Elemental Distribution in Reproductive and Neural Organs of the Epilachna nylanderi (Coleoptera: Coccinellidae), a Phytophage of Nickel Hyperaccumulator Berkheya coddii (Asterales: Asteraceae) by micro-PIXE

Abstract

The phenomenon of metal hyperaccumulation by plants is often explained by a pathogen or herbivore defense hypothesis. However, some insects feeding on metal hyperaccumulating plants are adapted to the high level of metals in plant tissues. Former studies on species that feed on the leaves of Berkheya coddii Roessler 1958 (Asteraceae), a nickel-hyperaccumulating plant, demonstrated several protective mechanisms involved in internal distribution, immobilization, and elimination of Ni from the midgut and Malpighian tubules. These species are mainly coleopterans, including the lady beetle, Epilachna nylanderi (Mulsant 1850) (Coleoptera: Coccinellidae), collected from the ultramafic ecosystem near Barberton in South Africa. By performing particle-induced X-ray emission microanalysis elemental microanalysis (PIXE), this study examined whether Ni may be harmful to internal body systems that decide on insect reactivity (central nervous system [CNS]), their reproduction, and the relationships between Ni and other micronutrients. Data on elemental distribution of nine selected elements in target organs of E. nylanderi were compared with the existing data for other insect species adapted to the excess of metals. Micro-PIXE maps of seven regions of the CNS showed Ni mainly in the neural connectives, while cerebral ganglia were better protected. Concentrations of other bivalent metals were lower than those of Ni. Testis, compared with other reproductive organs, showed low amounts of Ni. Zn was effectively regulated at physiological dietary levels. In insects exposed to excess dietary Zn, it was also accumulated in the reproductive organs. Comparison of E. nylanderii with other insects that ingest hyperaccumulating plants, especially chrysomelid Chrysolina clathrata (Clark) (Coleoptera: Chrysomelidae), showed lower protection of the CNS and reproductive organs.