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.
Becerril et al. (Tue,) studied this question.