Phage-host coevolution in natural populations

Adele James

Abstract:

Coevolution between bacteriophages (phages) and their bacterial hosts occurs through changes
in resistance and counter-resistance mechanisms. To assess phage-host evolution in wild
populations, we isolated 195 Vibrio crassostreae strains and 243 vibriophages during a five
month time-series from an oyster farm and combined these isolates with existing V.
crassostreae and phage isolates. Cross-infection studies of 81,926 host-phage pairs delineated
a modular network where phages are best at infecting cooccurring hosts, indicating local
adaptation. Successful propagation of phage is restricted by the ability to adsorb to closely
related bacteria and further constrained by strain-specific defence systems. These defences are
highly diverse and predominantly located on mobile genetic elements, and multiple defences
are active within a single genome. We further show that epigenetic and genomic modifications
enable phage to adapt to bacterial defences and alter host range. Our findings reveal that the
evolution of bacterial defences and phage counter-defences are underpinned by frequent genetic
exchanges with, and between, mobile genetic elements.