Fluctuating pathogen-mediated selection drives the maintenance of innate immune gene polymorphism in a widespread wild ungulate

Toll-like Receptors (TLR) play a central role in recognition and host frontline defence against a wide range of pathogens. A number of recent studies have shown that Tlrs often exhibit a large polymorphism in natural populations. Yet, there is little knowledge on how this polymorphism is maintained and how it influences disease susceptibility in the wild. We investigated the selective mechanisms by which multiple-pathogens drive the genetic diversity of Tlr genes, as well as the commonly-used Mhc-Drb in European roe deer (Capreolus capreolus), an abundant and widespread ungulate in Europe. In a previous work, we showed that some Tlrs exhibit contemporary signature of balancing selection in roe deer. To assess the relative contribution of 9heterozygous advantage9, 9negative-frequency dependence9 and 9fluctuating selection9, we tested associations between Mhc-Drb or Tlr variants and infection or co-infection with Toxoplasma and Chlamydia, two intracellular pathogens known to cause reproductive failure in ungulates. We showed that Toxoplasma and Chlamydia exposures vary significantly across space and time with few co-infection events and that the two pathogens act antagonistically on Tlr polymorphism leading to fluctuating selection. By contrast, we only observed Tlr heterozygous advantage for a single pathogen and we did not find evidence of negative-frequency dependence. We did not detect any association with Mhc-Drb. Our study confirmed the importance of looking beyond Mhc genes in wildlife immunogenetic studies. It also emphasized the necessity to consider multiple pathogen challenges and their spatio-temporal variations to improve our understanding of vertebrate defence evolution against pathogens.