Transcriptional analysis of insect extreme freeze tolerance.

Few invertebrates can survive cryopreservation in liquid nitrogen, and the mechanisms by which some species do survive are underexplored, despite high application potential. Here, we turn to the drosophilid Chymomyza costata to strengthen our fundamental understanding of extreme freeze tolerance and gain insights about potential avenues for cryopreservation of biological materials. We first use RNAseq to generate transcriptomes of three C. costata larval phenotypic variants: those warm-acclimated in early or late diapause (weak capacity to survive cryopreservation), and those undergoing cold acclimation after diapause entry (extremely freeze tolerant, surviving cryopreservation). We identify mRNA transcripts representing genes and processes that accompany the physiological transition to extreme freeze tolerance and relate cryopreservation survival to the transcriptional profiles of select candidate genes using extended sampling of phenotypic variants. Enhanced capacity for protein folding, refolding and processing appears to be a central theme of extreme freeze tolerance and may allow cold-acclimated larvae to repair or eliminate proteins damaged by freezing (thus mitigating the toxicity of denatured proteins, endoplasmic reticulum stress and subsequent apoptosis). We also find a number of candidate genes (including both known and potentially novel, unannotated sequences) whose expression profiles tightly mirror the change in extreme freeze tolerance status among phenotypic variants.