Chapter *17 



CHROMOSOMAL REARRANGEMENTS 

 IN NATURE 



Oenothera ' 



The evening primrose, Oenothera (Figure 

 17-1 ). is a common weed found along road- 

 sides, railway embankments, and in aban- 

 doned fields. It exists in nature in a number 

 of pure breeding, self-fertilizing strains — 

 each with a characteristic phenotype. These 

 strains can be cross-fertilized in the labora- 

 tory. If the two strains crossed are La- 

 marckiana and biennis the outcome in F x is 

 surprising. First, the F, are not all uniform 

 in phenotype as one would expect from pre 

 vious experience with crossbreeding two pure 

 lines, but three distinct types (which we will 

 call A, B, C). Second, upon self-fertiliza- 

 tion each of these three F, types is thereafter 

 pure breeding. If the Fi were hybrid, we 

 would expect self-fertilization to produce 

 recombinants and. therefore, more than one 

 phenotype in its progeny. These two pe- 

 culiarities are summarized in Figure 17-2 

 where the results obtained from similar 

 crosses with garden peas are shown side 

 by side. 



We must conclude from the Oenothera 

 results that self-fertilizing strains cannot au- 

 tomatically be considered pure homozygous 

 lines, despite any contrary impression gained 

 in Chapter 1. In order to obtain three dif- 

 ferent genotypes in F,. either Lamarckiana, 



1 Based upon work of H. DeVries. O. Renner. 

 R. E. Cleland. F. Oehlkers, A. F. Blakeslee, J. 

 Belling, S. Emerson, and A. H. Sturtevant. 



228 



or biennis or both must be heterozygous. 

 Assume that Lamarckiana is heterozygous 

 for a single pair of genes. If so, how can this 

 strain produce only Lamarckiana upon self- 

 fertilization'.' To do this would require that 

 the heterozygote produces only heterozy- 

 gote progeny. But suppose that self-fertili- 

 zation does, as expected, produce the two 

 homozygotcs, both of which are lethal. 

 ( Recall that for yellow mice — p. 69 — only 

 one homozygote is lethal; the other is viable. 

 In the present case the two different alleles 

 would both have to act as recessive lethals. ) 

 This hypothesis which predicts that one half 

 of the zygotes die before becoming mature 

 Lamarckiana, is supported by the finding 

 that approximately one half of the ovules 

 regularly fail to produce seed upon self- 



figure 17-1. Oenothera. (Courtesy of R. E. 

 Cleland.) 



