Mitochondria of cold hardy insects: Responses to cold and hypoxia assessed at enzymatic, mRNA and DNA levels

Winter survival for larvae of goldenrod gall insects, the freeze-avoiding Epiblema scudderiana, and the freeze tolerant, Eurosta solidaginis, includes entry into diapause (a torpid state of arrested development) and expression of a variety of cryoprotective adaptations. Diapause and cold winter temperatures, as well as freezing in E. solidaginis, all strongly reduce the need for mitochondrial activity. To evaluate the responses of mitochondria to these conditions, we assessed the maximal activity of cytochrome c oxidase (COX), transcript levels of COX subunit 1 (encoded on the mitochondrial genome), mitochondrial 12S rRNA levels and mitochondrial DNA content. COX activity decreased over the winter months in both species to levels that were about one-third of September values. COX activity also dropped significantly in E. scudderiana in response to cold acclimation (4,−4,−20 °C) or hypoxia exposure. COX activity was less sensitive to these stresses in E. solidaginis but rose by ∼50% when larvae were thawed after freezing. COX 1 mRNA transcripts and 12S rRNA levels were unchanged over the winter months in E. scudderiana, as was COX 1 DNA content; this indicates that changes in COX enzymatic activity are likely mediated mainly by post-translational modification. However, both COX transcript and 12S rRNA levels decreased in response to hypoxia exposure in both species, whereas COX DNA did not, which indicates that transcription of the mitochondrial genome is sensitive to oxygen levels.