A cause of variation in heavy metal tolerance: coevolution with metal-accumulating plants
Heavy metals are of great concern to public health and ecosystems because they are toxic to many plants and animals when concentrations exceed trace amounts. Environmental exposure and ingestion of heavy metals such as lead, copper, and zinc can result in chronic health effects such as carcinogenesis and impacts to neurological development. Presence of heavy metals can also have negative effects on the health of wild animals, including pollinators. Previous invertebrate studies have focused on established model systems and short-term exposure, but there are limited studies on a diversity of wild pollinators, such as butterflies, and on the long-term effects and evolution of heavy metal tolerance. In this research, we address this gap by focusing on long term heavy metal exposure and developing another pollinator group, butterflies, as an additional model system. We hypothesized that butterfly species differ in adaptations that allow them to tolerate high levels of heavy metals as a result of specialized larval feeding on plants that phytoextract high levels of metals from soils. We predicted that butterfly species whose caterpillars feed on plant families which accumulate heavy metals to high degrees will show stress tolerance to metals as adults. To explore this question, we used brain mass as a measure of performance as many heavy metals are neurotoxins. We focused on three species that vary in metal content of their larval host plants: Pieris rapae (cabbage whites) which feed on a high metal plant family (mustards), Colias eurytheme (orange sulphurs) which feed on a moderate metal family (legumes), and a third, more distantly related species, Danaus plexippus (monarchs), which feed on a low metal host (milkweeds). We found that heavy metal exposure did not impact brain mass in Pieris rapae and Colias eurytheme, and that exposure may correlate with reduced brain mass in Danaus plexippus with more statistical power from additional replicates. Future analyses will also control for variation in body size and sex.