186 



PAUL C. MANGELSDORF 



The four Pre-Columbian Exotic races, also believed to be relatively free 

 from contamination, have an average chromosome knob number of 4.3. The 

 thirteen Prehistoric Mestizos and the four Modern Incipient races (all except 

 one of which are assumed to involve teosinte introgression) have chromo- 

 some knob numbers of 7.1 and 8.0, respectively. 



It is interesting to note that in races for which hybridization is postulated 

 the hybrid race, although usually intermediate in chromosome knob number 

 between its two putative parents, resembles most closely the parent with a 

 high knob number. For the eleven hybrid races for which chromosome knob 

 numbers are available, not only for the hybrid races but for the two suspected 

 parent races, the data (Table 11.3) are as follows: the average of the lower- 



TABLE 11.3 



CHROMOSOME KNOB NUMBERS OF MEXICAN HY- 

 BRID RACES OF MAIZE AND OF THEIR 

 PUTATIVE PARENTS* 



Hybrid Race 



Tabloncillo 



Comiteco 



Jala 



Zapalote Chico. . 

 Zapalote Grande 



Tuxpeno 



Vandeiio 



Chalqueno 



Celaya 



Conico Norteno. 

 Bolita 



Averages . . . 



Race 



7.6 

 5.6 

 7.5 

 11.7 

 7.4 

 6.1 

 8.1 

 6.8 

 8.5 

 8.0 

 8.6 



7.8 



Parents 



Lower 



low 

 5.0 

 5.6 

 5.5 

 7.0 

 6.3 

 6.1 

 1.0 

 6.1 

 1.0 

 7.6 



5.1 



Higher 



8.0 



.0 

 .6 

 .0 



,7 

 .0 



7. 

 7. 

 9. 



11. 

 9. 

 7.4 

 6.1 

 7.6 

 8.5 



11.7 



8.5 



* Data from Wellhausen et al. 



numbered parent was 5.1 knobs, of the higher-numbered parent, 8.5 knobs, 

 of the hybrid, 7.8 knobs. The fact that the average knob number in the 

 hybrid races approaches the average knob number of the higher parents 

 suggests, perhaps, that natural selection has tended to retain the maximum 

 amount of teosinte introgression and hence the maximum number of knobs. 



The Effects of Hybridizing Maize and Teosinte 

 There is no doubt that maize and teosinte are hybridizing in Mexico and 

 Central America today, and there is at least a strong indication that they 

 have done so in the past. What have been the effects of that hybridization? 

 One valid way of determining what happens when teosinte introgresses 

 into maize is to produce such introgression experimentally. This has been 

 done on an extensive scale by crossing an inbred strain of maize, Texas 4R-3, 



