Temporal changes in Gpdh1 expression in Bombyx mori: its correlation with embryonic diapause initiation and maternal...

Cytosolic NAD-dependent glycerol-3-phosphate dehydrogenases (GPDHs) catalyze the reduction of dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate using NADH, representing the first step in the glycerol biosynthesis pathway. While it is well established that the onset of egg diapause in Bombyx mori is accompanied by the rapid conversion of glycogen into sorbitol and glycerol, the regulation of Gpdh1 gene expression during embryonic diapause remains less well understood. In the present study, we investigated changes in the expression levels of the Gpdh1 gene. In diapause eggs, Gpdh1 expression remained relatively high during the first three days after oviposition, then gradually declined. In contrast, HCl-treated developing eggs exhibited an abrupt decrease in Gpdh1 expression upon treatment, followed by persistently low levels. Non-diapause eggs exhibited a similarly abrupt decrease as observed in HCl-treated eggs, indicating a correlation between the sharp decrease in Gpdh1 expression and embryonic development. To further explore the potential mechanisms regulating Gpdh1 expression, we analyzed the expression of several genes-including Gpdh1, insulin signaling (bombyxin-A6 and -Z1), sugar transporters (Tret1, St1, St3, St4, and St5), and trehalases (Treh1 and Treh2)-in the mother ovaries during the pupal stage of diapause- and non-diapause-destined silkworms. Our results showed that the expression of Gpdh1, bombyxin-Z1, Tret1, and Treh2 genes in diapause-destined ovaries increased in a stage-dependent manner compared to non-diapause-destined ovaries. In contrast, diapause-destined mother ovaries exhibited significantly lower expression levels of bombyxin-A6, indicating reduced insulin signaling during the pupal stage. Our findings suggest that reduced insulin signaling, along with elevated expression of Gpdh1, bombyxin-Z1, Tret1, and Treh2 genes may promote glycogen accumulation in the ovaries of diapause-destined mothers during the pupal stage. This accumulation could, in turn, lead to increased Gpdh1 gene expression in diapause eggs laid by these mothers, thereby contributing to the initiation of diapause.