Robustness promotes evolvability of thermotolerance in an RNA virus

BMC Evolutionary Biology BioMed Central Research article Open Access Robustness promotes evolvability of thermotolerance in an RNA virus Robert C McBride*, C Brandon Ogbunugafor and Paul E Turner Address: Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut, 06520-8106, USA Email: Robert C McBride* - ; C Brandon Ogbunugafor - ; Paul E Turner - * Corresponding author Published: 11 August 2008 BMC Evolutionary Biology 2008, 8:231 doi:10.1186/1471-2148-8-231 Received: 3 June 2008 Accepted: 11 August 2008 This article is available from: http://www.biomedcentral.com/1471-2148/8/231 © 2008 McBride et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: The ability for an evolving population to adapt to a novel environment is achieved through a balance of robustness and evolvability. Robustness is the invariance of phenotype in the face of perturbation and evolvability is the capacity to adapt in response to selection. Genetic robustness has been posited, depending on the underlying mechanism, to either decrease the efficacy of selection, or increase the possibility of future adaptation. However, the true effect of genetic robustness on evolvability in biological systems remains uncertain. Results: Here we demonstrate that genetic robustness increases evolvability of thermotolerance in laboratory populations of the RNA virus φ6. We observed that populations founded by robust clones evolved greater resistance to heat shock, relative to populations founded by brittle (lessrobust) clones. Thus, we provide empirical evidence for the idea that robustness can promote evolvability in this environment, and further suggest that evolvability can arise indirectly via selection for robustness, rather than through direct selective action. Conclusion: Our data imply that greater tolerance of mutational change is associated with virus adaptability in a new niche, a finding generally relevant to evolutionary biology, and informative for elucidating how viruses might evolve to emerge in new habitats and/or overcome novel therapies. Background Evolvability may be defined as the capacity to adapt in response to selection [1-3], or alternatively as the ability to access evolutionary innovations [4,5]. These varied definitions echo the diverse opinions on how evolvability might be influenced by aspects of genetic architecture, especially genetic robustness – phenotypic constancy in the face of mutational change [6]. If robustness affects evolvability, it should impact the ability for organisms to access evolutionary innovations [4,5]. Robustness more easily allows for the accumulation of mutations that are neutral in the current environment; should the habitat change, this robust genetic architecture may then promote access to a relatively greater number of mutations that are beneficial for adaptation [5]. For example, a robust population may be envisioned as residing in a region of a fitness landscape that is relatively flat, owing to the high proportion of resident genotypes in the population that are equal (neutral) in fitness [7]. This creates a large 'neutral network' of genotypes that can efficiently traverse the landscape through random drift, due to their high degree of network connectivity. If environmental change alters the fitness landscape, a robust population may experience an evolvability advantage because newly-arising mutations occur in a Page 1 of 14 (page number not for citation purposes) BMC Evolutionary Biology 2008, 8:231 wider diversity of genetic backgrounds, creating more-varied epistatic combinations that may prove beneficial for adaptation [8]. Until recently, it was controversial whether biological populations could evolve genetic robustness as posed by theory [9]. However, empirical work confirms that robustness of RNA viruses can be altered through directional selection [10], and that elevated mutation rates in RNA viruses and viroids selects for fitness improvement via increased robustness despite concomitantly reduced replication rate [11,12]. In contrast, the relationship between robustness and evolvability remains elusive; although the literature contains anecdotal accounts of their purported link [5,13], these examples mostly derive from the molecular level of organization [5,14]. Furthermore, these data are inconsistent, with some studies suggesting a positive http://www.biomedcentral.com/1471-2148/8/231 relationship between robustness and evolvability [14,15] and others implying a negative relationship [16-18]. To date there are no empirical data from biological systems which examine this relationship [5,6]. An ideal approach would be to study the influence of robustness on evolvability, using an empirical system where relatively robust and brittle genotypes have been identified, and which is tractable for studying adaptation under strong selection in a novel habitat. To test whether robustness promotes evolvability, we used a collection of genetically robust and brittle strains of the lytic RNA bacteriophage φ6. These strains originally came from an experimental evolution study [10,19], where replicate virus populations were selected on the bacterium Pseudomonas syringae pathovar phaseolicola, under low versus high levels of virus co-infection (Figure 1). Three of Figure 1 of the propagation schemes for the low and high co-infection treatments in Turner and Chao (1998) Summary Summary of the propagation schemes for the low and high co-infection treatments in Turner and Chao (1998). Phage (●) adsorbed to bacterial cells (䊐) at a constant multiplicity-of-infection, and this mixture was used to seed a bacterial lawn. During overnight growth, the viral progeny formed visible plaques (❍) which were harvested to create a bacteria-free lysate. Plaques in the low co-infection treatment were produced as the result of single infections, whereas those in the high co-infection treatment resulted from co-infection by two to three viruses (on average). To control for differences in population size across treatments, one-fifth as many plaques were harvested in the high co-infection treatment. See text for details. Page 2 of 14 (page number not for citation purposes) BMC Evolutionary Biology 2008, 8:231 the populations were cultured at a low multiplicity-ofinfection (MOI; ratio of infecting viruses to bacterial cells) of 0.002, where ~99.9% of all infected cells should be infected by a single virus [19]. In contrast, the other three lineages were passaged at MOI = 5, where ~97% of infected cells should be infected by two to three viruses (the limit to co-infection in φ6; [20]). Co-infection was controlled by mixing viruses and bacteria at a given MOI (...truncated)