The Chromosomes of Drosophila Pseudoobscura

H E linear arrangement of genes within chromosomes is constant T from generation to generation in each line of descent. The degree of this stability is comparable to that of the gene structure; genes change by mutation, chromosomes change by the occurrence of chromosomal aberrations. Strains and races of the same species, as well as distinct species, may differ in gene arrangement. One of us (STURTEVANT 1917, 1931) has shown that strains of Drosophila melanogaster coming from the same or from distinct geographical localities may differ in having blocks of genes rotated by 180' (inversion). The method of detection of inversions used in these early studies was based chiefly on observations on the strength of linkage in hybrids between different strains, but the laboriousness of this method limits its applicability. The discovery of the salivary gland chromosome method has rendered the task much easier. TAN (1935) in Drosophila pseudoobscura, and DUBININ, SOKOLOV and TINIAKOV (1936) in melanogaster have observed numerous inversions in hybrids between strains and races. DOBZHANSKY and TAN (1936) and DOBZHANSKY and BAUER (unpublished) have found extensive differences between the gene arrangements in two pairs of species of Drosophila, namely pseudoobscura and miranda, athabasca and azteca. In the present article we shall report the results of comparisons of the gene arrangement in the chromosomes of strains of D. pseudoobscura coming from different geographical regions. This species has five pairs of chromosomes; one of them, the third, is especially variable in the gene arrangement. Other chromosomes are relatively more constant, yet some variation has been observed in every chromosome, except the small fifth. Some of the gene arrangements are encountered in populations inhabiting a major part of the species area, while others are more restricted in their distribution. In many localities the population is mixed with respect to the chromosome structure, and inversion heterozygotes are very common in nature. Moreover, as pointed out in our preliminary communication (STURTEVANT and DOBZHANSKY 1936a), a comparison of the different gene arrangements in the same chromosome may, in certain cases, throw light on the historical relationships of these structures, and consequently on the history of the species as a whole. * The cost of Plate I is paid for by the Galton and Meudel Fund.