Abstract
Multicellular hosts and their associated microbial partners (i.e. microbiomes) often interact in mutually beneficial ways. Consequently, hosts may choose to allocate resources to regulate and recruit appropriate microbes. In doing so, hosts may incur an energetic cost and, in turn, these costs can affect host fitness. It remains unclear how hosts have evolved to balance the costs of expending resources to manage their microbiomes against the benefits that might accrue by doing so. To address this question, we extend a previously developed agent-based computational model of host-microbiome evolution by incorporating a resource provisioning process, to determine how hosts have evolved to balance the costs and benefits of expending resources to manage their microbiomes. Our results indicate that resource provisioning will evolve when hosts provide a high percentage of microbes to their offspring and contribute a low percentage of their microbiome to the environment. In contrast, resource provisioning will not evolve when hosts contribute a high percentage of their microbiome to the environment. This is because hosts that do not provide resources to acquire microbes can nonetheless still acquire microbes from the environmental contributions of hosts that do provide such resources. Since resource provisioning incurs a fitness cost to the host, over evolutionary time, resource provisioning will not be favored. Our results also show that hosts are less likely to provide resources to acquire beneficial microbes if hosts can obtain a high proportion of these microbes from the environment.