Life history trait correlations within and among genotypes as a function of food quality

Background/Question/Methods

Faced with finite amounts of food, organisms with indeterminate growth must make fitness choices regarding how best to allocate acquired energy to various life functions. These choices can also be influenced by the quality of the available food. The combined effects of food quantity and quality can be observed in the life history traits of the organism. These traits are usually assumed to be negatively correlated, indicating a trade-off. Positive correlations among life history traits have, however, been demonstrated among individuals of the cladoceran zooplankton Daphnia.  These correlations have been found both among individuals within a genotype and among genotypes, but no single study has looked at both simultaneously. Here we study the life history traits of Daphnia genotypes that coexist in situ across a finely resolved range of food quality in order to determine how individuals and genotypes differ in life history covariation and to use these to understand and predict how evolution may play out in different environments. We raised nine genotypes of Daphnia pulicaria on Scenedesmus acutus across a range of carbon:phosphorus ratios (molar C:P of 200, 400, 600, 800, 1000).  The experiments controlled for differential ingestion and genetic variation.

Results/Conclusions

Among individuals within a genotype, growth, reproduction, and longevity were typically positively correlated, suggesting that the standard assumptions of life history trade-offs are not met. Among genotypes, growth and reproduction were positively correlated at high food quality though the relationship became more negative with decreasing food quality. The among-genotype correlations between longevity and both reproduction and growth matched their respective among-individual trends, which may indicate that longevity, which itself is related to energetic allocation to maintenance, is relatively less sensitive to decreases in food quality. The relative order of correlation strength among genotypes for growth against reproduction does not, however, remain constant. This may indicate that genotypes have different sensitivities to decreasing food quality, which may represent different allocation decisions regarding growth and reproduction. Those genotypes that are more sensitive to variable food quality may be less competitive in poor food environments.