The Expression of Alleles at the Cubitus Interruptus Locus in Hybrids between Drosophila Melanogaster and Drosophila Simulans.

H E problem of position effect has been studied with the purpose of T elaborating upon the larger problem of gene action. DUBININ and SIDOROPP (1934) first showed the position effect of the fourth chromosome locus, cubitus interruptus, ( c i ) , in Drosophila melanogaster. They found that in translocations involving the fourth chromosome, the normal allele both in hemior homozygous constitution produced a normal phenotype, but when in heterozygous constitution with the recessive mutant gene ci, the dominance of the normal allele was diminished and there was an expression of the otherwise recessive ci. Additional data, some supporting and others modifying the results of DUBININ and SIDOROFF, were furnished by STERN and his collaborators (1943-1946). These investigators used the ci gene because it lent itself favorably to studies of different doses of alleles as well as different positions. A hypothesis was developed according to which the different alleles possess different powers of combining with specific substrate and different efficiencies of converting it into a product which will lead toward a normal phenotype. I t was suggested that the position effect may be caused by the unchanged allele acting in a new region of the gene sequence, a region which may provide an amount of substrate different from that a t the normal locus. Following MULLER (1939, EPHRUSSI and SUTTON (1944) presented a “structural theory” of position effect in which they regard “position effect as a result of (disturbed) chromosome pairing that causes a modification of stress near the affected loci.” If this theory were correct then in complete absence of pairing or with complete normality of pairing, there should be no manifestation of the position effect. I n hybrids between melanogaster and the closely related species simulans, PATAU (1935), KERKIS (1936), and HORTON (1939) never found somatic synapsis in the salivary gland nuclei between the two fourth chromosomes. According t o HORTON, the fourth chromosomes of the two species are distinguished by a relatively long inversion, and a “terminal ring” in simulans which he considers to have no corresponding structure in melanogaster. Even in “partial hybrids,” when there is a single simulans fourth chromosome in a melanogaster background and where synapsis conceivably might occur more readily, SLIZYNSKI (1941) found only one case where there was synapsis of the inverted