Spatial patterns of C4 grass-associated fungal endophyte communities

Abstract

Foliar fungal endophytes represent ubiquitous and hyper-diverse symbionts of all lineages of plants. However, the factors that shape their diversity and composition from local to global scales has been little explored, relative to charismatic macro-organisms e.g. plants and animals. Additionally, certain regions of the globe have been scantly explored when it comes to micro-organisms in general, e.g. tropical and sub-tropical grassy biomes. This thesis aimed to fill these gaps by exploring the patterns and the factors that shaped the diversity and composition of foliar fungal endophytes associated with C4 grasses, sampled within tropical and sub-tropical grassy biomes at local, continental and international scales. More specifically, I examined whether foliar fungal endophyte diversity of the endophytes associated with two widely distributed and keystone grass species within tropical and sub-tropical regions of the Paleo-tropics followed the classical latitudinal diversity gradient in species richness. I then attempted to link the observed patterns to established macroecological theories commonly used to evoke the latitudinal diversity gradient in species richness for many taxa i.e. the species-energy hypothesis and the evolutionary-time hypothesis. Using this same dataset, I determined which factors drove the turnover of rare to increasingly common endophyte taxa at continental and intercontinental scales, by using a newly proposed zeta diversity framework which explicitly considers the frequency of occurrence and thus a taxa’s rarity or commonness. Lastly, I grew many different C4 grass species in a common garden to remove the effect of variable abiotic conditions and different inoculum sources could have on endophyte communities, to explicitly assess how plant traits effected the diversity and composition of foliar fungal endophytes. My thesis showed that foliar fungal endophytes associated with widespread C4 grass species did follow the classical latitudinal diversity gradient is species richness, and that a combination of energy availability (i.e. high growing season temperatures) and the evolutionary history of the grass hosts (i.e. grass hosts which were present in a location for longer periods of time, in millions of years) generally supported a higher diversity of foliar fungal endophytes. My findings highlighted that different factors shape the turnover or rare to increasingly common endophyte taxa at both the continental and intercontinental scales, with geographic distance being the only consistent driver of endophyte turnover for rare and common endophytes at both spatial scales. Lastly, plant traits, specifically leaf structural traits, had a significant effect on the diversity and composition of foliar fungal endophytes, and are thus important factors to consider for future research. My thesis demonstrated that many different factors shape the diversity and composition of foliar fungal endophytes, and that the factors that shape compositional turnover differ for rare and common endophytes at different spatial scales. Future efforts should be made to assess other macroecological patterns e.g. species-area relationships or distributional patterns of foliar fungal endophytes. This thesis advances our understanding of the factors that shape the diversity and composition of foliar fungal endophytes across different spatial scales, adding value sampling from under-represented tropical and sub-tropical grassy biomes.