Evaluating the endophytic fungal community in planted and wild rubber trees (Hevea brasiliensis)
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Gazis, R. O. 2012. Evaluating the endophytic fungal community in planted and wild rubber trees (Hevea brasiliensis). University of Maryland, College Park, PhD Thesis.
Abstract:
Even though Hevea brasiliensis represents one of the most economically important crops and has been qualified as a high pest risk, its associated fungal community has not been studied in detail. This is especially true for its endophytic community, which could harbor beneficial and mutualistic species with biological control potential and applications. Since there was no previous knowledge of any aspect of the fungal endophytic community for any species of Hevea neither in the wild nor in plantations when this project started (2007), Chapter Ι presents a preliminary assessment of the fungal endophytes inhabiting leaf and sapwood tissue of a wild rubber tree population distributed within the Peruvian Amazon. Sampling techniques were tested, especially in regard to sapwood endophyte isolation. Collection sites involving wild rubber populations were all located in remote areas, where facilities for media and culture preservation represented a constant challenge. Chapter I describes the techniques used in endophyte isolation and isolate identification, and gives a preliminary overview of the diversity and abundance of the fungal endophytic species associated to wild H. brasiliensis. It also illustrates the compositional difference between the fungal endophytic community of leaf and sapwood tissues.
From Chapter I, an appreciation of the high diversity of fungal endophytes
and the difficulty in their species delimitation process came to light. Many groups of fungi known to contain species complexes were found to be common components of the endophytic community of H. brasiliensis. Therefore, in Chapter II, species delimitation concepts and techniques commonly used in the study of fungal diversity were explored and tested. The main objective of Chapter II was to demonstrate how the choice of a species delimitation concept influences not only the estimation of diversity but also the biogeographic and ecological inferences drawn from those values. Based on several approaches and using three unlinked genetic markers, we were able to conclude that three of the most common groups of endophytes (i.e., Colletotrichum, Pestalotiopsis, and Trichoderma) are really species complexes, harboring more than one cryptic species. Based on the results from this chapter, a higher ITS (Internal Transcribed Spacer genetic marker) sequence similarity threshold was recommended (up to 99% in sequence similarity).
Sampling intensity can also affect the estimation of diversity and, like the
choice of species delimitation criteria, it can influence further inferences and hypothesis on ecology and biogeography. Chapter III explores the incongruence of sampling designs among fungal endophyte studies and its effects on diversity estimation. Based on two H. brasiliensis populations (for a total of 100 individuals), the sampling effort needed to reach an asymptotic species accumulation curve was estimated. These populations of H. brasiliensis were also used to compare the sampling effort needed in natural (wild) versus artificial ecosystems (plantations). Furthermore, in this chapter, the performance of the most commonly used diversity indices were evaluated, within the hyperdiversity context imposed by tropical fungal endophytes.
This dissertation represents one of the few studies involving a fungal
endophyte survey from a Neotropical tree species distributed within its native range. Moreover, is one of the few that has explored the fungal endophytic community of sapwood. The fungal Kingdom is believed to hold a great percent of unknown species; therefore, it was not surprising that through this research, new lineages were discovered. Chapter IV describes a new fungal lineage within the Pezizomycotina (Ascomycota), corresponding to a class rank. The new lineage, named Xylonomycetes showed distinctive morphological, ecological, and molecular characteristics that set it apart from all the other recognized ascomycetous classes. Chapter IV highlights the incompleteness of our current knowledge of the ―Fungal Tree of Life and stresses the need to conduct more explorations in remote tropical areas and poorly explored niches (e.g. sapwood).
One of the main objectives of this dissertation was to explore for potential
biocontrol agents that can be use in the fight against economically important rubber diseases. For this purpose, we sampled fungal endophytes extensively (endophytes were isolated from 190 individual Hevea trees). Chapter V describes and compares the fungal endophytic community of wild and planted Hevea trees and proposes potential strains for future in vitro and in planta assays. This last Chapter emphasizes the vast and distinctive diversity of fungal endophytes harbored only in wild Hevea trees, while it raises the awareness of the potential loss of these symbionts due to land use change, viz. deforestation.