Sterols are essential components of cellular membranes and are required precursors for important hormones regulating growth and development. Unlike most animals, insects lack the ability to synthesize sterols de novo and they must acquire sterols from their food. Cholesterol is the typical sterol recovered from animals, including most insects. Plant and fungal sterols differ structurally from cholesterol, mostly in side chain configuration and the number and position of double bonds. In the lab, Drosophila are reared on diets that contain 4 different sterols - cholesterol (animal sterol), sitosterol and stigmasterol (plant sterols), plus ergosterol (fungal sterol); ergosterol comprises nearly 75% of the dietary sterol content. Like vertebrates, Drosophila requires cholesterol for membrane structure and hormone production. However, their inability to synthesize sterols de novo makes them a model organism to study sterol use and metabolism. Two experiments were performed. First, using a recently developed holidic diet, larvae were individually reared (from hatch) on each of the 4 different sterols in standard Drosophila diet, each at a range of different concentrations. When individual sterols are incorporated at different concentrations into a holidic diet, performance and overall survival are significantly affected. Individuals reared on cholesterol only diets exhibited significantly faster developmental times to pupation and also to eclosion from pupation; additionally, overall survival to pupation and eclosion was significantly increased compared to sitosterol, stigmasterol, and ergosterol. In the absence of sterols, individuals arrested development. As an individual sterol ergosterol minimally supported adult survival at low concentrations and failed to support survival at high concentrations. Next, the extent to which sparing occurs in Drosophila melanogaster was examined using different cholesterol and ergosterol ratios in the diet. When ergosterol was supplemented with cholesterol in different ratios, survival was dramatically improved and in some instances exceeded that of only cholesterol. Survival to pupation was significantly reduced as the ratio of cholesterol increased. Collectively the results show that slight variations in sterol structure have pronounced effects on Drosophila growth and development, and that a small amount of dietary cholesterol, likely for metabolic purposes, is required.
