Study Systems

Current Research Study System and Interests

Dictyostelium fruiting bodies

Cell lineage conflicts in the social amoebae

The social amoeba, Dictyostelium discoideum, has long been a model system for studying development and cell biology. It also has enormous but unrealized potential as a model system for social evolution. This is because the multicellular stage of this organism forms in a different manner from the usual case, in which the multicellular organism descends clonally from a single cell. In D. discoideum, a multicellular fruiting body is achieved by the aggregation of numerous separate amoebae. It has been widely recognized that if these cells come from different clones, conflicts may ensue over which cells would become the sterile stalk, and which the fertile spores. Yet almost no empirical work has been directed towards this issue. Work in the Strassmann/Queller lab has recently established that different clones will normally mix to form chimeric fruiting bodies, and also that one of the two mixed clones often cheats by contributing less to stalks than to spores.

Further work will explore this phenomenon much more widely. Empirical research will answer three questions about the factors influencing the success or failure of cheater clones in nature. First, how often do small soil samples contain different D. discoideum clones, and how does this vary with density? In areas where co-occurrence is uncommon, cheating phenotypes may also be rare because they will rarely be in a position to cheat. Second, does a clone that successfully cheats one partner also succeed at cheating many other partners? Cheating will be less successful if each cheater is successful against a limited set of victims, and if cheaters in one combination become victims in others. Third, do cheaters experience fitness costs that might counterbalance the fitness advantage they obtain in mixtures? Disadvantages to be tested for include lower growth rates in the single-cell stage, and shorter stalks when cheater clones fruit by themselves. Laboratory selection experiments will be used to explore the spread and possible fixation of exploitative clones in simple two-clone systems. Strassmann and Queller will also test for two other strategies predicted to be adaptive in chimeric mixtures. First, clones might generally reduce allocation to stalks in mixtures, because they are less likely to share genes with the spores that benefit. Second, the rarer of the two clones is predicted to particularly avoid the stalk. This work will be complemented by construction of a game theoretic model that will explore optimal behavior of two clones when mixed in different proportions.

The overall goal is to discover the nature and extent of selective conflicts in D. discoideum. If these turn out to be common, then the basis for considering this as a model system for development is weakened. However, it would also lay the initial groundwork for D. discoideum becoming an extraordinary model system for social evolution. Because numerous tools for studying the genetics and cell biology are already available for this organism, the work outlined here may establish a unique system where the genes underlying conflict and cooperation can be identified and studied in ways not possible with other social organisms.

Past Research

  • Stingless Bees
  • Polistes Wasps
  • Large Colony Wasps