No effect of host-parasite co-evolution on host range expansion.

Area of Science:

• Microbiology
• Evolutionary Biology
• Genetics

Background:

• Antagonistic co-evolution between hosts and parasites is theorized to promote host range expansion.
• This expansion is hypothesized to occur through the selection of novel parasite infectivity alleles.
• Empirical evidence for this hypothesis, particularly regarding parasite generalism, remains limited.

Purpose of the Study:

• To investigate if experimental co-evolution between a bacterium (Pseudomonas fluorescens SBW25) and its phage (SBW25Φ2) influences the phage's interstrain host range.
• To determine if adaptation to a specific host strain confers broader infectivity against other strains.

Main Methods:

• Experimental co-evolution was conducted between Pseudomonas fluorescens SBW25 and its phage SBW25Φ2.
• Co-evolved phage variants were isolated and tested for infectivity against both sympatric (P. fluorescens SBW25) and allopatric (other P. fluorescens strains) bacterial hosts.
• Genetic and phenotypic diversity of phage variants was assessed.

Main Results:

• All co-evolved phage variants exhibited a broader host range than the ancestral phage.
• Co-evolved phages could differentially infect co-evolved variants of P. fluorescens SBW25.
• Crucially, none of the evolved phages could infect alternative P. fluorescens strains, irrespective of co-evolutionary history.

Conclusions:

• Parasite generalism at the scale of adaptation to a specific host strain does not necessarily translate to generalism across different host strains.
• Fundamental genetic constraints appear to limit parasite adaptation and host range expansion in this system.
• The study highlights the complexity of parasite adaptation and the potential for specialized evolution despite antagonistic co-evolutionary pressures.