Maintenance of Diversity

General theoretical models predict that diversity can be maintained in the absence of environmental heterogeneity if all genotypes have equal fitness, or if each genotype has an advantage when rare.

Our work on natural isolates of Steinernema nematodes and their Xenorhabdus bacteria has shown that interactions among these clones may be nonhierarchical or non-transitive, meaning that no one clone is superior to all others. When interactions are non-transitive, such as in the rock-paper-scissors game, each player can be invaded by another strategy, thus allowing all three strategies to coexist. Non-transitive interactions are thought to be common in nature, but few studies have demonstrated their role in maintaining diversity, especially in relation to parasites. In our system, we have the ability to pair a highly tractable experimental system with observations in nature to test some of the key assumptions and predictions of this theory. We have begun by examining pairwise competitions among our natural isolates in the caterpillar

These studies demonstrate the importance of bacteriocin-based antagonisms in determining competitive outcomes and in altering infection dynamics. In addition, these studies also reinforce the importance of exploitative competition in within-host interactions. Specifically, we see that faster killing parasites are competitively dominant in the absence of bacteriocin-based interactions. These results suggest there may be multiple strategies for success in this community.