Mechanisms of Diapause and Cold Tolerance in the Colorado Potato Beetle
Abstract
Many temperate insects enter diapause (a state of dormancy) and enhance their cold tolerance to survive the winter. During diapause, the Colorado potato beetle (CPB, Leptinotarsa decemlineata, Coleoptera: Chrysomelidae) stops developing, lowers its metabolism, and changes its physiology to avoid freezing. The extent to which diapause confers cold tolerance in CPB is currently unknown. In my thesis, I used CPB to improve our understanding of the mechanisms underlying metabolic suppression during diapause and cellular protection at sub-zero temperatures in insects. First, I used RNA-sequencing (RNA-seq) to compare gene expression in two metabolically important tissues (the fat body and flight muscle) of diapausing and non-diapausing CPB, and then generated testable hypotheses about diapause in CPB. Colorado potato beetles differentially modulate their fat body and flight muscle transcriptomes during diapause; fat body plays a larger role in driving hypoxia- and immune-related processes during diapause, whereas processes mediating proteostasis and mitochondrial metabolism are more important in the flight muscle. Next, I tested the hypothesis that flight muscle mitochondria modulate energy metabolism in diapausing CPB. Indeed, low metabolic rates coincided with a reduction in flight muscle mitochondrial function and density, increased expression of Parkin (a mitophagy-related transcript), and presence of autophagic structures inside flight muscle cells. Further, knocking down Parkin with RNA interference partially restored mitochondrial density and whole-animal metabolic rate suggesting that Parkin-mediated mitophagy drives metabolic suppression during CPB diapause. In anticipation of emergence from diapause, beetles reversed this mitophagy and increase mitochondrial biogenesis to re-grow their mitochondria. Finally, I used RNA-seq to explore the mechanisms underlying the acquisition of cold tolerance in diapausing CPB. The major transcriptomic shift associated with the acquisition of cold tolerance is related to chaperone protein-related expression, and cold-tolerant beetles activate the chaperone response to a greater extent than less cold-tolerant diapausing counterparts. Further, cold-tolerant beetles potentially have a greater capacity for chaperone-mediated protein repair. Together, these studies contribute to an updated framework for insect diapause and cold tolerance and improve our understanding of the mechanisms insects use to survive the winter.