Chromosomal Rearrangements in Nature 



235 



sis. That all the configurations in Figure 

 17-9 are found in meiosis of such hybrids 

 must mean that different gene complexes 

 differ from each other in the specific ways 

 that their chromosome arms have become 

 translocated. Many thousands of ways are 

 possible for 14 ends to be arranged in seven 

 groups of two. How can we determine the 

 number of these different arrangements oc- 

 curring in nature? 



We can start by choosing a particular gene 

 complex — calling it the "standard'" — and 

 considering its chromosome ends to be 1-2, 

 3_4, 5_6, 7-8, 9-10, 11-12, and 13-14. 

 Normally, that is, in nature, this complex 

 would form a circle of 14 with the other 

 gene complex, which would therefore have 

 no chromosome with the same pair of num- 

 bered ends as any chromosome in the stand- 

 ard complex. Proceeding further, we form 

 a series of interracial hybrids with the stand- 

 ard as one of the complexes and score the 

 meiotic chromosome arrangements of the 

 hybrids. Suppose in one case that the hy- 

 brid forms five pairs and a circle of four. 

 This result must mean that the ends of 5 

 chromosomes are in the same order in the 

 complex under test as in the standard, but 

 that they are in a different order in the re- 

 maining two chromosomes. Although there 

 was previously no reason to assign ends 1-2 

 and 3-4 of the standard complex to any par- 

 ticular chromosomes, we can presently assign 

 these ends arbitrarily to the two standard 

 chromosomes in the circle of four. The 

 chromosomes in the circle from the complex 

 under test can then be called 2-3 and 4-1 

 (or 2-4, 1-3). In this way the composition 

 of ends of two chromosome pairs is specified 

 permanently. The top of Figure 17-11 

 shows the standard and tested complexes (of 

 our example) synapsed according to iden- 

 tical numbers. 



Call the complex just tested A. Suppose 

 another complex, B, is made hybrid both 



A. COMPLEXES DIFFERING BY ONE RECIPROCAL TRANSLOCATION 



1.2 3.4 5.6 7.8 9.10 11.12 13.14 



\ / \ II II I I I I I I 



23 41 56 7.8 9.10 11 12 13.14 



B COMPLEXES DIFFERING BY SIX RECIPROCAL TRANSLOCATIONS 

 1.2 3.4 5.6 7.8 9.10 11.12 13.14 



2.3 4.5 6.7 8.9 10.11 12.13 14.1 



C. MURICATA RACE'S ACTUAL COMPOSITION OF Q 14 

 1.2 3.4 6.5 13.12 7.11 10.9 8.14 



\ / \ / \ / I / I / \ / 1 

 2.3 4.6 5.13 12.7 11.10 9.8 14.1 



A and B are theoretical 



figure 17-11. Arrangement of chromosome 

 ends in different Oenothera complexes. 



with the standard and with A. The meiotic 

 configuration of the hybrid may specify 

 other ends of A, B, and the standard com- 

 plexes. Such procedures can be carried out 

 until all of the standard's chromosomes are 

 specified and the complete order of all four- 

 teen ends determined for any other complex. 

 In this manner, we can verify that a circle 

 of fourteen is produced in many different 

 ways in nature; a hypothetical and an actual 

 example is shown in the central and lower 

 parts of Figure 17-11. In fact, of 350 com- 

 plexes analyzed, more than 160 different 

 segmental arrangements have been found. 

 All these results are consistent with the hy- 

 pothesis that during the course of evolution, 

 the ends of Oenothera chromosomes have 

 been shuffled many times in different ways 

 by reciprocal translocation. A most convinc- 

 ing test of the reciprocal translocation inter- 

 pretation would be the ability to predict the 

 meiotic chromosomal arrangement to be 

 found in a hybrid not yet formed. This 

 type of predicting has been done many times 

 and all such expectations have been verified. 

 At various points in this discussion Oeno- 

 thera's behavior has seemed exceptional, ap- 



