MeV-ion microprobe analyses of whole Drosophila suggest that zinc and copper accumulation is regulated storage not deposit excretion

We examined Drosophila spp. using a penetrative ion microprobe technique that allows us to quantify element contents in whole organs and organisms. Comparatively non-penetrative techniques, such as electron microscopy, could not have been used to make many of these measurements because material is lost during sectioning. We found that zinc was accumulated predominantly within a single organ: in the main segments of both the anterior and posterior Malpighian tubules. In contrast to zinc, iron and copper were more generally distributed throughout the body. Zinc concentrations as high as 2.8 % of dry mass were measured in cell-sized volumes of the Malpighian tubules. The large quantities of zinc (approximately 2x10(-8) g in 8-day-old male adults) were sequestered by an unidentified mechanism. We found less than 1 % of the estimated amount of consumed zinc and copper in the abdomen of flies fed food containing several hundred parts per million dry mass of these metals. Our results are inconsistent with the detoxification hypothesis that predicts that a large proportion of the heavy metals passing through the gut are absorbed and stored permanently. We found for both zinc and copper that the quantity in the abdomen was not proportional to the concentration of these metals in the consumed food but was, instead, relatively invariant. For these reasons, we suggest that regulated biological availability, not detoxification, may be the primary benefit of zinc and copper storage.