Evolutionary Robustness of an Optimal Phenotype: Re-evolution of Lysis in a Bacteriophage Deleted for Its Lysin Gene Richard H. Heineman, 1 Ian J. Molineux, 2,3 James J. Bull 1,3 1 Section of Integrative Biology, University of Texas, Austin, Texas 78712, USA 2 Section of Molecular Genetics and Microbiology, University of Texas, Austin, Texas 78712, USA 3 Institute for Cell and Molecular Biology, University of Texas, Austin, Texas 78712, USA Received: 19 October 2004 / Accepted: 21 March 2005 Abstract. Optimality models are frequently used to create expectations about phenotypic evolution based on the fittest possible phenotype. However, they often ignore genetic details, which could confound these expectations. We experimentally analyzed the ability of organisms to evolve towards an optimum in an experimentally tractable system, lysis time in bacte- riophage T7. T7 lysozyme helps lyse the host cell by degrading its cell wall at the end of infection, allowing viral escape to infect new hosts. Artificial deletion of lysozyme greatly reduced fitness and delayed lysis, but after evolution both phenotypes approached wild-type values. Phage with a lysis-deficient lyso- zyme evolved similarly. Several mutations were in- volved in adaptation, but most of the change in lysis timing and fitness increase was mediated by changes in gene 16, an internal virion protein not formerly considered to play a role in lysis. Its muralytic do- main, which normally aids genome entry through the cell wall, evolved to cause phage release. Theoretical models suggest there is an optimal lysis time, and lysis more rapid or delayed than this optimum decreases fitness. Artificially constructed lines with very rapid lysis had lower fitness than wild-type T7, in accor- dance with the model. However, while a slow-lysing line also had lower fitness than wild-type, this low fitness resulted at least partly from genetic details that violated model assumptions. Key words: Optimality — Experimental evolution — Evolutionary robustness — Lysis — T7 — Bac- teriophage — Genome evolution — Molecular evo- lution — Fitness — Adaptation Introduction A large body of work in evolutionary biology ad- dresses the adaptive value of phenotypes, such as life history and behavioral traits, in the context of ecol- ogy (Charnov 1982; Freeland et al. 2000; Smith 1983; Trivers 1983; Williams 1966). By necessity, genetics of phenotype are often ignored in these approaches, except to posit trade-off functions that establish boundaries on the set of possible phenotypes. These trade-offs often suggest optima, maximally adaptive phenotypic values under particular conditions. A potential limitation of this approach is that the ge- netic system producing a phenotype may constrain its evolution in ways not captured by the trade-off, thus preventing attainment of the optimum or directing evolution toward pathways not predicted by the purely phenotypic model (Lewontin 1989). For example, models of optimal behavior might fail if it is impossible to evolve to make a particular decision. The reliance on purely phenotypic models is often a necessity, because the genetic nature of phenotypes is almost always unknown, but the rapidly advancing science of genomics may allow us to accommodate their genetic bases. A precedent for this marriage of Correspondence to: Richard H. Heineman; email: heineman@ mail.utexas.edu J Mol Evol (2005) 61:181–191 DOI: 10.1007/s00239-004-0304-4