Further Studies on the Salivary Gland Chromosomes of Drosophila virilis

抄録

1. Genetical and cytological studies on ten translocations and four inversions found in D. virilis have been carried out. All the translocations are of the mutual type.
2. The recombination values between the genes located in the vicinity of the loci at which the chromosomes are broken or reattached, are lower than the corresponding standard values.
3. The loci of many genes determined by means of these chromosomal aberrations are illustrated in Plate 1.
4. The loci of the spindle fiber attachment have been determined for all the chromosomes including the second and the fourth for which no genetical data are available.
5. The chromocentral region of D. virilis is not a mere aggregate of the proximal ends of the chromosomes, but is composed of very conspicuous granular element-the α-heterochromatin.
6. The β-heterochromatin exists at the proximal end of all the chromosomes, except the third and the sixth.
7. The α-heterochromatin represents the inert regions of the mitotic chromosomes including the Y chromosome, while the β-heterochromatin represents the active genes occurring in the inert regions.
8. The nucleolus is not connected with the β-heterochromatin in the X chromosome, but it is connected with the α-heterochromatin by a bundle of spiral threads.
9. The morphologically ‘weak’ points found in the euchromatic region seem to have no genetical significance. The existence of some genetically weak points is assumed on the ground that the breakages in chromosome aberrations have a tendency to fall at some definite loci, and crossing over often takes place at a particular space on the sixth chromosome.
10. The fusion of the distal ends of the salivary gland chromosomes frequently observed in the smeared preparations, is no more than a random gathering. Studies on the sections of the salivary glands show that all the distal ends of the chromosomes are separate.