Evolutionary reversals during viral adaptation to alternating hosts.

Area of Science:

• Microbiology
• Evolutionary Biology
• Virology

Background:

• Bacteriophage phiX174 exhibits host specificity, primarily infecting Escherichia coli.
• Understanding phage adaptation mechanisms is crucial for host-pathogen interaction studies.
• Previous studies identified a putative binding region in the phiX174 major capsid protein.

Purpose of the Study:

• To investigate the evolutionary consequences of bacteriophage phiX174 host adaptation.
• To identify genetic changes underlying altered host growth and fitness recovery.
• To explore the role of specific capsid protein residues in phage adaptation.

Main Methods:

• Experimental evolution of bacteriophage phiX174 through serial passage on Salmonella and Escherichia hosts.
• Genetic sequencing to identify mutations in the major capsid gene.
• Phenotypic analysis of phage growth and fitness on different hosts.

Main Results:

• Adaptation to Salmonella significantly impaired growth on Escherichia, while adaptation to Escherichia had minimal impact on Salmonella growth.
• Growth inhibition on Escherichia was attributed to two to three amino acid substitutions in the major capsid gene.
• Fitness recovery on Escherichia predominantly occurred through reversions at the original mutation sites, not compensatory mutations.

Conclusions:

• Host-specific adaptation in bacteriophage phiX174 can lead to reciprocal fitness costs.
• Specific mutations in the major capsid protein, affecting attachment efficiency, drive host adaptation.
• Reversionary mutations, rather than compensatory evolution, are key for restoring fitness after host shifts in this system.