Genetics and resource availability shape divergence in life history and behavior between locally-adapted populations of Atlantic mollies (Poecilia mexicana, Poeciliidae)

Phenotypic variation is common along environmental gradients, but it is often unknown to what extent it results from genetic differentiation between populations or phenotypic plasticity. We studied populations of a livebearing fish that have colonized streams rich in toxic hydrogen sulfide (H 2 S). In nature, there is strong phenotypic differentiation between adjacent sulfidic and nonsulfidic populations. In this study, we varied food availability to pregnant mothers from different populations to induce maternal effects, a form of plasticity, and repeatedly measured life-history and behavioral traits throughout the offspring’s ontogeny. Genetic differentiation affected most of the traits we measured, as sulfidic offspring tended to be born larger, mature later, have lower burst swimming performance, be more exploratory, and feed less accurately. In contrast, maternal effects impacted few traits and at a smaller magnitude, even though offspring from poorly provisioned mothers tended to be born larger and be more exploratory. Population differences and maternal effects (when both were present) acted synergistically, and there was no evidence for population differences in plasticity. Overall, our study suggests that phenotypic divergence between these populations in nature is primarily caused by genetic differentiation, and that plasticity mediated by maternal effects accentuates—but does not cause—differences between populations.