Developmental Effects of X-Ray Induced Euploid and near-Euploid Mutants in Heterozygous Condition in Drosophila Melanogaster. I. Delay in Egg Hatching and Larval Delay and Death Prior to Pupation.

YPICALLY, mutant genes in homozygous condition are deleterious. The usual effect of mutant genes in heterozygous condition, however, is still a matter of controversy. The most convincing evidence for the position which holds that heterozygosity is usually harmful, is derived from work on lethals in heterozygous condition in Drosophila (STERN, CARSON, KINST, NOVITSKI and UPHOFF 1952; MULLER and CAMPBELL 1950 unpublished communication; others, and more recently, HIRAIZUMI and CROW 1960). These studies were concerned with lethality before eclosion, longevity and fertility. Further evidence for this view is also available from studies with yeast (JAMES 1960). To date, the largest detrimental effect of heterozygous mutants of any kind, excluding egg lethals, was reported in studies on length of larval period (HERSKOWITZ and BAUMILLER 1959). Other investigations have shown a detrimental effect of such mutants during the egg stage (for preliminary results see BAUMILLER and HERSKOWITZ 1959, 1960). The present paper reports extensive experiments undertaken to determine the effect on time of egg hatching of individuals carrying X-ray induced euploid and near-euploid mutants in heterozygous condition. In addition, those individuals hatching from the egg at the most normal time in both the treated and control groups were compared with regard to their time of pupation and ability to pupate. Late hatching individuals were also studied on the same basis.