Strong microbiome disruption reduces survival in damselfly larvae, and subsequent pathogen exposure tends to increase mortality across metamorphosis

Microbiomes are increasingly recognized as key contributors to host fitness, yet their role in mediating stressor effects, especially across metamorphosis, remain poorly understood. To address this knowledge gap, we strongly reduced the gut microbiome of Ischnura elegans damselfly larvae using antibiotics followed by inoculation with a donor gut microbiome or not, and afterwards exposed the larvae to the pathogen Escherichia coli. Both host fitness and gut microbiome diversity and community composition were assessed during the larval and adult life stages. Despite marked differences in gut microbiome community composition between both life stages, partial retention of larval taxa in adult microbiota suggests incomplete microbiome turnover in these hemimetabolous insects. Furthermore, microbiome disruption significantly increased larval mortality, an effect mitigated by microbiome inoculation, underscoring the functional importance of microbial associations in damselflies. Moreover, pathogen exposure elevated larval mortality and tended to induce delayed mortality in the adult stage, revealing carry-over effects across metamorphosis. Yet, we did not find evidence of the gut microbiome mediating these carry-over effects. Together, our results highlight the critical role of the microbiome in determining host fitness, and the importance of considering both immediate and delayed stressor effects in animals with complex life cycles.