Abstract:
Sirex noctilio is a major pest to Pinus spp. and threat to the Forestry industry in the Southern
Hemisphere. The varying level of efficacy of its primary biological control agent, Deladenus
siricidicola, is also of serious concern. There is a need to better understand the diversity of
both the pest and its biological control agent, as one of the factors that could influence this
biological control system. One of the short‐comings is that molecular markers did not exist
to study the population diversity of the nematode and wasp. As part of this study it was
illustrated how next generation sequencing (454 pyrosequencing) can be used to enhance
microsatellite marker discovery tools, and produce a more robust, rapid and economical
tool for the development of these markers. The developed markers for D. siricidicola and S.
noctilio proved to be highly efficient in characterizing the population diversity of both of
these organisms.It was shown that the populations of D. siricidicola in the Southern
Hemisphere is highly homozygous at all the loci developed, which likely resulted from
human selection of favorable strains, genetic bottlenecks during transfer and subsequent
inbreeding. This result likely suggested a reduced ability of the nematode to adapt to the
diverse environmental conditions, different Pinus spp. and S. noctilio populations found in
the various regions. The populations from Southern Hemisphere and Canadian sources were distinct at most loci and illustrate the unexplored potential of exploring diverse populations
of D. siricidicola in order to screen for useful phenotypic variation. The microsatellite
markers that were developed were also effectively used to characterize representative
samples of S. noctilio in the Southern Hemisphere. The alleles are largely shared amongst
countries, reflecting the shared history of introduction. It also shows a stepwise invasive
process in South Africa, leading to a genetic bottleneck which might have a significant
impact on the population ecology of the wasp, as it can influence its sex ratio. This thesis
contributes critical tools and knowledge necessary to understand the population diversity of
both D. siricidicola and S. noctilio. As argued in the final chapter, this information and the
use of these tools in future is critical to understand aspects such as the evolution of
virulence and population ecology of the organisms, which should be used to direct biological
control programs.
