STRATIFICATION 



through selection, of the general gene system; the switch gene, 

 where it exists, is thereby given a progressively stronger effect. The 

 evidence that this is at least sometimes the case arises from the 

 method of breakdown seen already in Primula sinensis. Sudden 

 breakdown can, of course, occur in the switch gene itself Various 

 species o£ Primula, such as the common primrose, sometimes have 

 the pin-thrum type replaced, in nature, by a homostyle type with 

 anthers and stigma at the same level. This change was found by 

 Ernst in one case to be due to a change of the switch gene to a 

 third allelomorph. The new type must inbreed with a regularity 

 which is equal to the outbreeding of its predecessor, because, of 

 course, the genetic background remains the same. Similarly, in 

 ordinary incompatibility, the usual 5 allelomorphs can be replaced 

 by a so-called fertility allelomorph, S, which no longer inhibits 

 self-pollination or even any kind of cross-pollination. 



Most fertihty allelomorphs have been found in self-compatible 

 species related to other species which are uniformly self-incompatible. 

 But mutations to Sj- have appeared in the otherwise regularly 

 incompatible red clover. Moreover, in Antirrhinum majus, but 

 nowhere else so far as is known, a fertility gene exists, overriding 

 the S system, but not allelomorphic to it. These various cases of 

 breakdown show that a system which is built up gradually of many 

 elements including an operator, the switch gene, can be knocked 

 down by removing any one of them, especially, of course, by 

 removing the operator. 



Stratification 



All these facts and arguments relate to changes in the breeding 

 system at one level, or at one time of operation. But, as we know, 

 control occurs at many levels. How do they interact ? 



One system can be superimposed on another, while leaving all 

 the evidence of the other plainly revealed to us. We then have a 

 stratified system. For example consider wheat, any species of 

 Triticum. The anthers and stigmata are thrust out into the wind 

 to allow of cross-pollination, or so one would suppose ; but in fact 

 the anthers burst and pollinate the stigma inside the flower before 

 it opens. Inbreeding is superimposed on outbreeding: a low 

 hybridity optimum can replace a high one. This sequence of systems 



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