Maarten P.M. Vanhove1, Nikol Kmentová1, Christel Faes2, Jorge M.O. Fernandes3,4, Christoph Hahn5, Niel Hens2,6, Antoine Pariselle7,8 and Stephan Koblmüller5 1Research Group Zoology: Biodiversity and Toxicology, Centre for Environmental Sciences, Hasselt University, 3590 Diepenbeek, Belgium 2Interuniversity Institute for Biostatistics and Statistical Bioinformatics (I-Biostat), Data Science Institute, Hasselt University, 3590 Diepenbeek, Belgium 3Faculty of Biosciences and Aquaculture, Nord University, 8026 Bodø, Norway 4Institut de Ciències del Mar, Spanish National Research Council, 08003 Barcelona, Spain 5Institute of Biology, University of Graz, 8010 Graz, Austria 6Centre for Health Economic Research and Modelling Infectious Diseases, Vaccine and Infectious Disease Institute, University of Antwerp, 2000 Antwerpen, Belgium 7Institute of Evolutionary Science of Montpellier, Centre National de la Recherche Scientifique, Université de Montpellier, Institut de Recherche pour le Développement, 34394 Montpellier, France 8Laboratory Biodiversity, Ecology and Genome, Faculty of Sciences, Mohammed V University in Rabat, 10000 Rabat, Morocco Correspondence: maarten.vanhoveuhasselt.be

(Adaptive) radiations have attracted evolutionary biologists for a long time as ideal model systems to study patterns and processes of often rapid speciation. However, whereas a wealth of (sometimes already genome-scale) data is available for host radiations, very few studies target the patterns of diversification in their symbionts, even though they would be excellent models to study symbiont speciation. Our review summarizes what little is known about general patterns of symbiont diversification in often iconic adaptive host radiations and to what extent these patterns are dependent on the evolutionary trajectories of their hosts. We identify research gaps that need to be addressed in the future and discuss the potential of approaches not yet typically used in these study systems, such as epidemiological disease modeling and new omics technologies, for significantly advancing our understanding of these complex eco-evolutionary relationships.