Optimization of dietary restriction protocols in Drosophila

Abstract

Dietary restriction (DR) extends life span in many organisms, through unknown mechanisms that may or may not be evolutionarily conserved. Because different laboratories use different diets and techniques for implementing DR, the outcomes may not be strictly comparable. This complicates intra- and interspecific comparisons of the mechanisms of DR and is therefore central to the use of model organisms to research this topic. Drosophila melanogaster is an important model for the study of DR, but the nutritional content of its diet is typically poorly defined. We have compared fly diets composed of different yeasts for their effect on life span and fecundity. We found that only one diet was appropriate for DR experiments, indicating that much of the published work on fly "DR" may have included adverse effects of food composition. We propose procedures to ensure that diets are suitable for the study of DR in Drosophila.

Figure 1

Effect of dietary sucrose concentration on life span and fecundity of mated Drosophila females. Increasing concentrations of sucrose were added to a standard food background of 1.5 SYBaker’s (Table 1). Over the range of sucrose tested, very little change in life span was observed, whereas a significant decrease in fecundity was observed between 50 g/L and 100 g/L sucrose. Gray bars: index of lifetime fecundity (sum of the eggs laid by an average female on the days counted) ± standard error of the mean; connected black points: median life span. Representative data from one of two experiments are shown.

Figure 2

Effect of a range of concentrations of different commercially available yeasts on life span and fecundity. Five different yeast concentrations were prepared for each of five different sugar/yeast (SY) recipes. SYBaker’s, SYBrewer’s, and SYTorula each refer to food made with different, inactivated whole-yeast preparations, whereas SYExtract and CSYExtract refer to diets based on a water-soluble yeast extract. The nutritional components in each food type were sucrose and yeast or yeast extract and cornmeal (for CSYExtract only). Bars: index of lifetime fecundity ± standard error of the mean; connected black points: median life-span values. We specified a linear model to describe fecundity (Materials and Methods), which found all factors and interactions to be significant. The predicted values are plotted against observed values in Supplementary Figure 2. Each food concentration range was performed once, except for SYBaker’s and SYBrewer’s, which were performed twice.

Figure 3

Effect of varying agar concentration on life span and fecundity of females on SYBrewer’s medium. The effect of food hardness on life span and fecundity was tested by altering the agar concentration while all other ingredients were held at fixed concentrations (Table 1). This medium contained Brewer’s yeast at 200 g/L (2.0 level) (agar concentration ranges were also tested at two other SYBrewer’s concentrations; data not shown). Bars: index of lifetime fecundity ± standard error of the mean; connected black points: median life span; connected gray points: maximum life span (median of the last 10% survivorship).

Figure 4

Effect of water addition on the dietary restriction (DR) response of flies on SYBrewer’s medium. Free access to water was provided in the form of 1% agar in a pipette tip inserted into the food. Bars: index of lifetime fecundity ± standard error of the mean; connected points: median life span. Experiment was performed twice.