Re-inventing pathogen passage for social microbes
Passage experiments that sequentially infect hosts with parasites have long been used to manipulate virulence. However, in many invertebrate pathogens passage has been applied naively without a full theoretical understanding of how best to select for increased virulence. This has led to very mixed results. Understanding the evolution of virulence is complex because selection on parasites occurs across multiple spatial scales with potentially different conflicts operating on parasites with different life-histories. For example, in social microbes, strong selection on replication rate within hosts can lead to cheating and loss of virulence, because investment in public goods virulence reduces replication rate. In contrast, selection acting at a between host scale maintains virulence by selecting on parasite population size. In this study we tested how different scales of selection and varying mutation supply affect evolution of virulence against resistant hosts in the specialist insect pathogen Bacillus thuringiensis., aiming to optimize methods for strain improvement against a difficult to kill insect target. We show that selection for infectivity using competition between sub-populations in a metapopulation prevents social cheating, acts to retain key virulence plasmids and facilitates increased virulence. Increased virulence was associated with reduced efficiency of sporulation, and loss of function in putative regulatory genes but not with altered expression of known virulence factors. Selection in a metapopulation provides a broadly applicable tool for improving the efficacy of biocontrol agents. Moreover, a structured host population can facilitate artificial selection on infectivity, while selection on life history traits such as faster replication or larger population sizes can reduce virulence can reduce virulence in social microbes.