DIVERSITY AND HOST RANGE OF FOLIAR FUNGAL ENDOPHYTES: ARE TROPICAL LEAVES BIODIVERSITY HOTSPOTS? A. ELIZABETH ARNOLD1 AND F. LUTZONI Department of Biology, Duke University, Durham, North Carolina 27708 USA
Abstract. Fungal endophytes are found in asymptomatic photosynthetic tissues of all major lineages of land plants. The ubiquity of these cryptic symbionts is clear, but the scale of their diversity, host range, and geographic distributions are unknown. To explore the putative hyperdiversity of tropical leaf endophytes, we compared endophyte communities along a broad latitudinal gradient from the Canadian arctic to the lowland tropical forest of central Panama. Here, we use molecular sequence data from 1403 endophyte strains to show that endophytes increase in incidence, diversity, and host breadth from arctic to tropical sites. Endophyte communities from higher latitudes are characterized by relatively few species from many different classes of Ascomycota, whereas tropical endophyte assemblages are dominated by a small number of classes with a very large number of endophytic species. The most easily cultivated endophytes from tropical plants have wide host ranges, but communities are dominated by a large number of rare species whose host range is unclear. Even when only the most easily cultured species are considered, leaves of tropical trees represent hotspots of fungal species diversity, containing numerous species not yet recovered from other biomes. The challenge remains to recover and identify those elusive and rarely cultured taxa with narrower host ranges, and to elucidate the ecological roles of these little-known symbionts in tropical forests. Key words: Ascomycota; Barro Colorado Island; diversity; endophytic fungi; host affinity; ITSrDNA; latitudinal gradient; richness; symbiosis; tropical forests.
Comprising interactions that range from mutualism to antagonism, fungal symbioses with plants are key determinants of biomass, nutrient cycling, and ecosystem productivity in terrestrial habitats from the poles to the equator (e.g., Clay and Holah 1999, Hawksworth 2001, Gilbert 2002). Most plant-associated fungi catalogued to date have been recognized because of the fruitbodies they produce in association with their hosts (e.g., plant pathogens, mycorrhizal fungi). Yet plants in all major lineages, including liverworts, mosses, seedfree vascular plants, conifers, and angiosperms, also form cryptic symbioses with fungi that penetrate and persist within healthy aboveground tissues such as leaves. Foliar fungal endophytes (i.e., endophylls or mycophyllas) are a fundamental but frequently overlooked aspect of plant biology: all plant species surveyed thus far harbor one or more endophytic symbionts in their photosynthetic tissues (Stone et al. 2000). The presence of obligately heterotrophic endophytes within photosynthetic tissues of plants raises the Manuscript received 20 September 2005; revised 24 April 2006; accepted 2 May 2006. Corresponding Editor: G. S. Gilbert. For reprints of this Special Feature, see footnote 1, p. 539. 1 Present address: Division of Plant Pathology and Microbiology, Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721 USA. E-mail: email@example.com 541
question of the ecological importance of these cryptic symbionts. Studies of the systemic, maternally inherited endophytes (Clavicipitaceae, Ascomycota) associated with over 300 species of grasses indicate that an array of plant phenotypic traits—including drought tolerance, leaf chemistry, tolerance of heavy metals in soils, and propensity for vegetative reproduction—are directly attributable to the presence of endophytes (Clay and Schardl 2002). However, this model system of endophyte biology, arguably the most familiar to ecologists, represents a special case. The photosynthetic tissues of the vast majority of...