Chromosomal Rearrangements in Nature 



231 



ing that flavens (or velans) is actually a 

 single linkage group which cannot undergo 

 crossing over with its partner gene complex 

 in the parent race, but which can do so when 

 its partner is velans (or flavens). 



Tests of the hybrid containing flavens. 

 curvans showed m and P still completely 

 linked but segregating independently of 5, 

 which was, in turn, segregating independ- 

 ently of Sp and Cu, so that in this case three 

 linkage groups existed. Perhaps more link- 

 age groups would have been found with 

 additional genetic markers. In all cases, 

 however, a given hybrid combination always 

 showed the same linkage groups in its game- 

 togenesis. 



Because at least three linkage groups can 

 be identified in certain interstrain hybrids 

 (even though these act as one in the self- 

 fertilizing parental strain), it is expected that 

 the diploid Oenothera has at least three pairs 

 of chromosomes and cytological examination 

 confirms this genetic expectation — all of the 

 Oenothera strains discussed in this chapter 

 having seven pairs of chromosomes. (Oeno- 

 thera gigas, the triploid mentioned on p. 

 151, has 2 1 chromosomes. ) If the balanced 

 lethal system is based upon a single pair of 

 genes located on a single pair of homologs, 

 this pair of chromosomes must be hetero- 

 zygous in viable progeny. But this hetero- 

 zygosity would not be expected for the other 

 six pairs of chromosomes, if they segregated 

 independently. Consequently, all gametes 

 of O. biennis, for example, which carry the 

 albicans complex recessive lethal should sim- 

 ilarly be expected to carry the rubens or the 

 albicans homolog in each of the other six 

 cases of independent segregation. However, 

 this distribution is not found. We could 

 then suppose that each of the seven chro- 

 mosome pairs is heterozygous for a different 

 recessive lethal. Upon self-fertilization, 

 such a genotype would produce only viable 

 Fi like the parent. Since this explanation 

 predicts that only about (U) 7 of all ovules 



should develop as seeds, it cannot be the 

 correct one for Oenothera in which, as men- 

 tioned, about 50% of all ovuies mature into 

 seeds. 



A clue to the orderly segregation oi com- 

 plete gene complexes in Oenothera may be 

 found by cytological study of meiosis. The 

 typical self-fertilizing Oenothera in nature 

 does not form seven separate bivalents as 

 expected, but, as seen clearly at metaphase 

 I, forms a closed circle of 14 chromosomes 

 synapsed end to end (Figure 17-6). At ana- 



figure 17-6. Circle of 14 chromosomes in 

 Oenothera. Chromosome number is clear in 

 upper cell where the circle has broken open. 

 (Courtesy of R. E. Cleland.) 



