Abstract
In the paper referenced above we presented evidence that P. aeruginosa undergoes extensive genetic diversification during growth in biofilm communities (Boles et al., 2004). The genetic changes arise via a recA-dependent mechanism and affect multiple traits. We also showed that biofilms that had genetically diversified were better able to withstand an applied physiological stress as compared to biofilms that were unable to diversify. We related these findings to the insurance hypothesis, an ecological hypothesis which posits that diversity generally increases the ability of biological communities to withstand changes in environmental conditions (Yachi & Loreau, 1999). Cooper et al. have raised questions about our interpretation of the data, and have objected to a portion of the discussion in which we discuss mechanisms that could generate the observed diversity. Here we respond to the points they have raised.
(1) Cooper et al. acknowledge that we demonstrate increased stress resistance in high (as compared to low) diversity biofilms. However, they state, What also needs to be demonstrated (to claim an insurance effect) is a trade-off over different environments such that some members are better in some environments while others are better in others. We agree that the differential responses of community members to environmental conditions are fundamental to the insurance hypothesis. This point is made in our manuscript. On p. 16634 we state:
Indeed, the fitness of each specialized population is likely to depend on prevailing conditions; a given phenotype may be advantageous in certain circumstances and detrimental or neutral in others. The insurance hypothesis relates to the effects of diversity on the community as a whole. It predicts that a community composed of functionally diverse populations is likely to perform better in general because of the likelihood that some subpopulation will thrive as prevailing conditions change.
We considered this point intuitive and thus did not highlight it further; it is generally thought that most specialized phenotypes will have advantages in some conditions and disadvantages in others (MacLean et al., 2004; Wilson & Yoshimura, 1994).
Though this point was not stressed, data presented in our paper did provide evidence for such a trade-off. To emphasize this point we have combined data found in Figs 2(b) and 5(a) of our paper in the accompanying Fig. 1. To use the terminology of Cooper et al., we call our standard biofilm medium Environment #1, and medium with peroxide added after 3 days of growth, Environment #2. In Environment #1 typical cells were most prevalent in the biofilm community, whereas in Environment #2 (assayed after the same period of growth), wrinkly variants dominated. These results demonstrate a fitness trade-off in different environments and suggest that the wrinkly variant is not simply a superior competitor...regardless of environment as Cooper et al. suggest.