Life history diversity and signals of trade-offs in a large group of chloroviruses

Life cycle traits shape viral evolution through their effects on epidemiology, their diversity, and their trade-offs. Despite their importance, characterizations of viral life history diversity are hampered by the difficulty of measuring viral phenotypes. In this study, we overcame these challenges to describe the life history diversity of the genus Chlorovirus, which contains lytic dsDNA viruses that infect unicellular green algae. We developed efficient methods to measure adsorption rate, depolarization probability, lysis completion probability, lysis time, burst size, specific infectivity, and mortality rate. We used these methods to quantify the life history traits of 35 chloroviruses from three host range groups (NC64A, Osy, and SAG viruses). The trait estimates varied between 5- and 75-fold across all the virus strains, with most traits varying substantially both within and across groups. We then used the largest virus group in our study, the NC64A viruses (n = 20), to look for correlations that could signal trade-offs between traits. This revealed that viruses with high depolarization probabilities also had higher burst sizes, but lower probabilities of completing lysis. We also found a strong negative correlation between growth rate and survival. Finally, we discovered biphasic or ‘tailing’ decay in all the chlorovirus groups. The nature and diversity of these traits have major implications for the ecology and evolutionary potential of the chloroviruses, and reinforce the importance of studying viral life histories.