Genetic adaptation of earthworms to copper pollution: is adaptation associated with fitness costs in Dendrobaena octaedra?

Abstract

Exposure to copper pollution affects reproduction, growth and survival of earthworms. It is known that earthworms can cope with high copper burdens, but the distinction between physiological acclimation and evolutionary heritable changes and associated fitness consequences of the adaption to long-term copper exposure has rarely been studied. To investigate adaptation of earthworm populations of Dendrobaena octaedra to copper contamination, three populations from polluted soil were studied and compared to three unpolluted reference sites. Adult worms were collected at all six sites and cultured in uncontaminated control soil in the laboratory, where life-history traits were studied and F1-generations were produced. The newly hatched F1-generation worms were placed in uncontaminated control or copper-spiked soil to study if the adaptation was due to acclimation or genetic inheritance. This experiment showed that populations from polluted areas generally had a higher individual growth rate, reduced time to maturity, increased reproduction, and also increased mortality compared to the reference populations in both control and copper-spiked soil. The differences in life-history traits indicate that natural selection has resulted in genetic adaptation to copper pollution in the exposed populations. The population growth rates suggest a weak detrimental effect on population growth rate of being exposed to copper for both type of populations, but no sign of cost. On the contrary, estimates of population growth rates integrating all life-history traits showed that copper adapted populations perform on average relatively better than reference populations in both uncontaminated and copper-spiked soil.