Newly Discovered Reproductive Phenotypes of a Marine Copepod Reveal the Costs and Advantages of Resistance to a Toxic Dinoflagellate

We document for the first time toxin-resistant reproductive phenotypes of copepods and we describe a novel procedure to identify these phenotypes. Individual copepods of the species Acartia hudsonica were raised on two diets: a standard nontoxic diet and a diet containing the toxic dinoflagellate Alexandrium fundyense, both offered at nonlimiting concentrations. Resistant individuals were defined as those that survived on the toxic diet. We examined several life-history characters including survivorship, age at metamorphosis, age at maturity, fecundity, and fitness. During this study, we discovered five resistance-related reproductive phenotypes that appeared as discrete classes in a frequency distribution of fecundity. After grouping the data according to these phenotypes, we calculated the fitness of each phenotype on each diet. We also calculated the cost and advantage associated with resistance. On the standard diet, one phenotype had 46% lower fitness than the phenotype with the highest fitness, indicating that possessing resistance alleles can carry a substantial cost. A different phenotype showed maximum relative fitness on the toxic diet and reduced relative fitness on the standard diet. From these results, we argue that resistance is conferred by a simple genetic system showing heterozygote advantage and leading to a polymorphism for resistance. Such a polymorphism will prevent the fixation of resistance alleles in natural populations. It may also confound the interpretation of typical experiments that measure average population responses.