Temperature changes reveal different transcriptional responses in the larvae of the bark beetle Dendroctonus rhizophagus during the cold season.

Bark beetle, Dendroctonus rhizophagus, colonises and kills healthy sapling pine trees in the Sierra Madre Occidental, Mexico. In the autumn, its fifth-instar larvae migrate to the host's roots (hibernaculum) to overwinter; however, little is known about temperature changes in this hibernaculum during the cold season and the physiological responses related to cold tolerance in this species. A three-year temperature record was analysed to define thermal thresholds throughout the cold season in the hibernaculum. Fifth-instar larvae were collected from the thermal thresholds and sequenced using RNA-seq to assemble a global de novo transcriptome. Differential expression, gene enrichment and co-expression analyses were performed to determine the main metabolic pathways and biological processes taking place in these larvae during the cold season. Three thermal thresholds were defined: late-fall, mid-winter and late-winter. In late-fall, the transcriptional response was related to motility and feeding, possibly associated with the migration of larvae to the hibernaculum; in mid-winter, it was related to the physiological adjustments involved in the cold resistance phenotype; and, in late-winter, it was related to the processes involved in pupal chamber construction and the onset of metamorphosis. Our results show that the temperature in the hibernacula and the transcriptional response of fifth-instar larvae of D. rhizophagus change during the cold season, where lower temperatures coincide with the cold resistance phenotype.