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

Triggered by global warming there is increasing interest in understanding why conspecific populations differentiate along latitudinal gradients. Although microbiomes may shape host traits and vary geographically, how hosts differentially assemble microbiomes across latitudes and how this shapes latitudinal patterns in host trait values has been understudied. This is especially true for organisms with complex life cycles, where metamorphosis may reset the microbiome. We examined microbiome composition along a latitudinal gradient and assessed the relative influence of host and environment in shaping the microbiome of the damselfly Ischnura elegans . We sampled bacterioplankton from high‐latitude (southern Sweden) and low‐latitude (southern France) ponds, along with larval gut and adult abdominal microbiomes from I. elegans inhabiting these ponds. Additionally, we conducted a reciprocal translocation experiment in which larvae with a strongly depleted gut microbiome were transferred between three high‐ and three low‐latitude ponds, allowing microbiome reassembly from the local bacterioplankton, and quantified the growth rate of the larvae. We found latitudinal differences in 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 strong environmental effects on microbiome reassembly. Moreover, despite these differences, larvae and adults shared a small proportion of their microbiome (ca 15.83%). 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 the seven‐day 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.