Causes and consequences of variation in energy storage in Drosophila melanogaster

Variation in energy storage pools has a proximate cause variation in rates of influx and efflux from the pools, and it may, as an ultimate consequence, result in variation in fitness. The possibility of this chain of causal links motivates attempts to quantify the genetic correlations among pool sizes, enzyme activities and fitness components. In this report, homogenates from 83 second chromosome replacement lines of Drosophila melanogaster were analyzed to determine the amounts of stored triacylglycerols and glycogen as well as the activities of 11 enzymes in relevant metabolic pathways. The viabilities and fecundities of these same lines were determined by a segregation test using the SM5 balancer chromosome. Analysis of covariance revealed significant differences among lines in quantities of stored triacylglycerols and glycogen, as well as in activities of the assayed enzymes. Significant broad-sense genetic correlations were detected for a number of enzyme pairs. Some of the traits showed a significant correlation with viability and fecundity, including lipid and glycogen storage. Multiple regression models that fitted fitness components to linear and quadratic functions of the biochemical traits yielded highly significant fits. The partial regression coefficients indicate the shape of the selection gradient, and instances of significant directional and stabilizing selection were detected.