East and Hayes, Heterozygosis in Evolution and in Plant Breeding. ;; 



Crossing these inbred types invariably produced a great increase 

 in vigor in the first hybrid generation, vigor that again decreased 

 with self fertilization. 



Experiments on plants naturally self fertilized showed that 

 there existed: a) plants so different that they will not cross; b) crosses 

 that produce seed that contain no proper embryo; c) crosses that 

 produce seed with embryo, but which go no further than the rest- 

 ing stage of the seed; d) crosses less vigorous than either parent; 



e) crosses more vigorous than the average of the parents ; and 



f) crosses more vigorous than either parent. Fertile crosses are 

 nearly always more vigorous than the average of the parents, 

 sterile crosses are often more vigorous than the average of the 

 parents, but as the differences between the parent types becomes 

 greater a critical point is reached beyond which the cross is less 

 vigorous than either parent. The phenomenon of vigorous hybrids, 

 exhibited by crosses between plants usually self-fertilized naturally 

 is therefore the same phenomenon as the decline in vigor of plants 

 naturally cross-fertilized when they are inbred. 



The characters affected by hyterozygosis are only those which 

 are an expression of rapidity and amount of cell division. Even 

 some of these, such as size of flower, which might be expected 

 to respond to the action of heterozygosis, are not affected. 



After an analysis of the results in terms of modern genetics 

 is given, the same theories are shown to hold for the animal 

 kingdom. 



The value of the vigor due to heterozygosis during the process 

 of evolution is thought to be as follows: "It can hardly be doubted 

 that heterozygosis did aid in the development of the mechanisms 

 whereby flowers are cross-fertilized. Variations must have ap- 

 peared that favored cross-fertilization. These plants producing a 

 cross-fertilized progeny would have had more vigor than the self- 

 fertilized relatives. The crossing mechanism could then have become 

 homozygous and fixed, while the advantage due to cross-fertilization 

 continued. But was this new mechanism an advantage? It must 

 have been often an advantage to the species as a w r hole. In 

 competition with other species, the general vigor of those which 

 were cross-fertilized would aid in their survival. But the mechanism 

 may not have been useful in evolving real vigor in the species, 

 because of the survival of weak strains in combination. In self- 

 fertilized species, new characters that weakened the individual 

 would have been immediately eliminated. Only strains that stood 

 by themselves, that survived on their own merits, would have been 

 retained. On the other hand, weak genotypes in cross fertilized 

 species were retained through the vigor that they exhibited when 

 crossed with other genotypes. The result is, therefore, that self- 



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