Density-dependent natural selection in Drosophila: correlations between feeding rate, development time and viability

We have previously hypothesized that density-dependent natural selection is responsible for a genetic polymorphism in crowded cultures of Drosophila. This genetic polymorphism entails two alternative phenotypes for dealing wi th crowded Drosophila larval cultures. The first phenotype is associated wi th rapid development, fast larval feeding rates but reduced absolute viabil ity, especially in the presence of nitrogenous wastes like ammonia. The sec ond phenotype has associated with it the opposite set of traits, slow devel opment, slow feeding rates and higher viability. We suggested that these tr aits are associated due to genetic correlations and that an important selec tive agent in crowded larval cultures was high levels of ammonia. To test t his hypothesis we have examined viability and larval feeding rates in popul ations kept at low larval densities but selected directly for (i) rapid egg -to-adult development, (ii) tolerance of ammonia in the larval environment and (iii) tolerance of urea in the larval environment. Consistent with our hypothesis we found that (i) larvae selected for rapid development exhibite d increased feeding rates, and decreased viability in food laced with ammon ia or urea relative to controls, and (ii) larvae selected to tolerate eithe r ammonia or urea in their larval environment show reduced feeding rates bu t elevated survival in toxin-laced food relative to controls. It would appe ar that development time and larval feeding rate are important characters f or larvae adapting to crowded cultures. The correlated fitness effects of t hese characters provide important insights into the nature of density-depen dent natural selection.