Lipid digestion in the tsetse fly, Glossina morsitans

Abstract Examination of faecal material showed that 33% of the lipids contained in a blood meal ingested by Glossina morsitans morsitans were excreted. Separation by TLC showed the lipid components of the faeces were mainly cholesterol (50%) and hydrocarbons (19%). Forty-eight hours after ingestion of a blood meal containing glycerol tri-[1- 14 C] oleate 50% of the radiolabel was not recoverable from the gut lumen of adult male or female G. m. morsitans but little radiolabel was detectable in the excreta. Changes in the radioactive content of various lipid fractions in the gut plus its contents, and in the gut wall alone were monitored for 96 h following ingestion of a labelled blood meal. A consistent feature was the very low levels of labelled free fatty acid detectable in the gut lumen. This suggested that extracellular digestion of triglyceride was followed by immediate uptake of labelled fatty acid and its conversion to the other components detected in the gut wall. Liberated diglycerides were probably taken up directly by the gut epithelium. The small amount of radioactivity detected in the haemolymph was attributed to diglyceride while that detected in the fat body was incorporated into triglyceride. Flies fed [U 14 C] palmitic acid in blood meals showed a similar conversion to phospholipids, diglycerides and triglycerides in the gut wall. Flies fed [4- 14 C] cholesteryl, [4- 14 C] cholesteryl oleate, or cholesteryl [1- 14 C] oleate in blood meals showed a signicant ability to absorb cholesterol through the gut wall and in the case of females, to pass it on to the larva in utero . Cholesteryl oleate was hydrolysed and the free fatty acids liberated from such hydrolysis were detectable in the same range of neutral lipids as those found following ingestion of free fatty acid. A high percentage of ingested cholesterol or cholesterol ester was excreted as cholesterol, very little being excreted in its esterified form. This provided strong evidence that the 19% lipid in excreta which co-chromatographs with cholesterol ester in the system used was hydrocarbon. Results are consistent with the view that the tsetse gut contains lipid-digesting enzymes. Numerous interconversions occurred in the gut epithelium and apart from cholesterol which passed unchanged into the haemolymph it is likely that the neutral lipids were exported from the gut epithelium mainly as diglycerides, although the role of free fatty acids in this respect cannot be disregarded. The relative importance of dietary lipids other than cholesterol, which is essential, has not been estimated previously in Glossina . However, the ingestion of between 150 and 400 μg lipid with each blood meal cannot be considered insignificant in an insect whose total store of lipids is only 3 mg in a male and 6 mg in a female.