Viruses' life history: towards a mechanistic basis of a trade-off between survival and reproduction among phages

Abstract

Life history theory accounts for variations in many traits involved in the reproduction and survival of living organisms, by determining the constraints leading to trade-offs among these different traits. The main life history traits of phages-viruses that infect bacteria-are the multiplication rate in the host, the survivorship of virions in the external environment, and their mode of transmission. By comparing life history traits of 16 phages infecting the bacteria Escherichia coli, we show that their mortality rate is constant with time and positively [corrected] correlated to their multiplication rate in the bacterial host. Even though these viruses do not age, this result is in line with the trade-off between survival and reproduction previously observed in numerous aging organisms. Furthermore, a multiple regression shows that the combined effects of two physical parameters, namely, the capsid thickness and the density of the packaged genome, account for 82% of the variation in the mortality rate. The correlations between life history traits and physical characteristics of virions may provide a mechanistic explanation of this trade-off. The fact that this trade-off is present in this very simple biological situation suggests that it might be a fundamental property of evolving entities produced under constraints. Moreover, such a positive correlation between mortality and multiplication reveals an underexplored trade-off in host-parasite interactions.

Figure 1. Mortality Rates of Phage Particles

(A) Representative survival curves of phage particles maintained in LB at 37 °C, in the absence of host cells. Phage stocks are obtained by infecting growing E. coli host culture followed by cell elimination. Lines show exponential regressions, with R ² values ranging from 0.87 for P2 to 0.99 for MS2. The mortality rate is not influenced by the initial concentration of the phage populations (unpublished data). Each experiment was repeated at least three times independently. (B) Relation between mortality rate and temperature. Symbols are the same as in (A). Lines show exponential fits between the mortality rate and 1/T. R ² values range from 0.937 for Mu to 0.999 for P2.

Figure 2. Observation of Virions by Electron Microscopy after 8 D of Incubation at 37 °C

The table gives the relative variation in the proportion of broken virions, as measured by electron microscopy, of more than a hundred particles, as well as the variation in the number of viable phages measured by plating on a susceptible host. Both observations have been made on the same samples. For each type of phage, the proportion of broken virions is significantly higher after incubation, and the variation in broken-virion particles measured by electron microscopy is similar to the variation in the number of infectious phages.

Figure 3. Representation of Various Phage Parameters in a PCA

We can notice two groups of parameters: virion structural characteristics (green) and life history traits of phages (red). Within each group, parameters correlate highly with a nonparametric Spearman Rho test. For others correlations, see text and Figure 4.

Figure 4. Correlations between Phage Life History Traits and Phage Particle Characteristics

(A) Positive correlation between mortality rate and ρ _pack, the volumic density of the packaged DNA (Linear regression, R ² = 0.67 and p = 0.001). ρ _pack has been calculated only for phages with a double-stranded DNA genome, because the volumes of single-strand DNA and double-strand RNA are different than the volume of double-stranded DNA. ρ _pack is calculated by dividing the volume of the genome by the internal volume of the capsid. (B) Negative correlation between mortality rate and the surfacic mass of the capsid, calculated by dividing the capsid molecular weight by capsid surface (linear regression, R ² = 0.35 and p = 0.031). Because the surfacic mass is an estimation of the thickness of the capsid, it should be related to its strength. Some phages are not represented because data are not available, or in the case of M13, because it possesses a helical geometry, and thus the constraints on the capsid are very different than for icosahedral phages. (C) Negative correlation between the multiplication rate and the surfacic mass of the capsid (linear regression, R ² = 0.46 and p = 0.011). (D) Positive correlation between the mortality rate and the multiplication rate. The log–log scale is for a better visualization of the results and does not modify the significance of the correlation. The line shows a linear regression characterized by R ² = 0.73 and p < 0.0001. Each measure was repeated at least three times independently for the determination of the multiplication and mortality rates.

Figure 5. Actual Mortality Rate against Predicted Decay Rate by a Model of Multiple Regression Using the Decay Rate, ρ _pack, and the Capsid Surfacic mass

The estimates were identified by a stepwise regression among all parameters used. The order of entrance of parameters is an increasing function of p-values. The model explains 91% of the variance of the mortality rate.

Figure 6. Potential Relations between Phage Survival, Multiplication Rate, and Capsid Characteristics

Evolutionary theory predicts that a high decay rate is associated with an elevated multiplication rate. Forces exerted on the capsid might lead to the rupture of the head of phages, leading to their inactivation. The multiplication rate of phages is possibly associated with properties of phage particles as a consequence of kinetic and energetic considerations involved in the assembly of the capsid and/or the encapsidation of the genome.