Nonessential Sequences, Genes, and the Polytene Chromosome Bands of DROSOPHILA MELANOGASTER

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From earlier work, there appears to be an underlying one-to-one correspondence of polytene chromosome bands and complementation groups within a sizeable, continuous X-chromosome segment, 3A1–3C7 ( Judd, Shen and Kaufman 1972; Lefevre and Green 1972). However, most of the data supporting this one-to-one relation of bands and genes were gathered from mutants that upset vital functional units, thus leading to lethality. Among this series of mutants, only four loci, zeste, white, roughest and verticals, have no known lethal alleles. If phenotypic changes less drastic than lethality result from the loss of other chromosomal segments, they probably would not have been recognized in the earlier studies.—We report here some chromosomal sequences localized in 3A, 3B, and 3C whose loss effects no lethal change in the development of the animal. A portion of the 3A3–3A4 region can be disrupted in a nonlethal fashion, yet this sequence does not seem to be a part of either the zeste locus or l(1)zw1, which are known to be located in these bands. Two more complementation groups have been discovered that have no lethal alleles and map to 3B4–3B6; a third falls within 3B1–2. The loss of a sequence in 3C2–3 is tolerated without any genetically observable effect. Between 3C7 and the boundary of 3D there is at least one more sequence that behaves in this manner.—The discovery of these units, which are not allelic to any of the loci previously known, makes it clear that division 3B contains more genes (i.e., complementation groups) than polytene chromosome bands, while portions of 3A and 3C seem to have no functional significance. Accordingly many polytene chromosome bands may be composites of several complementing functional units. This investigation also indicates that there are chromosomal segments that are seemingly dispensible and thus function in a manner that is difficult or impossible to define with available methods.

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