Chapter 21 



NATURAL AND INDUCED 

 CHROMOSOMAL CHANGES 





HAPTER 19 dealt primarily with 

 different types of structural 

 'change within chromosomes 

 and the manner in which their origin de- 

 pended upon chromosome breakage. How- 

 ever, little was said there concerning the 

 factors responsible for the production of 

 the key events of breakage and of union. 

 This is one of the matters to be taken up in 

 the present Chapter. Also, in that earlier 

 Chapter, relatively little detail was given con- 

 cerning the types of structural change ac- 

 tually found in nature. We did learn sub- 

 sequently, in Chapter 20, that reciprocal 

 translocations have played an important role 

 in the evolution of Oenothera. It might 

 be claimed, however, that Oenothera does 

 not furnish a representative test of the im- 

 portance of chromosomal rearrangements in 

 evolution, since its cytogenetical behavior is 

 so unorthodox. For the specific reciprocal 

 translocations in Oenothera involve the ends 

 of chromosomes and are retained in natural 

 populations in heterozygous condition. 



There are hundreds of different species of 

 Drosophila in nature. These species can 

 be compared ecologically, morphologically, 

 physiologically, serologically, and biochemi- 

 cally. They can also be tested for ability to 

 interbreed, and when they do crossbreed, their 

 genetics can be compared ; they can be com- 

 pared relative to the banding patterns of their 

 salivary gland chromosomes and the appear- 

 ance of their chromosomes at metaphase. 

 And when all known information of this kind 

 173 



is taken into account, it becomes possible to 

 arrange these species on a chart so that those 

 closest together are more nearly related in 

 descent, in evolution, than are those farther 

 apart. ^ This has been done in Figure 21-1, 

 which shows the haploid set of chromosomes 

 at metaphase. including the X but not the Y 

 chromosome, for different species or groups 

 of species of Drosophila. For example, the 

 chromosomes of the melanogaster species 

 group are drawn in row 2, column 1, the bot- 

 tom chromosome being the rod-shaped X, 

 the two V's being the two large autosomes (II 

 and III) and the dot representing the tiny 

 chromosome IV. In the other metaphase 

 configurations, chromosomes or their parts 

 which are judged to be homologous are 

 placed in the same relative positions. What 

 can we learn from a comparison of these 

 metaphase plates? 



Since the amount of detail in a metaphase 

 chromosome is limited to size and shape, we 

 cannot expect to detect any rearrangements of 

 small size at this stage. So, regardless of their 

 importance, small rearrangements involving 

 duplications, deficiencies, shifts, transposi- 

 tions, inversions, and translocations cannot 

 be detected in the Figure. Even a large para- 

 centric inversion is undetected here, since it 

 does not change the shape of the chromo- 

 some. However, other gross structural 

 changes can be detected. In row 4, the chro- 

 mosome patterns in columns 2 and 3 seem 

 identical, except that a pericentric inversion 

 has changed a rod to a V, or the reverse. 

 (Pericentric inversions will always change the 

 relative lengths of arms when the two breaks 

 are different distances from the centromere.) 

 Compare the plate for melanogaster (r.2, c.l) 

 with the plate to its right (r.2, c.2). A V-shaped 

 autosome in melanogaster appears as two 

 rods in its relative. (Note also that the dot 

 chromosome is missing.) In the next plate to 

 the right (r.2, c.3), two rods have combined 



^ Based upon work of C. W. Metz and others. 



