Microbe Profile: Bacteriophage ϕ6: a model for segmented RNA viruses and the evolutionary consequences of viral ‘sex’

 Graphical abstract 

Phage ϕ6 is a model for segmented RNA viruses, and for testing evolutionary biology. (a) Phage ϕ6 attaches to type-IV pili of susceptible host bacteria and selects for phage-resistance via altered/deleted pili, causing a ‘trade-off’ that should reduce pathogenicity (leaf adherence and plant invasion) of Pseudomonad bacteria in the wild. (b) Muller’s ratchet predicts that asexual populations of small size should accrue harmful mutations that reduce mean fitness over time, depicted as rightward shift in a hypothetical histogram (blue bars) of individuals harbouring greater average genetic load. Whereas ϕ6 studies showed segment reassortment (viral sex) can create hybrids with fewer mutations (horizontal red bars on RNA segments) than those carried by co-infecting ‘parent’ viruses, causing leftward shift of histogram, and reduced average genetic load over time. (c) Wild-type phage (black) is advantaged in particle production, relative to a cheater phage (red). But frequent co-infection can favour selection for cheater variants of ϕ6 that outcompete the wild-type; overall productivity decreases but cheaters are over-represented among the viral progeny compared to their production when infecting cells alone. This result is consistent with the Prisoner’s Dilemma outcome in evolutionary game theory, which shows a cheating strategy (red line depicting cheater fitness) can invade a population at any initial starting frequency (exceeding horizontal black line for wild-type fitness defined as 1.0) despite causing mean fitness to decline through time (red arrow pointing down and to the right as cheaters approach fixation when frequency equals 1.0).