Cytogenetics of Oenothera 



163 



FIGURE 20-2 {Above). Comparative breeding re- 

 sults from garden peas and Oenothera. 



FIGURE 2Q-3 (Below). Balanced lethal systems 

 that enforce heterozygosity. 



ZYGOTIC 

 LETHAL 



SAMETOPHYTIC 

 LETHAL 



homozygote is lethal, the other is viable. In 

 the present case the two different alleles would 

 have to act as recessive lethals.) This hy- 

 pothesis would require that of all zygotes, one 

 half die before becoming mature Lamarcki- 

 ana. This view is supported by finding that 

 approximately one half of the ovules regular- 

 ly fail to produce seed upon self-fertilization, 

 and this is evidence that Lamarckiana in 

 nature is a permanent heterozygote in this 

 respect, as the result of a balanced lethal 

 system. In this case, since ovules fail to 

 produce seed, the lethals must kill prior to 

 this stage. In fact, the lethal may kill at the 

 time of fertilization, or very soon thereafter, 

 being in effect a zygotic lethal (Figure 20-3). 



But, we have ignored another possible time 

 for lethal action. Recall that in some plants, 

 including Oenothera, there is a haploid game- 

 tophyte generation. Permanent heterozygos- 

 ity could be maintained also, if one allele 

 were lethal to the male gametophyte and the 

 other to the female (Figure 20-3). So game- 

 tophytic lethals can also provide a balanced 

 lethal system. Further study shows that half 

 of the ovules fail to produce seed in biennis 

 also, and, in general, in all strains of Oeno- 

 thera found in nature, and that, in fact, both 

 zygotic and gametophytic lethals are involved 

 in these balanced lethal systems. 



Does the establishment of the existence of 

 a balanced lethal system in Oenothera explain 

 how it is that the phenotype, say of La- 

 marckiana, is the only one produced in the 

 progeny from self-fertilization? Since all 

 sexual organisms so far studied have many 

 pairs of genes, it would not seem reasonable 

 that Oenothera has only a single pair of 

 recessive lethal genes whose pleiotropic (mani- 

 fold) effects produce the entire phenotype. 

 It is more reasonable to believe that there are 

 many gene pairs but that these form a single 

 linkage group, so that the diploid has one 

 genome whose genes are all linked to one 

 recessive lethal, and another genome whose 

 senes are all linked to the allelic lethal. 



