Diversity in Sirex noctilio, Amylostereum areolatum and Deladenus siricidicola populations in Australasia and its implications for biological control

Abstract

The joint invasion of Sirex noctilio and its symbiotic fungus Amylostereum areolatum has threatened pine plantations in Australasia, and many other countries worldwide. The biological control nematode Deladenus siricidicola has been used to manage the populations of this pest complex for over 60 years. This thesis examined levels of genetic diversity, population structure and introduction history of the S. noctilio-A. areolatum-D. siricidicola populations in Australia and New Zealand, in comparison to global populations in the native and introduced regions. To gain insight into any potential ‘strains’ that may contribute to variable biocontrol success I further examined the reproductive rates of nematode diversity in Australia on different fungal strains and media types. Mitochondrial (mtCOI and mtSSU rRNA) sequence data and microsatellite markers were applied in the population genetics studies. This study identified unexpected diversity of D. siricidicola in Australia and New Zealand. Apart from the extensive diversity of known lineages (A, B and C) of the nematode, a previously unknown (lineage D) was discovered. The shared history between D. siricidicola populations in New Zealand and North America was unexpected. The higher-than-expected diversity suggests multiple introductions over time that have influenced these populations (either assisted or accidental), which increases the potential for the evolution of these populations. I found nonaligned diversity between the insect-fungal joint invasion in Australia and New Zealand, showing a similar pattern to previous studies. The diversity between the populations of S. noctilio was unique. Compared to other introduced populations worldwide, lower genetic diversity was found in Australasia, and there was no structure in populations between Australia and New Zealand. I was also in a unique position to compare the current populations of S. noctilio to that of museum collections in Australia and New Zealand. These collections of S. noctilio illustrated the link between the current and historical populations, but there was also a distinct shift evident in the diversity, likely due to the genetic drift in the population. Unlike the S. noctilio populations, widespread clonal populations of A. areolatum were found in the field in Australia and New Zealand, which formed a cluster distinct from fungal populations in all other countries. The high level of clonality in the A. areolatum population could be exploited for the management program using the nematode by rearing the nematode on the dominant clone. The behavioral study between the lineages B and D strains of D. siricidicola indicates significant variation in the reproductive rates when feeding on four different fungal strains and two media types. Lineage D strains show higher reproductive rates than the Kamona strain and other strains in lineage B, on all treatments in our trials. Taken together, the diversity identified in D. siricidicola populations in this thesis underscores the importance of post-release monitoring in the biocontrol program. The diversity of the D. siricidicola population in this thesis has significant consequences to re-visit applications of the biological control program. At the same time, the diversity results in the woodwasp and the fungus populations reflects an efficient quarantine system in recent decades, that isolated the woodwasp-fungal populations and reduced their complexity compared to other parts of the world.