Genetic basis of a between-environment trade-off involving resistance to cadmium in Drosophila melanogaster

In a replicated, laboratory, natural selection experiment Drosophila melanogaster populations were maintained for 20 generations either on unpolluted medium or on polluted medium containing cadmium chloride at a concentration of 80 μg/ml. Lines maintained on polluted medium evolved resistance. In comparison with unpolluted lines, their juvenile survivorship increased from 35% to 46%, developmental period decreased from 13.7 days to 13.0 days, and fecundity increased from 3 to 29 eggs per two-day period. Emergence weights, however, did not change. By contrast the "environmental" effect of moving susceptible flies onto polluted medium was that after two generations survivorship fell 62%, developmental period increased 40%, and fecundity fell 97%. Emergence weights fell 31% in females and 28% in males. Resistant lines paid a fitness cost in unpolluted environments, with fecundity being reduced by 44% and emergence weights being reduced by 4% in females and 6% in males. Developmental period, however, was unaffected. Analyses of crosses and backcrosses between the lines suggested that the evolved cadmium resistance was due to a single sex-linked gene. Levels of dominance were calculated, and in each life-history character the resistant allele was found to be completely dominant. Because the life-history effects appear to be produced by a single gene, it is probable that they all depend on the same metabolic pathway. Metallothionein production is a likely candidate because this is known to be controlled by genes on the X-chromosome. The study adds to a small number of examples of single or closely linked genes with large antagonistic pleiotropic effects on life histories. The result here is a between-environment trade-off, allowing animals increased fitness in polluted environments, but only at the cost of reduced growth and reproduction in unpolluted environments.