192 PAUL C. MANGELSDORF 



Stephens (1950) for species of Gossypium, although the extent of this cannot 

 be great, otherwise some combinations would be lethal. But whatever the 

 cause, there is little doubt about the reality of the unfavorable effects. 

 Therefore, if the repeated hybridization of corn and teosinte which has oc- 

 curred in the past has had any permanent effect, one of two things or both 

 must have happened: (1) The undesirable effects of teosinte have become 

 recessive as the result of natural selection for modifying factors. (2) The 

 regions of the chromatin involving teosinte genes have been kept heterozy- 

 gous. There is some evidence that both may have occurred. 



There is some evidence, by no means conclusive, that maize varieties of 

 today have absorbed teosinte germplasm in the past and are now bufered 

 against the effects of teosinte genes. There is at least no doubt that when the 

 same variety of teosinte is crossed on a series of maize varieties, considerable 

 variation is displayed by the Fi hybrids in the potence of the maize parents. 



In general, varieties which show some evidence of previous contamination 

 with teosinte are more likely to produce maize-like Fi hybrids than those 

 which do not show evidence of such contamination. Corn Belt inbreds as a 

 class produce the most maize-like Fi of any of stocks tested. Figure 11.6 illus- 

 trates a case where a South American stock (an inbred strain derived from 

 the Guarany corn of Paraguay) is less potent in crosses with two varieties of 

 teosinte than is a North American stock (a genetic tester). I also have ob- 

 served that blocks of teosinte genes introduced into an inbred strain of 

 Guarany by repeated backcrossing have a greater effect than these same 

 blocks introduced into Texas 4R-3 or Minn. A158, both of which seem al- 

 ready to contain appreciable amounts of teosinte. 



If the increased potency of teosinte-contaminated maize proves to be gen- 

 erally true, then the reason for it is that there has been a selection of modify- 

 ing factors which have tended to suppress the most unfavorable conspicuous 

 effects of the teosinte introgression. Otherwise, varieties of maize containing 

 teosinte germplasm should produce hybrids which are more teosinte-like, 

 rather than more maize-like, than the average. This is convincingly demon- 

 strated experimentally by crossing the original inbred 4R-3 and one of its 

 modified derivatives with the same variety of teosinte (Florida type). The 

 results are illustrated in Figure 11.7. 



The Fi of 4R-3 X teosinte is a typical Fi hybrid, intermediate between 

 its parents. It has both single and double spikelets and, although the fact is 

 not revealed by the illustration, it has approximately the same type of re- 

 sponse to length of day as does maize. In marked contrast, when a derived 

 strain of 4R-3 (in which a block of teosinte genes on chromosome 3 has been 

 substituted for a corresponding block of maize genes) is crossed with the 

 same teosinte, the Fi hybrid is scarcely distinguishable in its pistillate spike 

 from pure teosinte. Furthermore, it has teosinte's response to length of day. 

 Plants of this hybrid started in the greenhouse in February did not flower 



