Chromosomal Rearrangements in Nature 



229 



fertilization — evidence that in nature La- 

 marckiana is a permanent heterozygote in 

 this respect, with a balanced lethal system. 

 In this case both lethals kill the individual 

 sometime before seed formation, in fact, the 

 lethal kills at the time of fertilization or very 

 soon thereafter, being in effect a zygotic 

 lethal (Figure 17-3). 



Recall that some plants, including Oeno- 

 thera, have a haploid gametophyte genera- 

 tion. Permanent heterozygosis could be 

 maintained also, if one allele were lethal to 

 the male gametophyte and the other to the 

 female (Figure 17-3). Consequently, game- 

 tophytic lethals can also provide a balanced 

 lethal system which prevents half of the 

 ovules from producing seed. We have al- 

 ready seen an example of this kind of lethal 

 in the self-sterility gene in Nicotiana (p. 

 60). In general, all strains of Oenothera 

 found in nature, including biennis, have en- 

 forced heterozygosity due to the zygotic and 

 gametophytic lethals which produce bal- 

 anced lethal systems. 



Does a balanced lethal system explain 

 why the phenotype of Lamar ckiana, for ex- 

 ample, is the only one produced in the prog- 

 eny after 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 



PEAS 



Tall Dwarf 



I t 



Tall Dwarf 



I * 



Tall Dwarf 



\ / 



F Tall 



/ \ 



F 3 Tall; 1 Dwarf 



OENOTHERA 

 Lamarckiana Biennis 



t I 



Lamarckiana Biennis 



♦ t 



Lamarckiana Biennis 

 F A B C 



'III 



F A B C 



ZYGOTIC 

 LETHAL 



# 



© 



figure 17-2. Comparative breeding results 

 from garden peas and Oenothera. 



GAMETOPHYTIC 

 LETHAL 



figure 17-3. Balanced lethal systems that 

 enforce heterozygosity. 



lethal genes whose manifold (pleiotropic) 

 effects produce the entire phenotype. It is 

 more likely that many gene pairs exist which 

 form a single linkage group, so that the dip- 

 loid individual has one genome whose genes 

 are all linked to one recessive lethal and 

 another genome whose genes are all linked 

 to the allelic lethal. 



In other words, Lamarckiana behaves as 

 though it contains two complexes of linked 

 genes. Within a strain these genes are com- 

 pletely linked by some mechanism that pre- 

 vents recombination, leaving the gametes 

 with only two kinds of genotypes. The two 

 gene complexes are so constant in natural 

 populations of a strain that in the case of 

 Lamarckiana they are given the names gau- 

 dens and velans, and identified as gaudens. 

 velans; the biennis strain as described by its 



