Co-infection dynamics: Major Histocompatibility Complex modulation of Batrachochytrium dendrobatidis and Ranavirus infections in amphibians

Genetic variation in immune genes is an important component of genetic diversity. The genes in the Major Histocompatibility Complex (MHC) provide an excellent model system for studying the mechanisms that generate and maintain genetic diversity in natural populations. While both demographic factors and pathogen-mediated selection processes contribute to the extreme diversity observed in the MHC systems, determining the relative importance of these evolutionary mechanisms has remained challenging. We investigated the role of pathogen-mediated selection in driving MHC diversity in three amphibian species: <i>Ichthyosaura alpestris</i>, <i>Pleurodeles waltl</i> and <i>Pelophilax perezi</i>. Our study examined the relationships between individual MHC diversity, infection status, infection intensity, and co-infection with two major amphibian pathogens: <i>Batrachochytrium dendrobatidis</i> (<i>Bd</i>) and <i>Ranavirus</i> sp. (<i>Rv</i>) in natural populations. Our research demonstrated significant differences in <i>Bd</i> and <i>Rv</i> infection intensities among individuals with varying numbers of MHC loci. However, co-infection showed no discernible influence on infection intensities. We observed stronger associations of specific MHC alleles and supertypes with infection intensity and status in <i>I. alpestris</i>. These findings suggest that, in the context of multi-host infections, MHC genes may provide valuable insights into the evolutionary forces shaping MHC diversity, although the specific effects of individual MHC alleles on disease dynamics are yet to be clarified.Keywords: amphibian, co-infections dynamics, Chytrid fungus, viruses, infection status