Resolving the conflict: The ecology and evolution of horizontal versus vertical transmission strategies in a model insect-virus interaction

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Diseases infect plants and animals throughout the natural world. They may be horizontally transmitted from an infected individual to a susceptible individual either by direct contact or through the transfer of an infectious particle. Alternatively, the disease can be vertically transmitted following adult reproduction, when it is transferred from the parent to its offspring. This kind of transmission may continue for many generations with the infected animal appearing to be uninfected. These are called covert disease infections because the pathogen does not cause any visible symptoms. The abundance of uninfected hosts in a pathogen's environment is a key factor affecting how efficiently it is transmitted. When uninfected host density is high, horizontal transmission of lethal pathogens occurs frequently because individuals can come into contact with each other frequently. Conversely, when host densities are low, horizontal transmission is limited or may not occur at all. Vertical transmission of non-lethal covert infections can occur as this form of transmission does not depend on contact between infected and uninfected individuals. Since horizontal transmission of a lethal disease kills the host, it will clearly impede the vertical transmission of non-lethal covert infections, which need their host to survive and reproduce. Resolving this conflict between transmission modes is the central aim of the proposal. We will investigate the ecological conditions that favour different transmission modes and determine the link between transmission mode and pathogen virulence. Baculoviruses are pathogens that infect arthropods, especially butterflies and moths, and these pathogens are transmitted both horizontally (and lethally) and vertically (non-lethally). Lethal virus outbreaks are often associated with high densities of forest insect pests, but these outbreaks are typically separated by years of low host density and no obvious presence of pathogens. What happens at these times is an important part of the pathogen life cycle that is very poorly understood. We aim to test the idea that host density is a key factor affecting whether horizontal or vertical transmission routes are favoured. We will do this by monitoring the prevalence of lethal and covert infections in long term laboratory populations that differ markedly in patterns of abundance. We will also carry out selection experiments where we determine how the different transmission routes affect the life history traits of the host, such as development and fecundity, and of the pathogen, such as pathogenicity and virus production. Vertically transmitted infections are likely to be especially important in those species that are rare, or those species that have highly variable densities, such as pests and invasive species. Identifying the conditions that favour one or other transmission mode and their impact on both host and disease ecology is crucial.