Date of Award

Spring 5-15-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Advisor(s)

Segraves, Kari A.

Keywords

herbivory, performance, polyploidy, preference, whole genome duplication

Subject Categories

Ecology and Evolutionary Biology | Entomology | Evolution | Life Sciences

Abstract

Herbivory by insect herbivores is a strong selective force in many communities. Although herbivore-plant interactions have been a focus of interest for decades, we are still exploring how large-scale genetic changes in the host plant alters their interactions with insect herbivores. One such prevalent change in plants is whole-genome duplication (WGD), or polyploidy, but we are still learning how WGD affects ecological interactions. In particular, studies of herbivory on polyploids have primarily focused on established polyploid plant populations. Although this approach has yielded exciting results that suggest WGD affects insect herbivores feeding on polyploids, this approach also confounds the effects of WGD and evolutionary change in established polyploids. As a result, a critical gap in our understanding is to discern the direct effects of plant polyploidy on herbivore-plant interactions. One way to approach this problem is to study newly formed polyploids, or neopolyploids, because these plants will express the phenotypic effects of WGD with little evolutionary change. Thus, a primary goal of this dissertation was to investigate the immediate effects of WGD on herbivore-plant interactions by conducting experiments using neopolyploid host plants. In the first chapter, I examined how an insect herbivore responds to neopolyploid host plants by testing if preference and performance of a specialist aphid differed between diploids and neopolyploids. The results showed that aphid preference was unaffected by WGD, but some of the aphids experienced performance trade-offs when feeding on the neopolyploid host. This suggests that specialist herbivores will incorporate novel polyploid hosts into their diet, but that these host shifts may come at a fitness cost. These results demonstrated that a specialist herbivore was impacted by the immediate effects of WGD, yet we know that herbivore preference and performance are also dictated by herbivore feeding style and diet breadth. Therefore, in the second chapter, I assessed if WGD in the host plant and plant population of origin altered preference and performance of two chewing herbivore species that differed in diet breadth. I also explored if changes in herbivore performance corresponded with changes in trichome morphology, an important physical defense against insect herbivores. The results corroborated the findings of Chapter 1 by showing that preference of both herbivore species was unaffected by WGD in their host plant. There were, however, modest effects of WGD on herbivore performance. The herbivore with the broadest diet breadth was more sensitive to the changes caused by WGD and population of origin than the herbivore with the narrower diet breadth. WGD also appeared to alter physical defenses, as diploid plants had higher trichome densities and fewer branches than neopolyploid plants, and neopolyploids often had higher trichome densities and more branches than established tetraploids. These changes corresponded roughly to the observed changes in herbivore performance on neopolyploids, suggesting that WGD may alter plant traits in ways that benefit herbivores. Together, Chapters 1 and 2 highlight that WGD does not prevent herbivores from colonizing neopolyploid hosts or successfully developing on them, suggesting that WGD creates an opportunity for herbivores to expand their diet breadth. Given that the phenotypic effects of WGD in the host plant do not prevent herbivore attack, then this also means that neopolyploids are unlikely to escape herbivory. As a result, neopolyploids will experience the negative effects of herbivory, and this may impact their ability to persist in new populations. One way that neopolyploids could ameliorate the effects of herbivory is by being tolerant. That is, some plants can compensate for the damage caused by herbivores and attain similar levels of fitness as plants that are not damaged. This idea was tested in the final chapter where I examined if neopolyploids were tolerant to herbivory by comparing fitness of diploids and neopolyploids across multiple damage levels. This experiment showed that neopolyploids were as tolerant to herbivore damage as their diploid counterparts, indicating that neopolyploids do not gain a fitness advantage over diploids when faced with increased herbivory pressure; thus, changes in tolerance following WGD are unlikely to aid in the establishment of neopolyploids in new environments. This dissertation demonstrates that WGD does not convey any fitness advantages to neopolyploids when they are attacked by herbivores, and insect herbivores experience little to no effects when feeding on neopolyploids. Together, the results indicate that WGD may act as a mechanism that facilitates host shifts in insect herbivores, a critical first step in diversification for many insects. Since WGD induces instantaneous reproductive isolation in host plants, WGD likely plays a role in speciation of both insect herbivores and their hosts.

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