Employing laboratory experiments of three pairs of clade A and clade BT7-like cyanophages infecting the same cyanobacterial host, we assessed the physiology of their infection properties. We found that one lineage of the T7-like cyanophages (clade A) is more aggressive, as it has a more rapid infection cycle, greater reproduction of viral progeny and kills more cyanobacteria per infection cycle than the second lineage of cyanophages (clade B). These findings clearly show that the separation into phylogenetic lineages is not random and likely resulted through the evolutionary process of adaptation.
We next developed methods for quantification of viruses floating freely in the water column as well as the quantification of cyanobacteria infected by the different virus lineages in the marine environment, at the single cell level. Using these methods, we found that the T7-like cyanophages are very abundant in nature over seasonal cycles of changing water column conditions in the Red Sea as well as along transects of environmental change in the North Pacific Ocean. Of particular interest is the increase in the abundance of these cyanophages and in their infection to form a virus hotspot in the region between the subtropical and subpolar gyres. These results revealed that the slower and less aggressive cyanophage lineage (clade B) is the one that is more abundant in the world’s oceans under nearly all environmental conditions. In addition, the less aggressive cyanophage lineage infects more cyanobacteria than the more aggressive cyanophage lineage in nearly all waters. Therefore, it is the less virulent cyanophage lineage with the slower infection cycle and lower progeny production that is more successful in nature. Employment of a model to reconcile this apparent contradiction between infection characteristics and environmental infection and abundance patterns showed that the less aggressive cyanophage lineage maintains a more sustainable host population. Ultimately this allows for more stable host-virus coexistence and a larger population size of the less aggressive cyanophage lineage. This is directly linked to the requirement of viruses to infect their hosts to reproduce and therefore overuse of the host as a resource leads to lower population sizes in nature.
These above findings raise the question as to the genetic underpinnings for the observed differences in infection properties and environmental distribution patterns between the two lineages of T7-like cyanophages. To this end, we developed a genetic inactivation system for the T7-like cyanophages. Using this method we discovered two genes that impact the speed of the infection cycle or the number of cyanophage progeny produced, one that enhances clade A infection and one that dampens clade B infection. These genes are not part of the core set of genes found in all T7-like cyanophages, but have been horizontal acquired from their cyanobacterial hosts and are specific to only one of the cyanophage lineages. These findings indicate that the acquisition of genes from the cyanobacterial host has played a major role in the differential adaptation and evolution of the two cyanophage lineages. This has influenced not only their infection physiology, but also their populations sizes, infection patterns and coexistence with their cyanobacterial hosts.
This work has been presented at many national and international meetings. The methods have been published in, or submitted to, scientific journals. The findings obtained with these new methods are currently being prepared for publication. In addition, we have hosted members of the international scientific community to train them in these methods.
