Adaptation to Hot and Humid Climates in the Silkworm: Energy Reallocation and Cuticle Transpiration

The silkworm (<i>Bombyx mori</i>) is rich in germplasm resources, including thermotolerant strains that live in tropical/subtropical humid climates. In this study, two thermotolerant strains and one sensitive strain were used as materials, with the former exhibiting higher critical thermal maximum (CTmax) values. Under different temperature and humidity stresses, physiological and transcriptomic responses of the fifth instar larvae were compared. It was confirmed that high humidity exacerbates harmful effects only under high temperature conditions. Based on transcriptome and co-expression network analysis, 88 evolved thermoplastic genes (Evo_TPGs) and 1338 evolved non-plastic genes (Evo_non-PGs) were identified, which exhibited specific responses or expressions in the two thermotolerant strains. Eighteen of the Evo_TPGs encode cuticular proteins, 17 of which were specifically downregulated in the two thermotolerant strains after short-term exposure to 35 °C. This may promote cuticular transpiration to dissipate internal heat, thus compensating for the suppression of tracheal ventilation in hot and humid climates. For the Evo_non-PGs, most of the metabolic genes showed lower expression at background levels in the thermotolerant strains, while oxidative stress genes showed the opposite trend, suggesting that silkworms can enhance heat tolerance by suppressing metabolic rates and allocating more resources to overcome heat-induced oxidative damage. Furthermore, the heat resistance-related genes showed higher single nucleotide polymorphisms (SNPs) between resistant and sensitive strains compared to randomly selected genes, suggesting that they may have been subjected to natural selection. Through long-term adaptive evolution, thermotolerant silkworms may reduce their internal temperature by dynamically regulating cuticle respiration in response to high temperature and humidity, while allocating more energy to cope with and repair heat-induced damage. Overall, these findings provide insights into the evolution of heat-resistant adaptations to climate change in insects.