> Research Interests & Current Projects
Ecology and evolution of intraspecific aggression in clonal organisms

Many species of colonial marine invertebrates exhibit dramatic intraspecific variation in patterns of expression of allorecognition-dependent behaviors such as fusion and aggression. Several years ago, David Ayre and I began to explore the processes that maintain this variation in the clonal sea anemone Anthopleura elegantissima.

The paradigm for how cnidarians resolve intergenotypic conflicts over habitable space comes from studies of agonistic behavior in Anthopleura elegantissima. A. elegantissima clones consist of variable numbers of aggregated, asexually produced polyps. As clones expand and contact other conspecific clones, interacting polyps deploy specialized fighting tentacles (acrorhagi), and protracted battles ensue. This can lead either to the elimination of one clone, or to the formation of distinctive interclonal borders. Along these borders, the polyps sometimes differentiate into well-armed "warriors" that are smaller than the more central "reproductives", and that lack gonads.

The long-standing dogma for Anthopleura was that individual genotypes were uniformly aggressive toward nonself, and passive toward clonemates. At least on a local scale, space should be dominated by single clones, and especially those which are most aggressive.

Our population genetic studies tell a very different story: even on very fine spatial scales (say, centimeters), clonal diversity is high (Grosberg and Cameron, in prep.). Moreover, Dave Ayre and I showed that at the level of individual polyps nearly all clones differ with respect to aggressiveness, tolerance, enhancement, and highly specific, inducible memory (Ayre & Grosberg 1995, 1996). One of the most surprising outcomes of these studies was that even members of clones that shared an interclonal border, which we assumed would be equally good fighters, were rarely evenly matched. This suggested that the pairwise contests individual pairs of polyps that we used to assess fighting ability did not reflect the fighting abilities of entire clones.

These observations raise three important questions related to the maintenance of behavioral polymorphism in this, and other, species. First, do clones differ in their patterns of allocation to warrior versus reproductive individuals (i.e., social organization)? Second, to what extent does interclonal variation in social organization reflect phenotypically plastic responses to variation in the social environment (e.g., the size, genotype, and behavior of neighboring clones) versus underlying genetic variation? Third, how stable are interclonal boundaries?

Over the last few years, David Ayre and I have begun to assemble a detailed picture of how clones differ with respect to allocation to fighting and reproductive morphs, and how these differences translate into competitive ability. We found that clones differ dramatically in their patterns of allocation to specialized fighting morphs (warriors) along interclonal borders (see photo) versus reproductive morphs: some clones produce relatively few and poorly armed warriors, whereas others produce many well-armed warriors.

We recently completed the first long-term observations of a natural interclonal border, documenting a spectrum of extraordinary behaviors never previously observed. ( Ayre and Grosberg 2005) Among other things, these studies revealed how clonemates communicate with each other, despite the physical separation of individual polyps. We also completed a series of laboratory induction experiments, adding and removing the stimulus of allogeneic clones to determine whether individual polyps can shift their morphologies. These studies show that allogeneic stimulus induces a transformation of reproductive to warrior polyps. Inducibility, however, varies from clone-to-clone.