GENETIC-VARIATION AND COVARIATION FOR CHARACTERISTICS ASSOCIATED WITHCADMIUM TOLERANCE IN NATURAL-POPULATIONS OF THE SPRINGTAIL ORCHESELLA-CINCTA (L)

Heavy metals can be strong and stable directional selective agents for metal-exposed populations. Genetic variation for the metal-tolerance characteristic ''cadmium excretion efficiency'' was studied in populat ions of the collembolan Orchesella cincta from a reference- and a meta l-contaminated forest soil. Previously it has been shown that ''excret ion efficiency'' influences tolerance through midgut-mediated immobili zation and excretion of toxic metal ions, and that an increased mean e xcretion efficiency is present in animals inhabiting metal-contaminate d litter. In the present research, offspring-parent regressions showed that additive genetic variation for cadmium excretion efficiency was present in the population from the reference site. The heritability es timate was 0.33. In the natural population exposed to heavy metals fro m an industrial source, additive genetic variation was not significant ly different from zero. Differences in the heritability between the re ference and the exposed population were not significant. Genetic varia tion for cadmium excretion efficiency allows for a response to selecti on in the reference population. Such a response has probably occurred in the metal-exposed population. Half-sib analysis with animals from t he reference population was used to estimate genetic variation and mat ernal effects for excretion efficiency, relative growth rate and molti ng frequency, and to determine genetic correlations between these char acteristics. Additive genetic variation was demonstrated for all three characteristics, heritability estimates were 0.48, 0.75 and 0.46, res pectively. Maternal effects were low for excretion efficiency and molt ing frequency, but may be present for relative growth rate. Phenotypic and genetic correlations among these characteristics were positive. T he environmental correlation between relative growth rate and molting frequency was positive, others were negative. Direct selection for any of the characteristics, or genetic correlations between tolerance cha racteristics and growth characteristics, or both may have caused the r esponses previously observed in field populations.