New Evidence for the Homology of the Short Euchromatic Elements of the X and Y Chromosomes of Drosophila Busckii with the Microchromosome of Drosophila Melanogaster.

length from the proximal end. In salivary gland cells this chromosome consists of two euchromatic elements: a long distal one, and a short, proximal piece approximately l/20th as long. Both of these elements are connected with the nucleolus. It has also been proven that the nucleolus of the polytene X is located at a site corresponding to the secondary constriction of the metaphase chromosome, and the short euchromatic polytene element corresponds to the smaller block of the metaphase chromosome that lies proximal to the secondary constriction. In salivary gland cells the centromere of the X chromosome is located at the very proximal end of the short euchromatic element, and it is probably terminally located in the metaphase chromosome as well (KRIVSHENKO 1955). The metaphase Y chromosome is also rod-shaped, but has a primary constriction near the proximal end which divides the chromosome into two arms: a long left arm and a short, knob-like, right arm. In the salivary gland cells the Y chromosome is usually observed as a short euchromatic element that is approximately equal to the short euchromatic element of the X chromosome ( SIROTINA 1938; KRIVSHENKO 1939). It has been proven that this euchromatic part of the Y chromosome is genetically active and corresponds to the right arm of the metaphase chromosome, whereas the long left arm is heterochromatic ( KRIVSHENKO 1950, 1952). It was also found that XO males of this species are not viable (KRIVSHENKO 1941a, b), and that viability factors are located in the small, euchromatic right arm of the Y chromosome ( KRIVSHENKO 1952). In previous investigations of D. busckii, cytological and genetical evidence was obtained pointing to the homology of the short euchromatic elements of the X and Y chromosomes with each other, and indeed also with the microchromosome of D. melanogaster (KRIVSHENKO 1952,1955). The basis for this conclusion lies in: 1) the observed somatic pairing of X and Y by their proximal ends in ganglion cells, and the conjugation of the short euchromatic elements of these chromosomes at their centromeric regions in salivary gland cells; 2) the presence in the short Y chromosomal element of normal allelomorphs to four different