Field translocation across latitudes reveals local conditions drive microbiome assembly in a damselfly

Triggered by global warming there is a surge of interest in understanding why conspecific populations differentiate along latitudinal gradients. Despite the insight that microbiomes may contribute to host traits and show geographic variation, how hosts differentially assemble their microbiomes across latitudes and how this shapes latitudinal patterns in host trait values has been understudied. This is especially true for the many animals that show a complex life cycle with aquatic larvae and terrestrial adults, and where metamorphosis may reset the microbiome. To address this knowledge gap, we examined microbiome composition along a latitudinal gradient and assessed the relative influence of host and environmental factors in shaping the microbiome of the damselfly Ischnura elegans. We collected bacterioplankton samples from high-latitude (Southern Sweden) and low-latitude (Southern France) ponds, along with larval gut and adult abdominal microbiome samples from I. elegans inhabiting these ponds. Additionally, we conducted a reciprocal translocation experiment in which larvae with a strongly reduced gut microbiome were transferred between three high- and three low-latitude ponds, enabling them to reassemble their gut microbiome from the local bacterioplankton, and tested the larvae for their fitness. We found clear latitudinal differences in the community composition of the bacterioplankton and host-associated microbiomes in adults and to a lesser extent in larvae, highlighting differences in microbiome composition across the latitudinal gradient and suggesting a strong influence of environmental factors on microbiome assembly. Moreover, our results showed that metamorphosis did not completely reset the microbiome, since larvae and adults shared part of their microbiome. The reciprocal translocation experiment revealed that microbiome reassembly in microbiome-depleted larvae was primarily shaped by the local environment, rather than the host source population or latitude, underscoring the strong influence of environmental filtering in shaping microbiome composition. Moreover, phenotypic differences in larval growth rate between high- and low-latitude populations disappeared after translocation, suggesting that local environmental conditions overruled any underlying genetic differentiation. By combining microbiome sampling across latitudes with a novel reciprocal translocation approach using microbiome-depleted larvae, our study provides key insights into how the environment shapes host-microbiome associations along geographic gradients.