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



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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.



Lambda phage, Lysogeny, Temperate, Decision-making