Cadmium ingestion does not disrupt the social transmission of the microbiome in juveniles of a subsocial insect

Symbiotic microbes are essential for host health and fitness. In family-living species, these microbes are often acquired through vertical transmission from parents and horizontal transmission from siblings. However, it is unclear how environmental stressors, such as chemical pollution, influence these contributions to the juvenile microbiome. Here, we tested the independent and interactive effects of social environment and cadmium ingestion - a common heavy metal pollutant - on the microbiome of juvenile European earwigs. We reared juveniles either alone, with siblings or with siblings plus the mother, exposed them to cadmium at 0, 25 or 100mg/L, and analysed their microbiome composition and diversity. Our results showed that both social environment and cadmium independently influenced the phylogenetic beta-diversity of the juvenile microbiome although neither factor nor their interaction affected alpha-diversity. Moreover, we found that observed specific bacterial taxa in the social groups, including the overabundance of putatively pathogenic (Serratia) and mutualistic (Lactobacillus) symbionts were more abundant in juveniles reared with family members than isolated ones. Overall, our findings suggest that while both social environment and cadmium shape the juvenile microbiome, family does not amplify the effects of chemical exposure. This highlights the resilience of microbial transmission within families, even under environmental stress.