The virulence–transmission trade-off in vector-borne plant viruses: a review of (non-)existing studies

Abstract

The adaptive hypothesis invoked to explain why parasites harm their hosts is known as the trade-off hypothesis, which states that increased parasite transmission comes at the cost of shorter infection duration. This correlation arises because both transmission and disease-induced mortality (i.e. virulence) are increasing functions of parasite within-host density. There is, however, a glaring lack of empirical data to support this hypothesis. Here, we review empirical investigations reporting to what extent within-host viral accumulation determines the transmission rate and the virulence of vector-borne plant viruses. Studies suggest that the correlation between within-plant viral accumulation and transmission rate of natural isolates is positive. Unfortunately, results on the correlation between viral accumulation and virulence are very scarce. We found only very few appropriate studies testing such a correlation, themselves limited by the fact that they use symptoms as a proxy for virulence and are based on very few viral genotypes. Overall, the available evidence does not allow us to confirm or refute the existence of a transmission -virulence trade-off for vector-borne plant viruses. We discuss the type of data that should be collected and how theoretical models can help us refine testable predictions of virulence evolution.

Frequency-dependent assistance as a way out of competitive exclusion between two strains of an emerging virus

“…Finally, with quantitative models, we were able to estimate key parameters of the complex interaction between the two strains and to link our experimental results with TYLCV epidemiological dynamics. A link between within-plant viral load and transmission rate is generally accepted even if the studies on circulative viruses are rare [33]. Here, we demonstrated on TYLCV, a circulative nonpropagative virus, that in single infection the IL strain accumulates better (faster and in greater amounts) within the plant than the Mld strain and is also better transmitted.…”

Graphical analysis of evolutionary trade-off in sylvatic Trypanosoma cruzi transmission modes

“…The importance of contact process saturation (also suggested by [10]) in this weighting was seen in [20], where the contact process (leading to either stercorarian or oral transmission) whose saturation threshold was closer to the actual vector-host ratio was advantaged. The results of the present study highlight the difference in nature of the trade-off between horizontal host-vector transmission modes and vertical transmission in hosts, which bypasses vectors altogether.…”

“…Such a trade-off predicts that pathogens will evolve toward an optimal, nonzero virulence which maximizes the pathogen's overall reproduction between hosts. (The debate over this hypothesis centers around the assumed correlation of both virulence [as differential host mortality] and host-to-host transmission to within-host pathogen replication, for which supporting empirical data are limited, e.g., [10].) Alizon and van Baalen [2] used an embedded model (within-host dynamics linked to population-level dynamics) to study this hypothesis in the context of a vectorborne disease (namely, malaria).…”

“…Furthermore, the strains' differentiated adaptation to different transmission routes makes a single host population act as multiple resources at once, with the vector-host ratio weighting the importance of each transmission route through saturation in the contact processes. The importance of host ecology in mediating the coevolution of parasite transmissibility via multiple infection routes has been recognized [1, p. 251] but little studied, especially in the context of vector-borne pathogens [10]. These additional layers in the fitness landscape take the framework beyond its initial context in a way parallel to Rueffler et al's studies of frequency dependence.…”

Navigating the Devious Course of Evolution: The Importance of Mechanistic Models for Identifying Eco-Evolutionary Dynamics in Nature

“…Empirical support exists for a virulence-transmission trade-off, in particular in spore-transmitting (Agnew and Koella 1997;Jensen et al 2006;de Roode et al 2008) and vector-borne Read 1999, 2004;Chapuis et al 2012) pathogens for which transmission proxies are relatively easy to quantify, as well as in human immunodeficiency virus (HIV; Fraser et al 2007; for reviews, see Alizon et al 2009 andFroissart et al 2010). However, the functional form of the virulencetransmission trade-off, which is necessary to parameterize models, is difficult to determine from these data (Ebert and Bull 2003;Alizon et al 2009;Froissart et al 2010).…”