Surviving Alpine Winters Without Dehydration: Physiological and Transcriptomic Insights into the Freeze Avoidance of Megastigmus sabinae Xu et He (Hymenoptera: Torymidae)

Megastigmus sabinae Xu et He (Hymenoptera: Torymidae) is an oligophagous seed pest overwintering within Juniperus przewalskii cones in the alpine Qilian Mountains. Understanding its cold adaptation is critical for forest pest management. To investigate its overwintering mechanisms, we monitored field temperatures and larval development. Subsequently, larvae were subjected to a series of low-temperature treatments under laboratory conditions. We then assessed survival rates, supercooling points (SCPs), free water content, and the transcriptomic profiles of water-regulatory genes. Field data revealed that larvae enter a quiescent state when ambient temperatures drop below 5 °C. Laboratory acclimation at 0 °C significantly improved larval survival at −10 °C and markedly depressed the SCP by approximately 6 °C. Notably, free water content remained completely stable across all temperature treatments. Consistent with this dehydration-independent phenotype, transcriptomic analysis showed that the majority of 17 aquaporins (AQPs) and a Capa receptor gene lacked differential expression. However, two specific AQP genes (TIP1-1 and AQPN) were significantly upregulated. Collectively, M. sabinae larvae employ a freeze-avoidance strategy driven by dehydration-independent SCP depression and highly targeted transcriptional adjustments of specific water channels.