Cytogenetics of Oenothera 



169 



hybrid always giving the same meiotic con- 

 figuration. (The top cell in Figure 20-7 shows 

 an inner circle of four and an outer circle of 

 ten chromosomes.) If what has been sup- 

 posed is true, it also ought to follow that 

 while alternate chromosomes within a circle | 

 show complete linkage with each other, such I 

 linkage groups would not be prevented from I 

 segregating independently from those linkage 

 groups made up of chromosomes either in 

 separate circles or in separate pairs. This^ 

 expectation can be tested by comparing the 

 number of linkage groups found in the dif- 

 ferent hybrids of Figure 20-5 with the chro- 

 mosome arrangements seen cytologically 

 during their meiosis. When this is done, it is 

 found that the number of separate groups of 

 chromosomes observed in meiosis is always 

 equal to, or greater than, the number of link- 

 age groups detected genetically. In fact, 

 whenever enough genetic markers are used, 

 the number of linkage groups always equals 

 the number of chromosome groups. 



While all of the preceding satisfactorily 

 demonstrates that segregation of alternate 

 chromosomes in a circle to the same pole and 

 the presence of balanced lethal systems can 

 explain most of the unusual genetic behavior 



of Oenothera, other matters still need explana- 

 tion. What causes these chromosomes to 

 form circles in the first place? A clue to this 

 is contained in the observation made near the 

 end of the last Chapter, namely, that two 

 pairs of nonhomologs will be associated to- 

 gether during synapsis if a reciprocal trans- 

 location is present between two of the non- 

 homologs and absent in their homologs, i.e., 

 if these nonhomologs are heterozygous for a 

 reciprocal translocation. This can be il- 

 lustrated in Oenothera by means of Figure 

 20-10. In Oenothera all chromosomes are 

 small, of the same size, with median centro- 

 meres. To help us identify homologous 

 chromosomes, the ends of each chromosome 

 in a genome are given diff'erent numbers. 

 Suppose, some time in the past, a eucentric 

 reciprocal translocation occurred between the 

 tips marked 2 and 3 (top left of Figure). This 

 rearrangement in heterozygous condition 

 (middle left) would produce an X-shaped 

 configuration at the time of synapsis in pro- 

 phase I (bottom left), and a circular appear- 

 ance at metaphase I — early anaphase I. In 

 this way a circle of four chromosomes would 

 be produced. 



If now a second reciprocal translocation 





2"2 



FIGURE 20-10. Heterozygous 

 reciprocal translocations and 

 circle formation. 



3 3 

 2' '2 



