Fitness costs constrain the evolution of resistance to environmental stress in populations. We earlier reported on a rapid response to laboratory selection for cadmium resistance in the least killifish (Heterandria formosa). By the sixth generation, the three selection populations were threefold more resistant to cadmium than the control populations. Here, we report the fitness costs and trade-offs associated with this evolution of resistance. In the F3 and F4 generations, the selection populations produced smaller-sized offspring than the control populations. A comprehensive life-history traits study in the F7 generation showed that the selection populations had, on average, an 18% decrease in fecundity. The selection populations also had a smaller brood size, longer time to first reproduction, and shorter female life expectancy than the control populations. Our results strongly suggest that fitness costs and trade-offs were associated with the evolution of resistance to cadmium in the least killifish. The fitness costs and trade-offs may result from maintenance of the underlying resistance mechanisms, leading to changes in resource allocation in the cadmium-adapted fish.
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