Neotropical fungal endophytes: Diversity and ecology

Abstract

Fungal endophytes associated with leaves of woody plants are thought to be diverse and abundant, especially in tropical forests. However, associations of endophytes with tropical woody angiosperms have not been characterized in detail. In this dissertation, I assess: (1) the scale of endophyte biodiversity in a tropical forest; (2) ecological, temporal, and host-mediated factors influencing endophyte abundance in leaves of tropical trees; (3) utility of endophyte morphospecies as functional taxonomic units; and (4) the nature of endophyte-host interactions. To characterize endophyte biodiversity, I measured endophyte richness in asymptomatic leaves of two distantly related, but co-occurring, host species in lowland forest in central Panama. I found that endophytes are highly diverse within leaves, plants, and host species, and that they demonstrate both host preference and spatial heterogeneity. To determine factors influencing abundance of tropical endophytes, I assessed effects of inoculum abundance, duration of exposure to inoculum, phylogenetic position of host, and leaf traits (e.g., chemical and structural defenses) on rates of endophyte infection. I found that endophyte abundance is sensitive to inoculum abundance in the short term, but to duration of exposure over the long term. In turn, inoculum abundance is sensitive to canopy cover, time of day, and time since precipitation. Generally, neither leaf traits nor phylogenetic position of hosts is related to rates of endophyte infection: endophytes occur with consistently high abundance and diversity among representatives of 14 orders of angiosperms. To assess utility of morphospecies as taxonomic units, I compared diversity and taxonomic composition among morphospecies of endophytes as delineated using several criteria, and among species as inferred using nrDNA data. I found that conservatively designated morphospecies can approximate species boundaries as defined by nrDNA data for diverse Ascomycota. Finally, I examined effects of endophytes on (1) minimum leaf conductance, a measure of water loss from leaves under drought conditions; and (2) leaf mortality and necrosis in the presence of a foliar pathogen for a focal host. I found that endophytes may cost their hosts by increasing water loss under conditions of severe drought, but that they also may play an important role in host defense against pathogens.