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

Metals and metal isotopes incorporation in insect wings: Implications for geolocation and pollution exposure

Anthropogenic activities are exposing insects to elevated levels of toxic metals and are altering the bioavailability of essential metals. Metals and metal isotopes have also become promising tools for the geolocation of migratory insects. Understanding the pathways of metal incorporation in insect tissues is thus important for assessing the role of metals in insect physiology and ecology and for the development of metals and metal isotopes as geolocation tools. We conducted a diet-switching experiment on monarch butterflies [Danaus plexippus (L.)] with controlled larval and adult diets to evaluate the sources of 23 metals and metalloids, strontium isotopes, and lead isotopes to insect wing tissues over a period of 8 weeks. Concentrations of Ca, Co, Mo, and Sb differed between the sexes or with body mass. Ni and Zn bioaccumulated in the insect wing tissues over time, likely from the adult diet, while increases in Al, Cr, Cd, Cu, Fe, and Pb were, at least partially, from external sources (i.e., dust aerosols). Bioaccumulation of Pb in the monarch wings was confirmed by Pb isotopes to mainly be sourced from external anthropogenic sources, revealing the potential of Pb isotopes to become an indicator and tracer of metal pollution exposure along migratory paths. Concentrations of Ba, Cs, Mg, Na, Rb, Sr, Ti, Tl, and U appeared to be unaffected by intrinsic factors or additions of metals from adult dietary or external sources, and their potential for geolocation should be further explored. Strontium isotope ratios remained indicative of the larval diet, at least in males, supporting its potential as a geolocation tool. However, the difference in strontium isotope ratios between sexes, as well as the possibility of external contamination by wetting, requires further investigation. Our results demonstrate the complexity of metal incorporation processes in insects and the value of studying metals to develop new tools to quantify pollution exposure, metal toxicity, micronutrient uptake, and insect mobility.

Ecophysiology of nickel hyperaccumulating plants from South Africa - from ultramafic soil and mycorrhiza to plants and insects

An overview of 30 years of studies related to South African nickel hyperaccumulators is presented. Only five species have so far been identified as Ni hyperaccumulator plants among very rich and diversified South African flora. All of them occur on soils derived from ultramafic (serpentine) rocks and belong to the family Asteraceae: Berkheya coddii Roessler, Berkheya zeyheri subsp. rehmannii var. rogersiana, Berkheya nivea, Senecio coronatus, Senecio anomalochrous. Several techniques and methods were used to investigate ecophysiological aspects of the Ni hyperaccumulation phenomenon, from basic field and laboratory studies, to advanced instrumental methods. Analysis of elemental distribution in plant parts showed that in most cases the hyperaccumulated metal was stored in physiologically inactive tissues such as the foliar epidermis. However, an exception is Berkheya coddii, which has a distinctly different pattern of Ni distribution in leaves, with the highest concentration in the mesophyll. Such a distribution suggests that different physiological mechanisms are involved in the Ni transport, storage location and detoxification, compared to other hyperaccumulator species. Berkheya coddii is a plant with high potential for phytoremediation and phytomining due to its large biomass and potentially high Ni yield, that can reach 7.6% of Ni in dry mass of leaves. Senecio coronatus is the only known hyperaccumulator with two genotypes, hyperaccumulating and non-hyperaccumulating, growing on Ni-enriched/metalliferous soil. Detailed ultrastructural studies were undertaken to characterize specialized groups of cells in the root cortex of Ni-hyperaccumulating genotype, that are not known from any other hyperaccumulator. The occurrence of arbuscular mycorrhiza (AM) in Ni-hyperaccumulating plants was found for the first time in South African hyperaccumulator plants, and this type of symbiosis has been proved obligatory in all of them. There is a significant influence of mycorrhiza on the concentration and distribution of several elements. Three highly specialized herbivore insects feeding only on Ni hyperaccumulator plants were identified: Chrysolina clathrata (formerly Chrysolina pardalina), Epilachna nylanderi and Stenoscepa sp. The Ni-elimination strategies of these specialised insects have been established. Microbiological studies have revealed several genera of fungi and bacteria isolated from B. coddii leaves as well as presence of specialised, Ni-resistant yeasts in the C. clathrata gut. Understanding ecophysiological response to harsh environment broadens our knowledge and can have practical applications in cleaning polluted environments through phytomining/agromining. Finally, conservation aspects are also discussed and lines for future research are proposed.

Divergent biology of facultative heavy metal plants

Among heavy metal plants (the metallophytes), facultative species can live both in soils contaminated by an excess of heavy metals and in non-affected sites. In contrast, obligate metallophytes are restricted to polluted areas. Metallophytes offer a fascinating biology, due to the fact that species have developed different strategies to cope with the adverse conditions of heavy metal soils. The literature distinguishes between hyperaccumulating, accumulating, tolerant and excluding metallophytes, but the borderline between these categories is blurred. Due to the fact that heavy metal soils are dry, nutrient limited and are not uniform but have a patchy distribution in many instances, drought-tolerant or low nutrient demanding species are often regarded as metallophytes in the literature. In only a few cases, the concentrations of heavy metals in soils are so toxic that only a few specifically adapted plants, the genuine metallophytes, can cope with these adverse soil conditions. Current molecular biological studies focus on the genetically amenable and hyperaccumulating Arabidopsis halleri and Noccaea (Thlaspi) caerulescens of the Brassicaceae. Armeria maritima ssp. halleri utilizes glands for the excretion of heavy metals and is, therefore, a heavy metal excluder. The two endemic zinc violets of Western Europe, Viola lutea ssp. calaminaria of the Aachen-Liège area and Viola lutea ssp. westfalica of the Pb-Cu-ditch of Blankenrode, Eastern Westphalia, as well as Viola tricolor ecotypes of Eastern Europe, keep their cells free of excess heavy metals by arbuscular mycorrhizal fungi which bind heavy metals. The Caryophyllaceae, Silene vulgaris f. humilis and Minuartia verna, apparently discard leaves when overloaded with heavy metals. All Central European metallophytes have close relatives that grow in areas outside of heavy metal soils, mainly in the Alps, and have, therefore, been considered as relicts of the glacial epoch in the past. However, the current literature favours the idea that hyperaccumulation of heavy metals serves plants as deterrent against attack by feeding animals (termed elemental defense hypothesis). The capability to hyperaccumulate heavy metals in A. halleri and N. caerulescens is achieved by duplications and alterations of the cis-regulatory properties of genes coding for heavy metal transporting/excreting proteins. Several metallophytes have developed ecotypes with a varying content of such heavy metal transporters as an adaption to the specific toxicity of a heavy metal site.