Salivary Gland Chromosomes in the Two Races of Drosophila Pseudoobscura.

The two races of Drosophila pseudoobscura, known as race A and race B, produce sterile male offspring when crossed (LANCEFIELD 1929), have visibly different shapes of the Y chromosomes (DOBZHANSKY and BOCHE 1933), and differ in their physiological characteristics (POULSON 1934, DOBZHANSKY 1935a, BOCHE unpublished). They appear, however, indistinguishable morphologically. LANCEFIELD (1929) and KOLLER (1932) found, in female hybrids between these two races, a suppression of crossing over as observed in both ends of the X chromosome, while in its middle crossing over is nearly normal. Such a suppression of crossing over is usually due to the presence of inverted sections (STURTEVANT 1926, 1931). It is, therefore, probable that race A differs from race B in having two inverted sections in the X chromosome. Nothing has been reported concerning the conditions of autosomes in this respect. The technique of studying the salivary gland chromosomes (HEITZ 1933 and PAINTER 1934a) permits a more precise comparison of the gene arrangement in the chromosomes of related species and races of Diptera than has been possible heretofore. The results of such a comparison of the chromosomes of A and B races of Drosophila pseudoobscura are presented in this paper. The author wishes to express his gratitude to Professors TH. DOBZHANSKY and A. H. STURTEVANT for their encouragement, advice and use of their stocks