Equilibrium in virus populations
Virus populations are a common feature of perennial crop infections, but we know very little about how they come to be or how they might change over time. This is important because the structure of a population can determine the type and severity of disease produced. Our end goal is to manipulate virus populations to prevent disease induction, but first we need to understand the behavior of viruses in populations. Here we examined how populations of Citrus tristeza virus (CTV) in Citrus change over time from their initial inoculation. We found that CTV populations reach equilibrium, a stable population structure of one strain versus another, regardless of how much virus was initially inoculated, or the order of virus introduction. On further investigation, we found that the host species and variant fitness were important factors that determine what a population would look like over time.
We started off with the premise that vector transmission is a bottleneck event that alters both composition and titer of the CTV populations transmitted. We found that this was indeed the case with aphid transmission of CTV; each transmission produced a new population that was different from the original. But then we wondered, are the proportions of strains present in a population stable? Will the population structure change with the passage of time? So we examined what would happen if we left these populations, which contained the same three strains at different titers, to develop. We found that the titers of the strains in each plant changed to achieve the same ratio of one strain to another. This was also true for the order of inoculation; it didn’t affect the final, stable equilibrium. Why is this important? It means that virus population structures are not random and conform to a series of rules or selective pressures.
An example of the change in CTV strain titers as the population structures reach equilibrium. The three citron plants started off with different titers of strains T36 (blue), T30 (orange), and VT (white), but over six months reached the same equilibrium.
Because we were working with seedlings under experimental conditions, we next examined populations in commercial citrus. Trees in a single grove, of the same cultivar and age, possessed near-identical CTV populations with little variation in structure, and none in composition. Yet, when we examined different groves, each had a different population profile: populations are uniform within a grove but different between groves. This was our ‘gotcha’ moment, for if field populations are not random, and behave the same as experimental populations, we can manipulate them; it also led to our current work to identify the levers by which we can shift the structure of a population.
About the authors
Dr. Scott Harper is a Research Assistant Scientist at the Citrus Research and Education Center, University of Florida. Dr. Sarah-Jane Cowell (not pictured) is a Postdoctoral Associate at the University of Florida, and Dr. William Dawson (also not pictured) is a Distinguished Scholar at the University of Florida.
About the research
Virology, Volume 485, November 2015, Pages 205–212
S.J. Harper, S.J. Cowell, W.O. Dawson