X-ray absorption spectroscopy imaging can be used to spatially resolve and quantitate chemical forms of elements with absorption edge features that differ in energy by a few electron volts (Pickering et al, 2000). We used this new technique to examine the nutrition, ecology and biochemistry of metals and metalloids in insects. At SSRL, live insects immobilized by nitrogen gas were rastered over a highly collimated 50 micron X-ray beam tuned to the K-edge absorption energy of the element of interest. Elemental mapping of other elements with absorption energies below that of the element of interest were collected simultaneously. Organic selenium and selenate were spatially resolved in Spodoptera exigua (Lepidoptera: Noctuidae) 3rd instar larvae raised on a selenate-enriched diet. Some larvae were parasitized by Cotesia marginiventris (Hymenoptera: Braconidae). Selenium levels in a juvenile parasitoid, imaged within the hemocoel of its host are similar to levels in the host and we present the first in situ elemental mapping of iron, zinc and manganese in these species. Images were also collected of copper (I) and (II) species in Drosophila melanogaster 3rd instar larvae (Diptera: Drosophilidae) raised on a copper-enriched diet. Cuprophilic cells were identified in the X-ray images by their high copper content and proximity to the midgut iron region. The cuprophilic cells were moved into the 50 micron beam and copper K-edge spectra were collected. These resembled the copper K-edges of cuprous thiolates but X-ray absorption fine structure (EXAFS) of isolated gut tissue did not show Cu-Cu bonds characteristic of metallothionein.
Species 1: Lepidoptera Noctuidae Spodoptera exigua (beet armyworm)
Species 2: Hymenoptera Braconidae Cotesia marginiventris
Species 3: Diptera Drosophilidae Drosophila melanogaster (fruit fly)
Keywords: copper, selenium
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