Djahoui, E. | bioRxiv Science

bioRxiv2025

The role of pathogens in creating and maintaining host polymorphism

Terrestrial and marine ecosystems exhibit remarkable species diversity and richness. Among the hypotheses put forward to explain this diversity --its origins and the forces sustaining it-- one prominent hypothesis posits that pathogens could be pivotal in generating and maintaining species diversity within ecosystems. To explore this hypothesis, we analyzed a host-pathogen system, examining the specific conditions that might facilitate pathogen-driven diversification within the host population, with a focus on both the emergence and persistence of diversity. By applying a fecundity-transmission trade-off in which a non-evolving pathogen influences its evolving host, we observed that pathogens only infrequently drive host diversification, and only around intermediate trade-off conditions. Furthermore, while these rare diversification events result in stable coexistence, evolved diversity is low, consisting of two coexisting host morphs. Because the sole evolution of hosts in response to pathogens offers limited opportunity for diversification, we propose additional mechanisms that could foster higher levels of diversity within the system.

ecology↗

bioRxiv2025

The strength of the fecundity-immunity trade-off modulates host evolutionary dynamics, pathogen propagation, and host abundance

In most species, life history trade-offs generate conflicts between biological functions because of resource allocation constraints. During pathogenic infections, hosts can deploy various immune defenses, but these responses usually carry fitness costs, most often expressed as reduced fertility. This fecundity-immunity trade off has been widely studied for its evolutionary implications, yet its effects on other emergent properties --such as host abundance and pathogen spread-- remain poorly understood. Here, we develop an SIR model incorporating three immune strategies --reduced host susceptibility, increased host recovery, and reduced pathogen virulence-- and show that the evolutionary impact of the fecundity-immunity trade off depends on both its shape and the specific immune mechanism under selection. For reduced susceptibility and increased recovery, disease spread and host abundance evolve in opposite directions along evolutionary trajectories, such that when disease spread increases, host abundance decreases, and vice versa. For reduced virulence, however, this relationship is more complex and does not follow a simple pattern. Across all three mechanisms, trade offs with low cost intensity, whether concave or convex, favor the joint evolution of high fecundity and high immunity. In contrast, concave or convex trade offs incurring high cost consistently select for high fecundity at the expense of immunity. Trade offs with moderate cost intensity generate two alternative outcomes: one strategy with high fecundity and low immunity, and another combining low fecundity with enhanced immunity. Together, these findings underscore the importance of accounting for the specific immune mechanism evolving and the associated cost intensity when predicting the eco-evolutionary consequences of fecundity-immunity trade-offs. HighlightsO_LIImpacts of host evolution on R0 and total host abundance N are rarely investigated C_LIO_LIHost evolution accessed under various components of a fecundity-immunity trade-off C_LIO_LIWeak fecundity-susceptibility / recovery trade-offs decrease R0 and increase N C_LIO_LIStrong fecundity-susceptibility / recovery trade-offs increase R0 and decrease N C_LIO_LIMore complex patterns are observed for the fecundity-virulence trade-off C_LI

ecology↗