A TEST OF HOST-TAXON, ENVIRONMENT, AND DISTANCE EFFECTS ON LEAF FUNGAL ENDOPHYTES IN METROSIDEROS ON THE ISLAND OF O‘AHU

Abstract

Tree fungal endophyte (FE) communities may be influenced not only by abiotic environmental conditions, but also by varying degrees of affinity to their host plants. The landscape-dominant woody genus Metrosideros (Myrtaceae) comprises many morphologically distinct taxa that occupy different habitats throughout the Hawaiian Islands. This study used Metrosideros on O‘ahu to test the relative importance of environment versus host taxon on FE composition and diversity. Variation in FE communities due to geographic distance (across and within sites) was also examined. From each of four elevation gradients (sites), leaves were collected from four Metrosideros taxa, two sympatric taxa occurring at high elevations, representing a wetter and slightly cooler environment, and two sympatric taxa occurring at lower elevations, representing a drier and slightly warmer environment (11 trees/taxon/gradient, n = 176 trees). DNA was extracted from surface-sterilized leaf samples. Fungal DNA was amplified using barcoded internal transcribed spacer (ITS) forward and reverse primers, and the barcoded amplicons underwent next-generation sequencing (IonTorrent). The sequences were filtered in R, using RStudio and bioinformatically processed with the vegan and dada2 packages. The remaining samples (n = 113 trees) yielded 1,637 unique ESVs (exact sequence variants). Permutation tests, diversity indices, and Akaike information criterion models revealed that variation in FE diversity was significantly explained by Metrosideros taxon, site, and geographic distance. Non-metric multidimensional scaling (NMDS) plots showed considerable overlap in FE communities among host taxa and among sites, however, and evidence for host-specificity of leaf FEs was weak and restricted to 700-1,000 m above sea level. FE communities did not vary with elevation (environment); however, the elevation ranges examined may be too narrow for the detection of elevation/environmental effects. Lastly, a significant pattern of isolation by distance on FE community composition was detected across the island as well as within each of the four sites. These results suggest that within O‘ahu Metrosideros, leaf FE communities vary in diversity and composition across space, some of this variation is associated with host taxonomic effects and distance, and very little is associated with environmental variation across the island`s elevation gradients.