • D. discoideum has sexual recombination regularly, as indicated by recombination rates (Flowers et al. 2010).
  • The first cells to starve that call in the other cells become spore, not stalk, consistent with an advantage of being the first to turn on genes for competition  (Kuzdzal Fick et al. 2010).
  • Resistance against cheaters can be easily evolved, and these resistors are not themselves cheaters on wild-type (Khare et al. 2010)
  • We found a huge field in Texas containing a single, abundant clone of D. discoideum  (Gilbert et al. 2009).
  • Different clones can recognize each other and segregate during development; this is mediated by lagC and LagB (trgC and tgrB) genes, which are highly variable (Benabentos et al. 2009; Ostrowski et al. 2009; Mehdiabadi et al. 2006, 2009).
  • Sometimes two species form fruiting bodies together, but the fruiting bodies are not intermediate morphologically (Jack et al. 2008).
  • There are over a hundred knockouts that cause their bearers to cheat against intact clones suggesting that there are many factors contributing to sociality (Santorelli, et al. 2008).
  • A mutation accumulation experiment demonstrated that microsatellite mutation rates are extremely low in D. discoideum (McConnell et al. 2007).
  • Wild fruiting bodies have very high relatedness, sufficient to suppress costly cheaters (Gilbert et al. 2007).
  • As D. discoideum slugs move, they drop off cells capable of exploiting newly-found food sources (Kuzdzal-Fick et al. 2007).
  • Chimeras have a cost; they move less far than clones on agar and soil (Foster et al. 2002; Castillo et al. 2005).
  • An apparent cheater knock-out lost late in the social stage, because of pleiotropic effects, indicating that pleiotropy is a powerful stabilizer of cooperation (Foster et al. 2004).
  • Wild co-occurring clones form a linear dominance hierarchy in which the loser produces the fewest spores, and the most dead stalk cells (Fortunato et al. 2003a, 2003b).
  • We found a greenbeard gene that recognizes and favors others with the same gene (Queller et al. 2003).
  • Multiple clones join into the same fruiting body in social amoebae, where one clone contributes more than its fair share to spores (Strassmann et al. 2000).

More detail on some topics:

  • Multicellularity and social cheating in Dictyostelium discoideum: Social amoebae have a solitary stage in which they eat bacteria and divide by binary fission. They also have a multicellular stage that begins with aggregation, so genetically different clones can come together and compete to become spore and avoid being stalk. Different clones do co-aggregate and contribute differently to spore vs. stalk, establishing this as a good system to study social evolution (Strassmann et al. 2000).
  • Single-gene greenbeard effects in the social amoeba Dictyostelium discoideum: A greenbeard gene is one that combines a trait, its recognition, and an altruistic act all in one. It is something that W. D. Hamilton predicted, R. Dawkins named, and many thought impossible, because of the effects of such a gene on the rest of the genome. We identified a cell adhesion gene that functions as a greenbeard, and demonstrated its pleiotropic effects on the multicellular organism differed according to substrate.
  • Phylogeny of Dictyostelium: There are few morphological characters to sort social amoebae into species. It is very important for studies of social interactions to know if individuals are of the same or different species. Since we can freeze spores of Dictyostelium, and do experiments repeatedly on the same clones, it is worth the effort to put individual clones in a phylogeny. We have found that many clones of D. discoideum fall into one clade. We have also found that there are likely to be several cryptic species closely related to D. discoideum in the neotropics.
  • Agriculture in Dictyostelium We have discovered that several species of Dictyostelium have clones that carry bacteria through the social stage, then disperse them when the Dictyostelium  encounter conditions favorable for proliferation. There are multiple species of bacteria carried. Some are used for food, but some are not. Farmers are prudent in the sense that they enter the social stage before they have eaten all the bacteria. Much of our future research will follow this exciting development.
  • Organismality We have defined an organism in a new and conceptually cohesive way as having high cooperation and low conflict among its parts. This definition puts animals that develop through a single cell bottleneck as organisms, but also claims that some symbioses and other collaborations formerly not understood as organisms in fact are.
  • Cheating The multicellular stage of Dictyostelium begins with aggregation, setting the stage for one clone to force the other into becoming the dead stalk. We have defined three kinds of cheating, examined their frequency, and shown that under experimental evolution with very low relatedness, altruism can be lost.