BRLEDING SYSTEMS 



relationships : self-pollination rarely succeeds. P. axillaris, on the other 

 hand, shows no trace of incompatibility : selfing and crossing succeed 

 equally well. In the F^ different 5 allelomorphs from violacea vary 

 in eftectiveness and plants differ in the degree of self-compatibility. 

 By crossing together F^ plants with different S allelomorphs, or by 

 backcrossing them to their violacea parent, we can get plants with 

 the same 5 constitution as violacea but with other genes, half in the 

 F2 and a quarter, or nearly a quarter, in the backcross, from axillaris. 

 Although alike in regard to S these three types of plants show 

 incompatibility relations differing in two ways (Fig. 64). 



First, the amount of seed set on self-fertihzation increases with the 

 proportion of axillaris genes. These genes must therefore be under- 

 mining the operation which the 5 genes control. 



Secondly, the amount of seed set on pollination of backcross 

 or Fo plants by violacea is greater than any produced by self-pol- 

 lination, although the same 5 genes are at work. Thus the axillaris 

 genes not merely weaken the operation of the S genes; they shift 

 their operation so as to put it out of step in plants with different 

 proportions o£ axillaris genes. Or, the other way round, we may say 

 the same 5 genes can not merely control systems of different 

 strengths, but systems wliich are less efficient with one another than 

 each is within itself. 



Now, we may ask, what happens when we cross two self-incom- 

 patible species, each with the S genes? This has been done for 

 Nicotiana alata and N. forgetiana in the production of the garden 

 form N. sanderae. Pseudo-compatibility, that is successful fertiliza- 

 tion with pollen having an S allelomorph the same as in the style 

 and therefore not legitimately capable of growth, is unknown in 

 forgetiana. It occurs only rarely in alata. In their derivative it is 

 common, especially with certain of the weaker allelomorphs. Thus 

 the recombination between the general gene systems of two species 

 has robbed each of its efficiency as a basis for the action of the 5 

 series. 



The best known example of all switch genes, or gene complexes, 

 is of course that determining sex. We have already seen how on 

 crossing the two sub-species of Melandriwn dioicinn Winge was able 

 to obtain in Fg plants male in regard to the sex chromosomes but 

 bearing female as well as male organs. As with Petunia and Nicotiana 



2^6 



