# Virulence and Vertically Transmitted Pathogens: The Role of Costs Paid by Co‐Evolving Hosts in a Self‐Regulating Population

## TL;DR

The study explores how host and pathogen co-evolution affects virulence and vertical transmission, finding that assumptions about immune costs are crucial.

## Contribution

The paper resolves conflicting predictions by modeling co-evolution under specific assumptions about host population growth and immune costs.

## Key findings

- A positive relationship between host-pathogen antagonism and vertical transmission is possible under certain assumptions.
- Density-dependent growth and cost-free immune function in uninfected hosts can lead to pathogen extinction.
- Immune cost assumptions are critical in determining co-evolution outcomes.

## Abstract

Many pathogens transmit horizontally through usual routes and vertically from parent to offspring. Co‐evolution is predicted, under certain circumstances, to produce a positive relationship between host‐pathogen antagonism and the rate of vertical transmission. We cannot disentangle the roles of host demographics and the costs of host immune function in establishing this pattern. On one hand, models that assume no density‐dependent growth of host populations and limit the cost of immune function to infected hosts only predict that the positive relationship is possible. On the other hand, models that assume density‐dependent growth of host populations and impose the cost of immune function on all hosts, regardless of infection status, suggest that the positive relationship is not possible. Here, we seek to resolve the confusion. We model the co‐evolution of a host and its pathogen when the latter can transmit both vertically and horizontally. We assume host population growth is self‐limiting, and we impose the cost of immune function only on infected hosts. We find that a positive relationship between host‐pathogen antagonism and vertical transmission is possible under our assumptions. Our finding points to the critical role played by assumptions about when hosts pay the cost of immune function. We also find that the combination of density‐dependent host population growth and the cost‐free lifestyle of uninfected hosts raises the possibility of selection‐driven pathogen extinction. We discuss our findings in relation to previous theory and empirical findings.

Theory suggests that coevolution can lead to a positive relationship between host‐pathogen antagonism and vertical transmission, but it’s uncertain whether host demographics or the nature of immune costs drive this pattern. We address this uncertainty with a mathematical model of host‐pathogen co‐evolution that allows for both vertical and horizontal transmission of the pathogen. We find that self‐regulation of the host does not always lead to lower virulence of vertically transmitted pathogens.