| Popis: |
Host-plant resistant (HPR) crop varieties provide effective and inexpensive pest control through the expression of natural resistance genes. However, the durability of HPR is often short lived as insect populations evolve virulence, i.e. the ability to bypass these natural defenses. The evolutionary and genetic mechanisms that produce virulence govern its proliferation are largely unstudied. Understanding these mechanisms could provide valuable insight into how virulence emerges and spreads within populations, allowing for the development of more effective virulence management tactics. In this dissertation I have integrated population genetic and genomic analyses with field ecology to investigate the evolutionary and genetic roots of virulence in the soybean aphid (Aphis glycines), and how this information can be applied to increase the durability of HPR soybean. Population genetic analysis in chapter two revealed no significant genetic differentiation between avirulent and virulent biotypes. This pattern is consistent with the uninhibited gene flow and long distance migration found in past studies, and suggests biotypes regularly interbreed. These patterns do not align with a simple gene-for-gene interaction, but are indicative of polygenic or non-genetic adaptation. This prediction was clarified in chapter three where population genomic analyses revealed regions of the genome under divergent selection between avirulent and virulent biotypes clustered both spatially and functionally, in agreement with the polygenic model. Further, transposable elements and transcription/translation regulators were over represented within these divergent genomic regions, suggesting their probable role in virulence evolution. Chapter four describes that construction of a draft genome assembly of A.glycines. The final assembly represents the smallest known aphid genome (317.2Mb) and is highly continuous and complete with a length of 302.9Mb (95.5% of predicted genome) spread across 8,397 scaffolds. Additionally, 92% of A. glycines transcripts mapped to the assembly, suggesting a high coverage of known genes. Gene modeling and transposable element prediction suggest that both gene count (17,558 models) and repetitive sequence (9.36% of genome) are reduced in A. glycines relative to other aphid genomes, potentially accounting for the smaller size. While gene annotation is required to complete the genome assembly, this draft has revealed much regarding the genomic architecture and gene structure of the species.Chapter two and three revealed unrestricted gene flow between biotypes and a polygenic adaptive mechanism, suggesting that virulence could spread rapidly under selection. In chapter 5 these findings were applied to the field, investigating the use of susceptible refuges to manage the proliferation of virulence through the reduction of selection pressure. Susceptible refuges in microcosms were found to significantly increase the fitness of avirulent biotypes relative to 100% HPR soybean plantings. However, they were not capable of reversing the proliferation of virulent biotypes. The increase in avirulent fitness suggests that refuges may effectively slow the proliferation of virulence, and may be synergized by abiotic and biotic variables in the field. Unexpectedly, infesting plants with a mixture of biotypes significantly altered biotype fitness and movement, with biotype 3 experiencing a 90.30% fitness increase, indicating a potential synergistic effect of virulence mechanisms. |
