Environmental Microbial Cues Alter Embryonic Development and Stress Responses in Vertebrates: Insights From the Zebrafish ( <i>Danio rerio</i> ) Model

Host-microbiome interactions shape key physiological processes, including bioenergetics, neurodevelopment and xenobiotic metabolism, and strongly influence the ecological fitness of the host. However, our understanding of host-microbiome interactions is primarily derived from post-embryonic free-living life stages, when the organism is in direct contact with microbes. Whether environmental microbial communities influence embryogenesis-particularly in oviparous organisms where embryos are encased in a protective chorion-remains unknown. Using zebrafish (Danio rerio) embryos reared germ-free or conventionalized with a defined microbial community at 6 and 24 h post-fertilisation, we demonstrate that environmental microbiota influence embryonic development prior to hatching, altering transcriptomic, proteomic and metabolomic pathways linked to energy metabolism, neurodevelopment and xenobiotic responses, including cytochrome P4501A (cyp1a) activation. Furthermore, embryos exposed to benzo(a)pyrene, a CYP1A-activator, exhibited microbiome-dependent changes in embryonic mitochondrial function and larval behaviour, revealing persistent developmental effects. These findings challenge the long-held assumption that embryonic development is insulated from microbial influence. Instead, our results reveal that host-microbe interactions begin earlier than previously recognised, with implications for developmental plasticity, xenobiotic stress responses and environmental sensitivity likely affecting the ecological fitness of the host. Our results advance understanding of molecular adaptation to microbial environments and provide a foundation for investigating how microbiota shape the developmental origins of host resilience and vulnerability in microbially-dynamic natural habitats.