A density threshold for the expression of longevity in Drosophila melanogaster.

Long- and short-lived strains, selected for longevity for up to 27 generations, were raised under two treatments of developmental density. Selected stocks, controls and F1 crosses between them are respectively long-lived, short-lived and intermediate, when raised at a high and uncontrolled developmental density. But when development takes place at a low density, longevity is sharply reduced, showing the existence of a developmental-density threshold for the expression of long life in selected strains and crosses. Selection for longevity is shown to have had effects in males comparable with those in females in the long-lived stock and in reciprocal F1 crosses with the short-lived control. No age-specific effects on longevity were found in progeny from young, middle-aged and old adults.