Holobionts - hosts together with their resident microorganisms - provide a framework for studying life as a network of interdependent partners. Within host-ectoparasite holobionts, the dialogue between the two microbiomes offers powerful clues to ecological balance, disease dynamics and evolution. Because each holobiont is structurally and functionally compartmentalised, microbes exchanged at the interface can elicit highly local, niche-specific effects that ripple through the system. This review synthesises evidence for microbiota-to-microbiota interactions in four models: Varroa mite-honeybee, tick-vertebrate, bat fly-bat and mosquito-vertebrate pairs. In all cases, microbes move passively during feeding or contact, then colonise, replicate and modulate physiology and immunity, exerting a longer-lasting influence than transient biochemical cues. We further introduce the idea of indirect modulation, whereby abiotic or biotic factors act on a recipient holobiont through the intermediary of transferred microbes, underscoring the adaptive plasticity of holobiont networks. Bidirectional cross-talk forms self-reinforcing feedback loops that can redefine a microbe as pathogen, symbiont or immunomodulator, and tune its virulence according to context. These mechanisms shape disease transmission, resistance traits and the overall health of both partners. A deeper grasp of such cross-holobiont dynamics will pave the way for microbiota-based vaccines, targeted microbiome engineering and other innovative tools for human, veterinary and environmental health.

• Keywords
• Ectoparasite, Holobiont, Host, Microbial interactions, Microbiome,
• MeSH
• Diptera microbiology   MeSH
• Host-Parasite Interactions * MeSH
• Ticks microbiology   MeSH
• Microbiota * MeSH
• Vertebrates microbiology   parasitology   MeSH
• Symbiosis * MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Review MeSH

Varroa destructor mite is a major threat to honeybee (Apis mellifera) populations, contributing to colony losses through parasitism and pathogen transmission. While extensive research has focused on Varroa biology and its role as a virus vector, its microbiome remains poorly understood, particularly regarding geographic variation. Here, we investigated the microbial diversity, composition, and functional potential of Varroa mite microbiota collected from two neighboring countries, Czechia and Slovakia. Using high-throughput sequencing and network analysis, we assessed alpha and beta diversity metrics, microbial co-occurrence patterns, and predicted metabolic functions. Our results revealed significant differences in microbial diversity between the two regions, with some bacterial taxa appearing more prevalent in specific populations. Network analysis suggested potential variation in the structural stability of microbial communities in Varroa mites, raising the possibility that geographic factors may influence microbial interactions. Functional profiling indicated region-associated differences in predicted metabolic pathways, possibly linked to certain bacterial taxa. While these findings provide new insights into the Varroa microbiome and its potential ecological role, the interpretation of geographic influence remains a subject of ongoing investigation to better understand its scope and underlying mechanisms. A deeper understanding of these microbial dynamics may contribute to the development of novel strategies for Varroa mite management and the conservation of honeybee health.

• Keywords
• Varroa destructor, Geographical variation, Microbial co-occurrence networks, Microbiome, Predicted functional pathways,
• Publication type
• Journal Article MeSH