HYBRIDIZATION IN THE EVOLUTION OF MAIZE 191 



The reason for the strong jjotence of maize over teosinte in blocks of 

 genes introduced from teosinte into maize, is to be found in a phenomenon 

 termed "antithetical dominance" which has been postulated by Anderson 

 and Erickson (1941) on theoretical grounds. These writers assumed that in 

 species hybrids such as that between maize and Tripsacum, the Fi would be 

 intermediate but that backcrosses to either parent would strongly resemble 

 the recurrent parent. The basis for this assumption is that the possibilities 

 for successful recombination of two such different systems is remote. 



The conception of antithetical dominance has some relationship to 

 Richey's opinion (1946) that dominance in some cases is no more than a con- 

 dition where one allele is capable of doing the entire job, or most of it, while 

 the other allele merely stands by. According to this interpretation, genes are 

 not favorable because they are dominant, but are dominant because they are 

 favorable. They reveal their presence by doing something. 



There is, in any case, little doubt that something of the general nature of 

 antithetical dominance or the kind of dominance postulated by Richey is 

 involved in the teosinte-maize derivatives. Both teosinte and maize are 

 about equally potent in the Fi hybrid, but a small amount of teosinte germ- 

 plasm incorporated into maize in the heterozygous condition is definitely 

 lacking in potence. 



Effects in Homozygous Condition 



Since a block of teosinte genes introduced into maize is largely recessive 

 in its effects when heterozygous, its effects should become much more ap- 

 parent in the homozygous condition. This is indeed the case. The ear on the 

 right in Figures 11.3, 11.4, and 11.5 illustrates the effects of one or more 

 blocks of teosinte genes incorporated in a homozygous condition in the inbred 

 strain 4R-3. 



The combination of traits from corn and teosinte which occurs in these 

 homozygous teosinte derivatives is characterized by a distinct lack of har- 

 mony in the development of the pistillate inflorescence. The husks are too 

 short for the ears, the glumes are too small for the kernels and tend to con- 

 strict the growing caryopses producing misshapen kernels. The vascular sys- 

 tem is inadequate for the number of kernels borne on the ear, and there are 

 many shrunken kernels as well as numerous gaps where no kernels have de- 

 veloped. Germination of the seeds is often poor, and viability of short dura- 

 tion. Homozygous combinations of this kind obviously have a low survival 

 value. Indeed it has been difficult to maintain some of them in artificial 

 cultures. 



These unfavorable effects of teosinte introgression in the homozygous con- 

 dition may be nothing more than the result of substituting parts of one well- 

 integrated system for corresponding parts of another. They may, however, 

 also involve "cryptic structural differentiation" of the kind suggested by 



