Diversity, specificity and admixture in the Sirex - Amylostereum - Deladenus symbiosis

Abstract

Biological control is an important management tool to deal with the rapidly increasing number of invasive pests of plantation forests globally. It is important to consider the genetic diversity of both the pest and biological control populations to understand geographic population structure, patterns of invasion, genotype-genotype interactions and potential adaptability. This thesis examined patterns of genetic diversity and specificity of the Sirex–Deladenus–Amylostereum complex. Mitochondrial sequence data and nuclear microsatellite markers were used to characterize the diversity in a global collection of D. siricidicola from both native and non–native regions. The data revealed the presence of three distinct lineages, from North America (Lineage A; non-native), the Southern Hemisphere (Lineage B; non-native) and Spain (Lineage C; native). Interestingly, samples from Chile represented an admixed population of lineages A and B. The global study showed evidence of substantially genetic diversity present globally which could be used to augment the reduced genetic diversity in the Southern Hemisphere biological control populations. The three D. siricidicola lineages were shown to be able to interbreed in culture. The admixed offspring of one of the crosses showed a significant increase in its reproductive rate on the slowest growing fungal isolate, when compared to the parental strains. Experimental admixture suggests the possibility and advantage of introducing more genetic diversity in biological control programs. As the symbiotic fungus A. areolatum of the pest wasp S. noctilio plays a crucial role in the mass production and influences the performance of the biological control agent, the fidelity of the Sirex –Amylostereum association was studied in native Siricids in Japan and their fungal associates. It was shown that the association was not species specific. Sirex nitobei was associated not only with Amylostereum areolatum, but also Amylostereum chailletii. Urocerus sp., previously associated with A. laevigatum, carried A. chailletii. Vegetative compatibility test revealed high clonality both among A. areolatum and A. chailletii in association with these wasps. Together with previous studies it seems that the host tree plays a more critical role in selection of Amylostereum species than the wasp. The thesis illustrates the importance of studying the genetic diversity of biological control agents and the potential of augmenting the genetic diversity in these populations as a mean to improve adaptability.