Selection for rate of larval development usingDrosophila melanogastercultured axenically on deficient diets

Abstract

1. The problem of improving rate of larval development ofDrosophilaby selecting for this ‘character’ on deficient diets is examined by culturing larvae axenically on low-casein and low-pyridoxine media. Under these conditions it is possible to develop strains which grow faster than the parent population.2. Selection for fast- and slow-growing larvae on a low-pyridoxine diet proceeds with a realized heritability of about 20%, but progress ceases after eight to nine generations. The selected larvae show no alteration of pyridoxine requirements up to the tenth generation, but the lines develop at different rates under optimal conditions. This difference is exaggerated when the diets are low in pyridoxine. By the fourteenth generation, requirements of the two lines for optimal growth have become distinct, the fast line requiring less pyridoxine than the control. Casein requirements show about the same optimum for the two lines but this is lower than that of the control, foundation population.3. Selection for fast- and slow-development lines on a low-casein diet continues to be effective throughout the fifteen generations of the test. The realized heritability in this case is about 10%. The optimal requirements of the two lines are the same, and there is little difference in their development rates when reared on this optimal diet. The response is found only under sub-optimal conditions, both of deficiency and of excess casein. Pyridoxine requirements do not seem to be altered in the two lines.4. Crosses between the selected lines show that each genotype has its own optimal environment, as judged by pyridoxine and casein requirements. Crosses among the lines after fourteen generations show that all the hybrids are superior to the mid-parent and three grew faster than the better parent. Other environments would have given different results.