Abstract
The Argentine stem weevil (Listronotus bonariensis) caused considerable damage to New Zealand pastures before the introduction of the parasitic wasp Microctonus hyperodae and ryegrass endophytes. Microctonus hyperodae biocontrol is starting to fail, as is indicated by lower parasitism rates and observed pasture damage. The Argentine stem weevil and Microctonus hyperodae have different reproductive modes, with the Argentine stem weevil reproducing sexually and Microctonus hyperodae reproducing through parthenogenesis. The Argentine stem weevil may be ‘out evolving’ Microctonus hyperodae as a result.
Although most Hymenoptera, including Apis mellifera, use a haploid-diploid method of sex determination, there are many parthenogenic species. Parthenogenesis results from both infection with endosymbionts and through genetic mechanisms. The literature focuses heavily on how parthenogenesis occurs through infection, with little research into genetic or molecular mechanisms.
Sequencing and annotating the genomes of multiple strains of Microctonus aethiopoides, which use either sexual or asexual reproductive strategies, and subsequent bioinformatic analysis could allow for the genetic and molecular mechanism through which Microctonus hyperodae became asexual to be discovered.
Sequenced and assembled genomes were annotated in order to further study the gene compliment of the Microctonus aethiopoides and Microctonus hyperodae and analyze how these genes relate to biocontrol fitness in these species.
One group of genes which was assessed in detail is the olfactory gene compliment for the Microctonus species as olfactory receptors are fundamental for a range of behavioral responses which are required for effective biocontrol. Variation in the olfactory receptor complement may be due to selective identification of preferred hosts or mate identification in sexual Microctonus. Hi-C analysis of the genomes allowed for chromosome level assembly of the “Irish” strain Microctonus aethiopoides and Microctonus hyperodae, revealing that the number of chromosomes varies in the Microctonus genus. The Divergence of Microctonus hyperodae and Microctonus aethiopoides was estimated at 17 MYA; A more ancient divergence date than expected. Kraken2 analysis of sequenced reads failed to reveal endosymbiont infections in the Microctonus. Asexuals likely result from genetic mechanisms instead of endosymbiont infections. Genomescope2 analysis revealed low levels of heterozygosity in the genomes of Microctonus. Variation in heterozygosity did not correlate with asexual reproduction. Microctonus genomes have low repeat levels and have a total genome size of ~120 Mb.
This thesis is a monumental step towards expanding our understanding of the Microctonus, host and mate identification processes, and the mechanisms of asexual reproduction in the Microctonus asexuals. This thesis should continue to broaden the understanding of the biology of this group of agriculturally significant biocontrol agents.