Host conservatism or host specialization? Patterns of fungal diversification are influenced by host plant specificity in Ophiognomonia (Gnomoniaceae: Diaporthales) DONALD M. WALKER 1,2 *, LISA A. CASTLEBURY 3 , AMY Y. ROSSMAN 3 and LENA STRUWE 2,4 1 Department of Natural Sciences, The University of Findlay, 1000 North Main Street, Findlay, OH 45840, USA 2 Department of Plant Biology and Pathology, Rutgers University, 59 Dudley Road, New Brunswick, NJ 08901, USA 3 Systematic Mycology & Microbiology Laboratory, USDA Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, USA 4 Department of Ecology, Evolution and Natural Resources, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901, USA Received 12 May 2013; revised 11 August 2013; accepted for publication 12 August 2013 In this study evolutionary host plant patterns at ranks from order to species were analysed using spatial evolutionary and ecological vicariance analysis (SEEVA), based on a multigene phylogeny of 45 ascomycete fungal species. The objective was to understand speciation events and host associations in Ophiognomonia (Gnomoniaceae). Species of this genus are perithecial fungi that occur as endophytes, pathogens, and latent saprobes on plants in the families of Betulaceae, Fagaceae, Juglandaceae, Lauraceae, Malvaceae, Platanaceae, Rosaceae, Salicaceae, and Sapindaceae. A second objective was to determine whether speciation events are influenced by host conservatism, host specialization, or host switching at different taxonomic host ranks. Host differences between sister clades were interpreted using the divergence index (D) from the SEEVA analysis, ranging from 0 for no divergence to 1 for maximum possible divergence. Several fungal subclades showed clear patterns of host order/family conservatism (D= 1.00) for hosts in Betulaceae, Fagaceae, Juglandaceae, and Rosaceae. Clear trends of host specialization at host genus and species ranks (D = 1.00) were suggested within these host families. Independent host jumps were observed for two species at the family rank and three at the order rank. As a result of this study, host specificity and specialization is hypothesized as a mechanism that can strongly contribute to speciation patterns in fungal pathogens. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 1–16. ADDITIONAL KEYWORDS: co-evolution – divergence index – ecological niche evolution – host evolution – multigene phylogeny – SEEVA. INTRODUCTION Understanding the trade-offs between generalist and specialist life strategies is important to interpret the coevolution of host–fungus systems over geological space and time (Whitlock, 1996; Kawecki, 1998). In addition, understanding phylogenetic niche conserva- tism in an evolutionary context within monophyletic clades of fungi/hosts, and how they behave over time, is critical to explain coevolutionary patterns among species of fungi and their host plants. Fungi use a suite of pathogenicity factors that determine host association and the potential to cause disease in their *Corresponding author. E-mail: walkerd@findlay.edu DNA sequence data deposited in GenBank. Data from SEEVA analyses available from Dryad and D.M.W.’s lab homepage: https://sites.google.com/site/findlaymycology/ Biological Journal of the Linnean Society, 2014, 111, 1–16. With 4 figures © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 1–16 1