Editorial: Epigenetics as a Deep Intimate Dialogue between Host and Symbionts

In ancient Greek, the term koiné refers to a common language, which spread in the Eastern Mediterranean region and the Near East following the conquests of Alexander the Great. In the 4th century BC, koiné quickly became a shared language that enabled people speaking different dialects to communicate with each other. Indeed, effective communication is always necessary for overcoming " barriers " between individuals and groups, tightening relationships, and enabling new ways of coping with a changing world. This is the case not only for interactions involving cultural relations. Different living entities communicate in many ways, with the most intimate and often the most long-lasting communication being the formation of mutually beneficial, obligatory symbiotic associations. As Lynn Margulis forcefully argued, symbiotic relations are found among virtually all living organisms, and were critical for the origin and diversification Since the partners in symbiotic relationships frequently belong to different kingdoms, and the intimacy may be so deep that the Symbionts reside in the tissues and/or cells of the host, their shared " language " should be a basic—and ancient—form of communication. Such effective communication blurs the boundaries between different living entities, giving rise to a single biomolecular network, a " holobiont " with a " hologenome " (Zilber-Rosenberg and Rosenberg, 2008; Bordenstein and Theis, 2015), thus problematizing the conventional notion of individuality. The series of papers presented in this topic-issue explore the fascinating developmental and evolutionary relationship between Symbionts and hosts, by focusing on the mediating epigenetic processes that enable the communication to be effective and robust at both the individual, the ecological, and the evolutionary time scales. One of the currently most researched cases illustrating the productive cross-talk between Symbionts and hosts is the symbiosis between mammals and their gut microbiome. As Gilbert stresses, the birth of a mammal is not merely the origin of a new distinct (traditional) individual, but the onset of a new community. Already in utero, the fetus interacts with a network of symbiotic