Effects of endophyte infection, environmental stress and competition on Lolium perenne populations from the Mediterranean region

My thesis is a study of the variable outcomes of the interaction between the common forage grass species Lolium perenne and its fungal symbiont Neotyphodium lolii to explain the existing variation in endophyte infection frequencies. N. lolii is a systemic fungal endophyte that colonizes the aerial tissues of its host plant and is vertically transmitted to the next generation via seeds. Neotyphodium endophytes in agronomic grass species are considered plant mutualists, because they improve the resistance of the host against various abiotic and biotic stresses. Empirical evidence suggests variable fitness effects of this endophyte on wild populations of L. perenne and this association may vary from mutualism to antagonism and may depend upon environmental conditions and the genotypes of these interacting species. I used wild infected grass populations which are part of the native distributional range of L. perenne, to understand how the overall ecological outcome of this interaction depends upon host genotype, water availability, inter-genotype competition between endophyte-infected (E+) and non-infected (E-) grasses, and the native habitat of this grass species. I also examined a possible relationship between climate in the Mediterranean and infection frequencies of N. lolii in natural L. perenne populations.;In some populations under drought, E+ plants had more tillers, greater tiller lengths, total dry mass and green shoot mass than non-infected E- plants, suggesting a positive effect of endophyte infection on host growth. Total tiller length and the number of tillers showed significant population*treatment*infection interactions for 4 of 6 populations which shows endophyte effects specific for particular accessions and cultivars under different water treatments. E+ genotypes outperformed E- genotypes under inter-genotype competition. Egenotypes experienced the effects of competition more severely than E+ genotypes although E+ genotypes didn't significantly outperform E- genotypes in all populations. In addition I found a negative relationship between infection frequency and a precipitation climate variable.;Interpretation of the results leads to the conclusion and support of the hypothesis that the interaction of infection, genotype and the environment is crucial in determining the outcome of the grass-endophyte symbiosis.