HYBRIDIZATION IN THE EVOLUTION OF MAIZE 185 



Rogers (1950) has shown that teosinte varieties differ quite markedly in 

 their genes governing the characteristics in which maize and teosinte differ, 

 especially characters of the j)istillate inflorescence, tillering habit, and re- 

 sponse to length of day. He attributes these differences to varieties in the 

 type and amount of maize germplasm which has become incorjiorated into 

 teosinte. 



If teosinte varieties differ in the amount and kind of maize contamination 

 which they now contain, it is difficult to escape the conclusion that maize 

 varieties must likewise differ in the amount of teosinte contamination. There 

 is little doubt that maize varieties do differ in this respect. 



Ancient Hybridization 



The prehistoric maize from Bat Cave has already been briefly mentioned. 

 The earliest Bat Cave corn, dated at approximately 2000 B.C., shows no 

 evidence whatever of teosinte introgression. Beginning about midway in the 

 series (which would be about 500 b.c. if the sequence were strictly linear but 

 which, according to unpublished radio-carbon determinations made by Libby , 

 is probably somewhat later) cobs make their appearance which are scarcely 

 distinguishable from the cobs which we have produced experimentally by 

 crossing corn and teosinte. \\'eatherwax (1950) regards this evidence of teo- 

 sinte introgression as far from conclusive, and it is, of course, quite impossible 

 to prove that a cob a thousand years or more old is the product of hybridiza- 

 tion of maize and teosinte. Nevertheless, it is true that teosinte introgression 

 produces certain definite effects upon the cob, as some of us who have studied 

 the derivatives of teosinte-maize crosses on an extensive scale are well aware. 



When it is possible to duplicate almost exactly in experimental cultures 

 specimens found in nature, the odds are at least somewhat better than even 

 that the resemblance between the two specimens is more than coincidence. 

 There is little doubt in my mind that the later Bat Cave corn is the product 

 of contamination with teosinte. Certainly it differs from the earlier Bat Cave 

 corn quite strikingly, and it is exactly the way in which it would be expected 

 to differ if it is the product of teosinte introgression. 



Significance of Chromosome Knobs 



Mangelsdorf and Reeves (1939) suggested some years ago that the deeply 

 staining heterochromatic knobs, characteristic of the chromosomes of many 

 varieties of maize, are the result of the previous hybridization of maize and 

 teosinte, or more remotely of maize and Tripsacum. There has been much 

 indirect evidence in support of this hypothesis (especially Mangelsdorf and 

 Cameron, 1942; Reeves, 1944), and the recent studies of Wellhausen ei al. 

 on Mexican races of maize provide additional evidence of this nature. 

 Chromosome knob number in Mexican races is closely correlated with the 

 characteristics of the races. The four Ancient Indigenous races, assumed to 

 be relatively pure corn, have an average chromosome knob number of 4.2. 



