SP6coli
Multiple genetic pathways to similar fitness limits during   viral adaptation to a new host
The gain in fitness during adaptation depends on the supply    of beneficial mutations. Despite a good theoretical understanding of    how evolution proceeds for a defined set of mutations, there is little    understanding of constraints on net fitness - whether fitness will    reach a limit despite ongoing selection and mutation, and if there is    a limit, what determines it. Here, the dsDNA bacteriophage SP6, a    virus of Salmonella, was adapted to Escherichia coli K-12. From an    isolate capable of modest growth on E. coli, 4 lines were adapted for    rapid growth by protocols differing in use of mutagen, propagation    method, and duration, but using the same media, temperature, and a    continual excess of the novel host. Nucleotide changes underlying    those adaptations differed greatly in number and identity, but the    four lines achieved similar absolute fitnesses at the end, an increase    of more than 4000-fold phage descendants per hour. Thus the fitness    landscape allows multiple genetic paths to the same approximate    fitness limit. The existence and causes of fitness limits have    ramifications to genome engineering, vaccine design, and ‘lethal    mutagenesis’ treatments to cure viral infections.