The tetrasomic for chromosome 4 in Drosophila melanogaster.

DROSOPHILA with one more or less 4th chromosome than the normal pair were first described by BRIDGES ( 1921a,b). Haplo-4 flies with only one 4th chromosome were discovered because of their obvious complex of abnormal characters. Most conspicuously they are Minute; that is they have short slender bristles. Triplo-4 flies do not differ greatly from normal flies in phenotype, but according to BRIDGES they do have smaller, smoother eyes, narrower and more pointed wings, darker body color and less pattern on the thorax than wild-type flies. Triplo-4 flies of both sexes are vigorous and fertile. One half of the gametes of triplo-4 flies are expected to carry two 4th chromosomes. A zygote resulting from the union of an egg nucleus possessing two 4th chromosomes with a sperm also carrying two 4th chromosomes will have four 4th chromosomes (tetra-4). BRIDGES (In MORGAN, BRIDGES and STURTEVANT 1925) interpreted genetic data from parents each of which were triplo-4 to indicate lethality of the tetra-4. LI (1927) made egg counts of triplo-4 females mated to triplo-4 males. Thirty to 40 percent more progeny from this cross died in the egg and larval stages than from crosses in which one or both parents were diplo-4. He concluded that all tetra-4 Drosophila die before reaching adulthood. He believed that most of them do not survive the embryonic stages and the remainder die during larval life. The resolution afforded by LI’S or BRIDGES’ experiments does not exclude infrequent survival into the adult stage. BRIDGES (1935) reported that he found one female pupa with four 4th chromosomes in oogonial cells. This indicated that tetra-4 Drosophila survive at least on occasion into the pupal stage. SCHULTZ (1935) states that he may have seen a tetra4 adult with four doses of the 4th chromosomes mutant, shaven. He described the fly as having extra bristles, but offered no genetic or cytological evidence that it was indeed a tetra-4. The difficulty in proving by genetic methods that flies are tetra-4 arises from the lack of suitable genetic markers on chromosome 4. To circumvent this dificulty, translocations of the 4th chromosome with other chromosomes can be used. The markers on the other chromosome parts of the translocations serve to indicate the presence of ~e 4th chromdsomes. This method has been used in the experiments reported here to demonstrate that flies carrying the genetic material of four 4th chromosomes survive, even though infrequently, into the adult stage.