Cytogenetics of Oenothera 



167 



FIGURE 20-7. Ralph E. Cleland points to zig-zag chromosomal arrangement at 

 the start of anaphase I of an Oenothera having a circle of 14 chromosomes at meta- 

 phase I. {Photograph courtesy of The Calvin Company.) 



A clue to the orderly segregation of com- 

 plete gene complexes is found in the study 

 of meiosis in Oenothera. Here it is found that 

 the typical Oenothera in nature does not form 

 seven separate bivalents, but, as seen clearly 

 at metaphase I, forms a closed circle of 14 

 chromosomes, synapsed end to end (Figure 

 20-6). At anaphase I, moreover, chromo- 

 somes that are adjacent to each other in the 

 circle go to opposite poles of the spindle, so 

 that at the start of the separation the chro- 

 mosomes assume a zigzag arrangement 

 (Figure 20-7). If you make the assumption 

 that paternal and maternal chromosomes 

 alternate in the circle, then all paternal chro- 

 mosomes go to one pole, and all maternal 

 chromosomes to the other. The complete 

 hnkage of all genes in a complex would be 

 explained (if crossing over is rare) by such a 



manner of chromosome segregation, and the 

 gametes produced by an individual would be 

 identical to those which united to form it 

 (Figure 20-8). 



If the alternate segregation procedure sepa- 

 rates maternal and paternal genomes, a circle 

 should always contain an even number of 

 chromosomes. Moreover, we could make 

 the prediction that when a genome no longer 

 behaves as a single linkage group it would 

 also no longer form, with the other linkage 

 group, a single circle of 14 chromosomes. 

 Theoretically, there is a total of 15 different 

 ways that 14 chromosomes can be arranged 

 in circles (composed of even numbers of 

 chromosomes) and pairs, as shown in Figure 

 20-9. And when various race hybrids are 

 made, indeed all 15 types and no others are 

 found in the metaphase I stage, any particular 



