Genomics and Evolutionary Origin of Longhorned Beetles in the Temperate Zone

Abstract

The latitudinal diversity gradient is a prominent pattern in the natural world, wherein species richness decreases from the equator towards the poles. Phytophagous insects are no exception to this rule, and their lower species diversity in the temperate zone is often attributed to the relatively young history of temperate lineages and their reduced host specialization and other biotic interactions. In this dissertation, I study the patterns and processes of adaptive evolution in phytophagous beetles of temperate forests within a phylogenetic framework. Through a detailed investigation of the phylogenetic and biogeographical history of temperate longhorned beetles, I examine the timing and genetic basis of their adaptation to cold and floristically depauperate habitats. In Chapter 1, I test a biogeographical hypothesis on the origin of the critically endangered Callipogon relictus in the temperate rainforest of East Asia and assess the potential impact of global warming on the distributional range of this longhorned beetle that serves as a relict of an ancient boreotropical distribution. In Chapter 2, I study another important longhorned beetle lineage of the genus Anoplophora to investigate whether the two temperate species in the genus represent independent adaptations to cold environments. To overcome the challenge of obtaining fresh materials that are suitable for DNA sequencing, I employ a target enrichment museomics approach and maximize the use of historical specimens. Finally, in Chapter 3, I use a comparative genomics approach to decipher the genetic basis of Monochamus longhorned beetles’ recent reverse shift to conifer-feeding, which represents a key innovation that allowed access to the biomass-rich resources of conifers in the Holarctic temperate forests. Based on the six newly assembled chromosome-level genomes of Monochamus and Anoplophora longhorned beetles, I find evidence of selection for genes that are associated with detoxification, innate immunity, and plant cell wall degrading enzymes in the conifer-feeding Monochamus, and suggest their possible coevolution with pinewood nematode.