AUTOSOMAL NONDISJUNCTION IN MALES OF DROSOPHILA MELANOGASTERl,'

B ECAUSE triploid females form large numbers of chromosomally unbalanced eggs it is possible to recover complementary types of unbalanced sperm. This technique revealed the occurrence of autosomal nondisjunction in males (STERN 1934; PONTECORVO 1940) and was also used by MULLER and PONTECORVO ( 1940) and PONTECORVO (1943) to recover sperm where radiation induced chromosome breaks caused one or more paternal chromosomes to be lost in the embryo. The same technique was employed for the present study of autosomal nondisjunction in males. There were two reasons for undertaking such a study, the first of which was to discover how males differed from females with respect to interchromosomal effects of inversions on segregation. Heterozygous inversions in females are known to have interchromosomal effects on crossing over and segregation, and COOPER, ZIMMERING and KRIVSHENKO ( 1955) have suggested that the interchromosomal effect of inversions on the frequency of X chromosome nondisjunction is caused by non-homologous pairing between the sex chromosomes and autosomes. If similar interchromosomal effects on segregation occurred in males, where crossing over is absent, the influence of inversions on segregation could be studied apart from their influence on crossing over. It was also hoped that a simultaneous study of sex chromosomal nondisjunction and autosomal nondisjunction might serve to test for the occurrence of non-homologous pairing. The second reason for studying autosomal nondisjunction in males was to reconsider STERN'S results. Over half the exceptions recovered by STERN required for their explanation the fertilization of a nondisjunctional egg (nullo I1 or nullo 111) from a triploid female, by a nondisjunctional sperm (diplo I1 or diplo 111) from a diploid male, and the number of exceptions recovered indicated that autosomal nondisjunction was fairly frequent. In order to understand the type of interchromosomal effects on segregation that might be expected in males, it will be necessary to review briefly the present knowledge of such effects in females. In females with structurally heterozygous X chromosomes the frequency of primary nondisjunction is increased when a