Role of trade-off between sexual and vertical routes for evolution of pathogen transmission

Environmental Conditions Modulate Plant Virus Vertical Transmission and Survival of Infected Seeds

Seed transmission is a major mode for plant virus persistence and dispersal, as it allows for virus survival within the seed in unfavorable conditions and facilitates spread when they become more favorable. To access these benefits, viruses require infected seeds to remain viable and germinate in altered environmental conditions, which may also be advantageous for the plant. However, how environmental conditions and virus infection affect seed viability, and whether these effects modulate seed transmission rate and plant fitness, is unknown. To address these questions, we utilized turnip mosaic virus, cucumber mosaic virus, and Arabidopsis thaliana as model systems. Using seeds from plants infected by these viruses, we analyzed seed germination rates, as a proxy of seed viability, and virus seed transmission rate under standard and altered temperature, CO2, and light intensity. With these data, we developed and parameterized a mathematical epidemiological model to explore the consequences of the observed alterations on virus prevalence and persistence. Altered conditions generally reduced overall seed viability and increased virus transmission rate compared with standard conditions, which indicated that under environmental stress, infected seeds are more viable. Hence, virus presence may be beneficial for the host. Subsequent simulations predicted that enhanced viability of infected seeds and higher virus transmission rate may increase virus prevalence and persistence in the host population under altered conditions. This work provides novel information on the influence of the environment in plant virus epidemics. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The importance of ecological dynamics in evolutionary processes: a host-bacteriophage model revisited

A recent study of adaptive dynamics of lysis propensity in temperate phages suggested that full lysogeny emerges as the outcome of bacteriophage evolution in a simple host-phage system. The conclusion is based on the premise that mutant strains necessarily appear in equilibrium host-phage environments. Revisiting the model, we show that the ecological system exhibits richer asymptotic dynamics and that, in a certain parameter regime, evolution may in fact drive lysis propensity towards an evolutionary singularity in which a non-zero proportion of phages initiate infection in a lytic cycle. These singularities act as points of evolutionary diversification, leading to periodic coexistence of two distinct phage strains on the evolutionary time-scale. One of the two strains in the dimorphic evolutionary singularity is fully lysogenic (in the sense that cell infection always leads to lysogeny), while the other is partially lytic. Our study thus highlights the importance of ecological interactions as a driver of evolution.

Sexually transmitted infections and mate-finding Allee effects

Infectious diseases can seriously impact dynamics of their host species. In this study, we model and analyze an interaction between a sexually transmitted infection and its animal host population affected by a mate-finding Allee effect. Since mating drives both host reproduction and infection transmission, the Allee effect shapes the transmission rate of the infection which we show takes a saturating form. Our model combining sexually transmitted infections with the mate-finding Allee effect in the host produces quite rich dynamics, including oscillations, several multistability regimes, and infection-induced host extinction. However, many of these complex patterns are restricted to a relatively narrow parameter range. We find that the host extinction occurs at intermediate levels of infection virulence, as well as for Allee effect strengths much lower than when the infection is absent. In both cases, a sequence of events comprising destabilization of an endemic equilibrium, growth of oscillation amplitude, and a heteroclinic bifurcation forms an underlying mechanism. We apply our model to the feline immunodeficiency virus (FIV) in domestic cats.

The evolution of plant virus transmission pathways

The evolution of plant virus transmission pathways is studied through transmission via seed, pollen, or a vector. We address the questions: under what circumstances does vector transmission make pollen transmission redundant? Can evolution lead to the coexistence of multiple virus transmission pathways? We restrict the analysis to an annual plant population in which reproduction through seed is obligatory. A semi-discrete model with pollen, seed, and vector transmission is formulated to investigate these questions. We assume vector and pollen transmission rates are frequency-dependent and density-dependent, respectively. An ecological stability analysis is performed for the semi-discrete model and used to inform an evolutionary study of trade-offs between pollen and seed versus vector transmission. Evolutionary dynamics critically depend on the shape of the trade-off functions. Assuming a trade-off between pollen and vector transmission, evolution either leads to an evolutionarily stable mix of pollen and vector transmission (concave trade-off) or there is evolutionary bi-stability (convex trade-off); the presence of pollen transmission may prevent evolution of vector transmission. Considering a trade-off between seed and vector transmission, evolutionary branching and the subsequent coexistence of pollen-borne and vector-borne strains is possible. This study contributes to the theory behind the diversity of plant-virus transmission patterns observed in nature.

And the nasty ones lose in the end: foliar pathogenicity trades off with asexual transmission in the Irish famine pathogen Phytophthora infestans

A trade-off between pathogenicity and transmission is often postulated to explain the persistence of pathogens over time. If demonstrated, it would help to predict the evolution of pathogenicity across cropping seasons, and to develop sustainable control strategies from this prediction. Unfortunately, experimental demonstration of such trade-offs in agricultural plant pathogens remains elusive. We measured asexual transmission of Phytophthora infestans isolates differing in pathogenicity in two sets of artificial infection experiments under controlled, semi-outdoor conditions. Higher foliar pathogenicity decreased mean daughter tuber weight, increased infection severity in daughter tubers, and increased stem mortality before emergence. The most pathogenic isolates thus suffer a double penalty for asexual transmission: a lower survival probability within small and severely infected tubers; and a lower infection probability of neighbouring healthy plants due to fewer infected stems produced by surviving tubers. Moderate tuber resistance favoured transmission of the least pathogenic isolates, while high levels of resistance almost abolished transmission of all isolates. These data demonstrate a trade-off between foliar pathogenicity and asexual transmission over seasons in P. infestans, which should stabilise pathogenicity over time in the potato late blight pathosystem and possibly favour clone replacement by less pathogenic lineages after demographic bottlenecks.

Beyond Mortality: Sterility As a Neglected Component of Parasite Virulence

Virulence is generally defined as the reduction in host fitness following infection by a parasite (see Box 1 for glossary) [1]. In general, parasite exploitation of host resources may reduce host survival (mortality virulence), decrease host fecundity (sterility virulence), or even have sub-lethal effects that disturb the way individuals interact within a community (morbidity) [2,3]. In fact, the virulence of many parasites involves a combination of these various effects (Box 2). In practice, however, virulence is most often defined as disease-induced mortality [1, 4–6]. This is especially true in the theoretical literature, where the evolution of sterility virulence, morbidity, and mixed strategies of host exploitation have received relatively little attention. While the focus on mortality effects has allowed for easy comparison between models and, thus, rapid advancement of the field, we ask whether these theoretical simplifications have led us to inadvertently minimize the evolutionary importance of host sterilization and secondary virulence effects. As explicit theoretical work on morbidity is currently lacking (but see [7]), our aim in this Opinion piece is to discuss what is understood about sterility virulence evolution, its adaptive potential, and the implications for parasites that utilize a combination of host survival and reproductive resources.

Evolutionary suicide through a non-catastrophic bifurcation: adaptive dynamics of pathogens with frequency-dependent transmission

Evolutionary suicide is a riveting phenomenon in which adaptive evolution drives a viable population to extinction. Gyllenberg and Parvinen (Bull Math Biol 63(5):981-993, 2001) showed that, in a wide class of deterministic population models, a discontinuous transition to extinction is a necessary condition for evolutionary suicide. An implicit assumption of their proof is that the invasion fitness of a rare strategy is well-defined also in the extinction state of the population. Epidemic models with frequency-dependent incidence, which are often used to model the spread of sexually transmitted infections or the dynamics of infectious diseases within herds, violate this assumption. In these models, evolutionary suicide can occur through a non-catastrophic bifurcation whereby pathogen adaptation leads to a continuous decline of host (and consequently pathogen) population size to zero. Evolutionary suicide of pathogens with frequency-dependent transmission can occur in two ways, with pathogen strains evolving either higher or lower virulence.