MEV-ION MICROPROBE ANALYSES OF WHOLE DROSOPHILA SUGGEST THAT ZINC ANDCOPPER ACCUMULATION IS REGULATED STORAGE NOT DEPOSIT EXCRETION

We examined Drosophila spp, using a penetrative iron microprobe techni que that allows us to quantify element contents in whole organs and or ganisms. Comparatively non-penetrative techniques, such as electron mi croscopy, could not have been used to make many of these measurements because material is lost during sectioning. We found that zinc was acc umulated predominantly within a single organ: in the main segments of both the anterior and posterior Malpighian tubules, In contrast to zin c, iron and copper were more generally distributed throughout the body , Zinc concentrations as high as 2.8 % of dry mass were measured in ce ll-sized volumes of the Malpighian tubules. The large quantities of zi nc (approximately 2x10(-8) g in 8-day-old male adults) were sequestere d 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 c ontaining several hundred parts per million dry mass of these metals. Our results are inconsistent with the detoxification hypothesis that p redicts that a large proportion of the heavy metals passing through th e gut are absorbed and stored permanently. We found for both zinc and copper that the quantity in the abdomen was not proportional to the co ncentration of these metals in the consumed food but was, instead, rel atively invariant, For these reasons, we suggest that regulated biolog ical availability, not detoxification, may be the primary benefit of z inc and copper storage.