First insight into the lipid uptake, storage and mobilization in arachnids: role of midgut diverticula and lipoproteins.

The importance of midgut diverticula (M-diverticula) and hemolymph lipoproteins in the lipid homeostasis of Polybetes phythagoricus was studied. Radioactivity distribution in tissues and hemolymph was analyzed either after feeding or injecting [1-(14)C]-palmitate. In both experiments, radioactivity was mostly taken up by M-diverticula that synthesized diacylglycerols, triacylglycerols and phospholipids in a ratio close to its lipid class composition. M-diverticula total lipids represent 8.08% (by wt), mostly triacylglycerols (74%) and phosphatidylcholine (13%). Major fatty acids were (in decreasing order of abundance) 18:1n-9, 18:2n-6, 16:0, 16:1n-7, 18:0, 18:3n-3. Spider hemocyanin-containing lipoprotein (VHDL) transported 83% of the circulating label at short incubation times. After 24h, VHDL and HDL-1 (comparable to insect lipophorin) were found to be involved in the lipid uptake and release from M-diverticula, HDL-2 playing a negligible role. Lipoprotein's labelled lipid changed with time, phospholipids becoming the main circulating lipid after 24h. These results indicate that arachnid M-diverticula play a central role in lipid synthesis, storage and movilization, analogous to insect fat body or crustacean midgut gland. The relative contribution of HDL-1 and VHDL to lipid dynamics indicated that, unlike insects, spider VHDL significantly contributes to the lipid exchange between M-diverticula and hemolymph.