The gut microbiome shapes latitudinal differences in host immunity and pathogen tolerance, and buffers against immunosuppression under future warming

Latitudinal patterns in fitness-related traits within species may inform how populations could evolve under global warming, yet are poorly understood. We investigated the novel idea that the gut microbiome drives latitudinal differences in immune function and pathogen tolerance by contrasting high- and low-latitude populations of Ischnura elegans damselflies. A reciprocal gut microbiome transplant was performed between high- and low-latitude larvae at two thermal regimes, whereafter larvae were exposed to the pathogen Escherichia coli. Pathogen exposure increased mortality, especially under warming. Our results confirmed latitude-associated thermal adaptation and a faster pace-of-life of the low-latitude larvae, and revealed this to be associated with a lower immune function and pathogen tolerance (higher E. coli body burden). Moreover, our results provide the first experimental evidence that the gut microbiome causally contributed to latitudinal differences in the host’s immune function and pathogen tolerance. As latitudinal patterns in the microbiome are widespread, this may be an important yet ignored proximate driver of latitudinal patterns in immune function and pathogen tolerance. As expected, warming reduced the immune function especially in high-latitude larvae. Notably, the integration of a gut microbiome transplant experiment with a space-for-time substitution suggested that the high-latitude host may avoid a strong immunosuppression under warming, when only its gut microbiome but not the host would converge to the current low-latitude host-microbiome combination. Taking into account how the gut microbiome shapes latitudinal patterns in fitness-related traits and integrating it into space-for-time substitutions will deepen our understanding of the evolution of host populations under warming.