CHROMOSOME ABERRATIONS IN ANIMALS 635 



approach involves inspection of the salivary-gland chromosomes of the 

 first generation (Fi) larval progeny of the irradiated fathers. Since 

 analysis of a given rearrangement by this method is restricted to the 

 chromosomes of a single individual, aberrations of special interest cannot 

 be perpetuated for genetic analysis and subsequent experimental use. 

 On the other hand, the large size and precise pattern of banding of the 

 salivary-gland chromosomes (as shown in Figs. 9-5, 9-6) offer unparalleled 

 opportunities for determining the complexity of a rearrangement and the 

 positions of the breaks involved in its production — as was first demon- 

 strated by Painter in 1933. Methods of preparing for cytological exam- 

 ination the aceto-carmine or aceto-orcein smears from which these photo- 

 graphs were made are briefly outlined in the "Drosophila Guide," by 

 Demerec and Kaufmann (1950). 



One limitation of the salivary-gland-chromosome method of diagnosis 

 is the difficulty of detecting rearrangements that are restricted to proximal 

 heterochromatic regions; these parts of the chromosome have poorly 

 defined bands, and aggregate to form a so-called " chromocenter " (illus- 

 trated in Fig. 9-5a). Exchanges involving breaks in proximal hetero- 

 chromatic regions can sometimes be detected in neuroblast cells of the 

 larva by the pattern of somatic pairing of the chromosomes. Thus the 

 cross-shaped configuration shown in the small inset of Fig. 9-5a (cf. Fig. 

 9-7b), results from the side-by-side association of unaltered second and 

 third chromosomes, maternal in origin, with second and third chromo- 

 somes of the paternal set that had exchanged parts (reciprocal transloca- 

 tion) as a result of irradiation of the spermatozoon. Identification of 

 intrachromosomal exchanges in neuroblast cells is difficult, but is some- 

 times possible because of the presence of constrictions, including those 

 associated with the formation of the nucleoli, that are visible in late 

 prophase stages (Fig. 9-7a, c, d, e). From these considerations it is 

 apparent that a comprehensive quantitative study of induced rearrange- 

 ments should include analysis of both salivary-gland and neuroblast 

 chromosomes from the same larva. The labor involved in cytological 

 examination of the neuroblasts is so considerable, however, that they are 

 rarely utilized for this purpose. Before the advantages of salivary-gland 

 chromosomes were recognized, cytological studies of genetically detected 

 rearrangements were made exclusively on chromosomes of neuroblast 

 cells of larvae or gonial cells of adults (see, for example, Dobzhansky, 

 1936; Stern, 1931). 



Genetic techniques for determination of induced chromosomal ex- 

 changes were described by Muller and Altenburg (1930) and Dobzhansky 

 (1929, 1930). Both intra- and interchromosomal rearrangements had 

 previously been known in Drosophila. Sturtevant (1926) had shown that 

 a reduction of crossing over in the third chromosome of D. melanogaster 

 was due to inversion (rotation through 180° as a consequence of breakage 



