Seasonal Dynamics in the Chemistry and Structure of the Fat Bodies of Bumblebee Queens

Abstract

Insects' fat bodies are responsible for nutrient storage and for a significant part of intermediary metabolism. Thus, it can be expected that the structure and content of the fat body will adaptively change, if an insect is going through different life stages. Bumblebee queens belong to such insects as they dramatically change their physiology several times over their lives in relation to their solitary overwintering, independent colony foundation stage, and during the colony life-cycle ending in the senescent stage. Here, we report on changes in the ultrastructure and lipid composition of the peripheral fat body of Bombus terrestris queens in relation to seasonal changes in the queens' activity. Six life stages are defined and evaluated in particular: pharate, callow, before and after hibernation, egg-laying, and senescence. Transmission electron microscopy revealed that the fat body contained two main cell types-adipocytes and oenocytes. Only adipocytes reveal important changes related to the life phase, and mostly the ration between inclusion and cytoplasm volume varies among particular stages. Both electron microscopy and chemical analyses of lipids highlighted seasonal variability in the quantity of the stored lipids, which peaked prior to hibernation. Triacylglycerols appeared to be the main energy source during hibernation, while the amount of glycogen before and after hibernation remained unchanged. In addition, we observed that the representation of some fatty acids within the triacylglycerols change during the queen's life. Last but not least, we show that fat body cell membranes do not undergo substantial changes concerning phospholipid composition in relation to overwintering. This finding supports the hypothesis that the cold-adaptation strategy of bumblebee queens is more likely to be based on polyol accumulation than on the restructuring of lipid membranes.

Fig 1. Ultrastructure of adipocytes in B. terrestris queens: pharate (A), callow (B), before hibernation (C), after hibernation (D), egg-laying (E), and senescent (F).

Scale bars represent 10 μm. Note the flat and highly condensed nucleus prior to hibernation and the initial phase of nucleus elimination in senescent queen. Abbreviations: cv, central vacuole; g, glycogen; ld, lipid droplet; m, mitochondria; n, nucleus; pg, protein granule; rer, rough endoplasmic reticulum.

Fig 2. Comparison of cell volume and the proportions of glycogen, lipid droplets, and proteins at each life stage from transmission electron micrographs.

Fig 3. Trends of changes in adipocyte size, total lipid weight, and triacylglycerol (TG) weight in queens at different life stages (mean values, cf. Table 1 and S1 Table).

Adipocyte volume in μm³; lipid weight in mg; TG weight in mg.

Fig 4

(A) Trends in proportions of the most abundant triacylglycerols (TG). Upper diagram shows the accumulation of particular TG in life stages involved in overwintering. Lower diagram shows a significant reduction in particular TG at the indicated life stages. (B) Trends in proportions of saturated and monounsaturated fatty acids (FA) obtained by recalculation of HPLC/MS-APCI data. Diagrams are focused on the most abundant FA, and some trends connected with overwintering are also observed. Generally, accumulation of monounsaturated FA and a decreased level of saturated FA in after- and before-hibernation life stages are obvious. Relative percentages were calculated from integrated peak areas of particular chromatographic peaks.