Chapter 20 



CYTOGENETICS OF OENOTHERA 



Ai 



T THE close of the last Chapter 

 you were introduced to some 

 .of the genetic and cytological 

 methods for detecting and identifying struc- 

 tural changes in chromosomes. In still 

 earlier Chapters you have learned the char- 

 acteristics and some of the properties of other 

 types of mutational events, as well as the 

 basic principles of transmission genetics. Let 

 us take the opportunity to use this informa- 

 tion in toto in an investigation ^ of a plant 

 called the evening primrose, Oenothera, which 

 has until now been mentioned only on one 

 occasion (p. 139). 



Oenothera (Figure 20-1) is a common weed 

 found along roadsides, railway embankments, 

 and in abandoned fields. It is self-fertilizing 

 in nature, where it exists in a number of pure 

 breeding strains, each having a characteristic 

 phenotype. These strains can be cross- 

 fertilized in the laboratory, however, and 

 progeny obtained. If the two strains crossed 

 are Lamarckiana and biennis, a surprising 

 result is obtained in Fi. In the first place, 

 the Fi are not all uniform in phenotype, as 

 we would expect from previous experience 

 with crossbreeding two pure lines, but are of 

 three distinct types which we can call A, B, C. 

 In the second place, each of these three Fi 

 types is thereafter pure breeding upon self- 

 fertilization. If the Fi were hybrid, we would 

 expect self-fertilization to produce recombi- 

 nants of more than one phenotype. These 



^ Based upon work of H. DeVries, O. Renner, R. E. 

 Cleland, F. Oehlkers, A. F. Blakeslee, J. Belling, S. 

 Emerson, and A. H. Sturtevant. 

 162 



FIGURL 20-1. Oenothera. (Cuurtesy of R. E. 



Cleland.) 



two peculiarities are summarized in Figure 



20-2, where the typical results obtained from 



similar crosses with garden peas are indicated 



side by side. 



What conclusions can we draw from this 

 information? We must conclude, contrary 

 to any such impression which might have 

 been gained in Chapter 1, that self-fertilizing 

 strains cannot automatically be considered to 

 be pure, homozygous lines. In order to ob- 

 tain three different genotypes in Fi, either 

 Lamarckiana, or biennis, or both, must be 

 heterozygous. Let us make the simplest as- 

 sumption, namely that Lamarckiana is heter- 

 ozygous for a single pair of genes. If so, how 

 can this strain produce only Lamarckiana 

 upon self-fertihzation? This would require 

 that the heterozygote produce only hetero- 

 zygote progeny. But suppose that self- 

 fertilization does, as expected, produce the 

 two homozygotes, but that these are never 

 observed because both types are lethal. (Re- 

 call that for yellow mice, see p. 65, only one 



