Abstract
<jats:p>The impacts of symbionts on their hosts can range from mutualistic to pathogenic, often changing in strength or even sign with context. When context varies spatially, spatial heterogeneity in species interactions can arise, including gradients in symbiont impacts on their hosts. One case where symbiont gradients have been well-studied is patterns of virulence for pathogens spreading across a stationary host population. However, less work has been done in the context where hosts themselves are moving, as is the case for range shifts or expansions, and considering mutualistic symbionts. Here, I develop a model to explore the formation of host-symbiont gradients (including both pathogenic and mutualistic strategies) in the context of a host population that is spreading spatially. I found that a key to generating gradients is the strength transmission-mortality trade-offs where both increasingly pathogenic symbionts have higher transmission and cause higher host mortality and increasingly mutualistic symbionts have lower transmission and cause increased host survival. I found that a transmission-mortality trade-off can generate symbiont gradients but only in one direction: with edge symbionts being less mutualistic (or more pathogenic) than those at the population core. In contrast, symbiont gradients in both directions were possible when the transmission-mortality trade-off was combined with symbiont effects on any of the host spatial processes considered: dispersal distance, survival during dispersal, or establishment after dispersal. This work expands on past theory by clarifying the details of the transmission-mortality trade-off, and it highlights symbiont impacts on establishment as a novel mechanism for generating spatial gradients.</jats:p>