Decision-Making In λ Phage And The Evolution Of The Temperate Strategy

Decision-making at the developmental level is ubiquitous in biology. The basis of the developmental decision between different phenotypic fates has been studied in the Enterobateriophage λ since the early days of molecular biology. The λ phage is called a temperate phage for its ability to decide between two developmental pathways: the lysogenic and the lytic. Here, I detail the life-history traits associated with the λ temperate strategy. I study the temporal gene expression patterns of the underlying genetic network involve in the decision between the two pathways. I used a stochastic computational model and captured how the multiplicity of infection (MOI) and mutations affect the probability of individuals following the lysogenic pathway. I show that the decision-making is robust to mutations and is achieved through differences in the temporal gene expression pattern. Furthermore, I study the stable lysogenic state. The lysogenic state is experimentally known as very stable and switches spontaneously from lysogeny to lysis at a low rate. I confirmed this in my model. I also found that the stability of lysogeny is achieved through different fine-tuning patterns of expression in different genotypes. In addition, I studied the conditions for the evolution of the temperate strategy in phages using an epidemiological model. I analyzed two epidemiological models of λ. I found the complex pattern of interaction of the probability of lysogeny, the rate of lysis, and the induction rate in different regimes that favored the different strategies of transmission of λ phage. Minimal levels of lysogenization are beneficial under bad ecological conditions, supporting the advantage of temperate strategies under those conditions. I also found the induction rate under selective pressure for plasticity in temperate phages. At conditions favoring the lysogenic strategy, the induction rate is under selective force to be minimal.