Symbiosis is an intimate relationship between different living entities and is widespread in virtually all organisms. It was critical for the origin and diversification of Eukaryotes and represents a major driving force in evolution. Indeed, symbiosis may support a wide range of biological processes, including those underlying the physiology, development, reproduction, health, behavior, ecology and evolution of the organisms involved in the relationship.

Although often confused with mutualism (when both organisms benefit from the association), symbiosis actually encompasses several and variable relationships. Among them is parasitism, when one organism benefits but the other is harmed, and commensalism, when one organism benefits and the other remains unaffected. Even if many symbiotic lifestyles do exist in nature, here we will refer almost exclusively to cases where the intimacy between the partners is so deep that the “symbiont” (sensu strictu) resides into the tissues and/or cells of the other partner. Such a lifestyle is commonly found in bacteria, fungi and protists and viruses, living in association with animal and plant hosts.

Deciphering the molecular mechanisms underlying the “dialogue” between hosts and symbionts is of extraordinary importance for many research areas, as well as for the possibility of therapeutic applications in animals and plants.

In recent years studies on the role of epigenetics in host-symbiont crosstalk have been flourishing. For example in many human pathogens epigenetic changes are involved in morphological and/or developmental plasticity that is cued by interactions with the host. On the symbiont side, many plant and animal pathogens seem or have been demonstrated to induce epigenetic modifications in the host’s genome, thus promoting their own survival and replication, as well as escaping from the host’s immune responses. Other data are provided by reproductive parasites of arthropods that interfere with the host's genetic imprinting for their efficient maintenance and spread.

Although these are promising beginnings, the role of epigenetic inheritance in host-symbiont associations must be clarified further. There is evidence that for symbionts that are located in the host's germ-cells, and thus vertically transmitted to the progeny, epigenetic changes are inherited by the host's progeny. However, epigenetic inheritance might also occur even if the symbionts do not establish intimate relationships with the host's germ-line, but the evidence is still scarce. We encourage authors to throw light on this question.

Another question of outstanding importance is to establish if the “epigenetic dialogue” between host and symbiont is direct or indirect. The existing data on symbiont molecules directly targeting the host’s epigenetic machinery are impressive but still scarce, and we do not yet know how widespread this phenomenon is. Furthermore, researchers are welcomed to explore and discuss other working hypotheses concerning the possibility that epigenetic changes in the host's genome may arise as a consequence of a “manipulation” made by the symbiont on host pathways such as those involved in hormonal signaling.
A large amount of data on the role played by hormones in epigenetic control of gene expression is available.

The aim of this research topic would be to enhance and promote interactions among high quality researchers from different disciplines such as genetics, cell biology, pathology, zoology, microbiology, endocrinology, and evolutionary biology in order to join forces and decipher the epigenetic dialogue between symbionts and hosts.

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