Author ORCID Identifier
Date Available
6-12-2023
Year of Publication
2022
Degree Name
Doctor of Philosophy (PhD)
Document Type
Doctoral Dissertation
College
Agriculture, Food and Environment
Department/School/Program
Entomology
First Advisor
Dr. Nicholas Teets
Abstract
Insects living in temperate and polar environments have developed numerous adaptations to increase survival at low temperatures. A majority of insects are freeze-intolerant and die from internal ice formation, but some are freeze-tolerant and can survive ice formation in extracellular spaces. Both categories of insects prepare for winter with a combination of seasonal and rapid acclimation responses, which differ both in time course and in underlying mechanisms. In this dissertation, I examine adaptations for winter survival in several insect species with a specific focus on molecular mechanisms. To better understand the underpinning mechanisms of these adaptations, I leverage functional genomics approaches and tools.
In my first research chapter, I established a cell culture system for studying rapid cold hardening (RCH) in Drosophila S2 cells. RCH is a process whereby a brief non-lethal exposure to cold greatly increases survival to a subsequent cold shock. Tissues are capable of undergoing RCH ex vivo, indicating that RCH is largely regulated at the cellular level. In my work, I demonstrated that cultured Drosophila S2 cells are also capable of RCH, which opens the door to use cell culture tools to identify the cell signaling mechanisms that underly RCH.
In my second research chapter, I characterized transcriptomic responses during distinct stages of reproductive diapause in the lady beetle Hippodamia convergens. Diapause is a programmed period of dormancy for surviving adverse seasons, and in the case of H. convergens, diapause creates challenges for its use as a biological control agent. Diapausing females either leave fields upon release or remain close by without feeding. Characterizing the molecular regulation of diapause may facilitate strategies to manipulate diapause in this economically important species. Further, molecular studies of diapause are currently dominated by studies in Diptera, so my work will contribute fundamental insights into the evolutionary physiology of diapause. For this study, I assembled and annotated a de novo transcriptome for H. convergens and found that diapause is accompanied by the upregulation of genes involved in locomotion to facilitate dispersal to overwintering grounds and by the downregulation of genes regulating reproduction.
In my third research chapter, I identified molecular processes specific to freezing by comparing gene expression profiles in frozen and supercooled larvae of Belgica antarctica larvae. This Antarctic species is freeze-tolerant, and in wet conditions, larvae freeze due to inoculation from ice crystals in the environment, while in dry conditions, larvae supercool their internal fluids to avoid freezing. These dual strategies offer a rare opportunity to directly compare gene expression changes following both freezing and supercooling, a commonly used strategy for freeze-intolerant species. Despite the challenges associated with ice formation, freezing did not elicit greater overall levels of differential expression or stronger expression of antioxidant and detoxification genes than supercooling. These results indicated that gene expression changes are largely driven by changes in temperature rather than ice formation.
Overall, my dissertation highlights that while insect overwintering appears passive on the surface, it is regulated by a dynamic web of gene expression, protein function, and hormone signaling. Furthermore, while some molecular hallmarks are shared across species, overwintering mechanisms can be highly unique to individual species. Thus, continuing to advance understanding of insect overwintering mechanisms will require careful coordination of study species, methodological approaches, and thorough data analysis. Together, my work provides critical insights into how insects survive winter at the molecular level.
Digital Object Identifier (DOI)
https://doi.org/10.13023/etd.2022.402
Funding Information
This study was supported by a United States Department of Agriculture National Institute of Food and Agriculture Hatch Project titled "Integrative Research on the Overwintering Biology of Insects" (no.: 1010996) from 2017 to 2021.
This study was supported by a United States Department of Agriculture National Institute of Food and Agriculture Hatch Project titled "Integrative Research on Insect Thermal Tolerance and Seasonal Biology" (no.: 7000545) from 2021 to 2022.
This study was supported by a National Science Foundation grant titled "National Science Foundation's Directorate of Geosciences and the National Environment Research Council: Mechanisms of Adaptation to Terrestrial Antarctica through Comparative Physiology and Genomics of Antarctic and sub-Antarctic Insects" (no.: OPP-1850988) from 2019 to 2022.
This study was supported by a Kentucky Science and Engineering Foundation grant titled "Calcium-dependent Signaling Mechanisms Governing Rapid Cold Hardening in Insects" (no.: KSEF-148-502-16-391) from 2016 to 2018.
Recommended Citation
Nadeau, Emily Allison Wheeler, "A Functional Genomics Approach to Overwintering Mechanisms in Insects" (2022). Theses and Dissertations--Entomology. 69.
https://uknowledge.uky.edu/entomology_etds/69
Chapter 2 Supplementary Data
Chapter_3_Supplemental.zip (2520 kB)
Chapter 3 Supplementary Data
Chapter_4_Supplemental.zip (2040 kB)
Chapter 4 Supplementary Data
