Host availability effects on evolution of generalist and specialist cyanophages
Obligate parasites such as bacteriophages are dependent on the availability of a sensitive host to persist. Natural environments pose challenges for survival, with periods of unavailability of such a host and gradual loss of infectivity of phages. Despite the importance of host availability to bacteriophages in natural environments, knowledge regarding evolution of ecologically prevalent bacteriophages is lacking. Here, we characterized the evolution of cyanobacteria-infecting bacteriophages (cyanophages) when propagating in the presence of a sensitive host. In addition, we characterized cyanophage evolution in the presence of increasing amounts of distinct resistant strains to which they can attach, while reducing the amount of sensitive host availability. Most cyanophage populations adapted to better infect the sensitive host. No evidence for changes in host range was observed. Whole-genome sequencing revealed patterns of convergent evolution, host availability-specific mutations, and genes associated with adaptation. Specifically, we found a putative holin, a common phage protein not previously characterized in marine cyanophages, that affects the length of the infection cycle. In addition, we found a decrease in attachment to both sensitive and resistant strains, suggesting a balance between infection characteristics beneficial in some host availabilities, and detrimental in others. In addition, we found that cyanophage evolution can have an unexpected effect on the growth of cyanobacteria not involved in the process of evolution. Together, these findings shed light on the complexity of cyanophage-cyanobacteria interactions in natural environments, highlighting some unexpected outcomes of these interactions for cyanophage persistence strategies and evolution of infection characteristics.
