Glycerol Production and Diapause Are Most Likely Not Linked in Manduca sexta

Several studies have demonstrated that diapausing insects upregulate glycerol production in response to cold temperatures. However, relatively few studies have investigated whether diapause can independently trigger increased glycerol production. In other words, little research has been dedicated to analyzing whether diapause and glycerol upregulation are linked. If a linkage between these two elements in insects exists, then photoinduced diapausing insects should upregulate glycerol production, even without exposure to cold temperatures. The current study examines whether this linkage exists in the tobacco hornworm pupae (Manduca sexta). Tobacco hornworms will enter a pupal diapause if presented with consistently short photoperiods, most notably 12 hours of daylight followed 12 hours of darkness. Regarding temperature, photoinduction of diapause in the tobacco hornworm is optimal at 26 degrees celsius. Using a spectrophotometric assay, glycerol levels in hemolymph were assessed in non-diapausing pupae shortly after pupation and in diapausing pupae after 16 days of pupation. Interestingly, non-diapausing pupae contained significantly higher glycerol levels in hemolymph compared to diapausing pupae. Furthermore, non-diapausing pupae produced approximately .099 M glycerol and diapausing pupae produced .085 M glycerol. This study provides evidence that glycerol upregulation and diapause are not linked in Manduca sexta, but are rather two separate events with two distinct causes. Nevertheless, the results are valuable since non-diapause specimens produced more glycerol than diapausing specimens, and moderately high concentrations of glycerol were found in both diapausing pupae and non-diapausing pupae.