# Dosage-dependent regulation of embryonic diapause by sorbitol dehydrogenase in the silkworm, Bombyx mori

## TL;DR

The study identifies BmSdh2 as a key enzyme regulating embryonic diapause in silkworms through dosage-dependent effects on metabolism.

## Contribution

BmSdh2 is shown to act as a molecular switch for diapause maintenance through sorbitol and lipid metabolism.

## Key findings

- Complete knockout of BmSdh2 eliminates diapause in silkworm embryos.
- BmSdh2 dosage modulates sorbitol accumulation and lipid metabolism during diapause.
- Heterozygous BmSdh2 mutants retain wild-type diapause phenotype.

## Abstract

Insects have evolved diapause to cope with harsh environmental conditions, during which their metabolism undergoes significant remodeling. The silkworm, Bombyx mori, enters diapause in the early embryonic stage, a process critically regulated by the sorbitol metabolism pathway and its key enzyme, sorbitol dehydrogenase (SDH). Nevertheless, the precise involvement of SDH in the diapause regulation of the silkworm remains to be fully understood. In this study, we identified that BmSdh2 is highly expressed in diapause-destined silkworm embryos through RNA-seq analysis. Genetic manipulation of BmSdh2 expression significantly influenced diapause progression: complete homozygous knockout of BmSdh2 led to diapause termination, whereas partial loss-of-function mutations maintained the wild-type diapause phenotype. Furthermore, integrative LC-MS/MS, metabolomic, and lipidomic analyses demonstrated that BmSdh2 dosage critically modulates diapause maintenance. These findings highlight BmSdh2 as a novel and potentially central molecular regulator in the silkworm diapause pathway.

Diapause serves as a crucial survival mechanism in insects, enabling them to withstand extreme environmental conditions by temporarily suspending development. In the silkworm (Bombyx mori), diapause involves metabolic reprogramming, including the conversion of glycogen into cryoprotectants such as sorbitol. Although sorbitol dehydrogenase (SDH) is known to reverse this process during diapause termination, its precise regulatory role remains unclear. Here, we identified BmSdh2 as a key gene that is highly expressed in diapause-destined silkworm embryos. Using CRISPR/Cas9-mediated mutagenesis, we discovered that BmSdh2 regulates diapause in a dosage-dependent manner: a complete knockout (BmSdh2−/−) eliminated diapause, whereas heterozygous mutants (BmSdh2+/−) retained the wild-type diapause phenotype. Metabolomic and lipidomic analyses revealed that BmSdh2 dosage critically modulates sorbitol accumulation and lipid metabolism, linking its activity to diapause maintenance. Our findings establish BmSdh2 as a molecular switch within the silkworm diapause pathway, shedding light on how metabolic sensing governs developmental plasticity. This work not only advances our understanding of insect diapause but also holds potential for manipulating diapause in sericulture, with implications for agricultural and biotechnological applications.