Phage (co)evolution along a parasitism-mutualism continuum
Figure 1: Experimental Design Phage lambda infects E. coli and can integrate itself into the bacterial chromosome at a relatively low rate. In our experiment, the phage genome contains the lacZα subunit, which complements the hosts lac operon allowing it to breakdown and consume lactose molecules. We evolved ten replicate populations of phage: five in media containing small amounts of LB and large amounts of fructose and five with lactose as the primary carbon source. We also coevolved 10
... coevolved 10 replicates of both E. coli and phage similarly. Prophage Immunity Lysogens are maintained by the expression of the cI gene, which produces Repressor protein. Repressor binds to operators that stop transcription and keeps phage genes off. Extra Repressor binds to superinfecting phage DNA and prevents integration or circularization, thus leading to its degradation. Prophage Induction Lysogens carry the cI857 mutation, and can be thermoinduced at 42°C. This facilitates non-mutagenic induction for phage evolution and assays for intact prophage. Bacterial sequencing has revealed abundant prophages in nature, and they often contain host-altering genes (i.e., virulence factors). When those genes increase the fitness of lysogens, temperate phages could have a fitness advantage over their lytic relatives. In those cases, selection may favor more and more temperate phages and even eventually establish mutualisims. To study adaptation along this parasitism-mutualism continuum, we used an engineered lysogenic λ phage (SYP045) containing the lacZα subunit and an E. coli B strain with the ΔM15 α-complementation deletion (REL606 ΔM15). By evolving and coevolving phage in environments with and without lactose, we tested predictions about movement along this parasitismmutualism continuum. Evolving phage did not become more temperate as predicted; instead, unexpected and interesting complexities arose. Coevolved phage did show signs of mutualistic interactions, though they were potentially short lived.