DETERMINANTS OF THE HIGH-METHIONINE TRAIT IN WILD AND EXOTIC GERMPLASM MAY HAVE ESCAPED SELECTION DURING EARLY CULTIVATION OF MAIZE

The 18 kDa high-methionine delta-class zein gene from maize has been c loned, and its regulation, structure, and map position studied. These studies have shown that (i) zein genes may also contain tryptophan and lysine codons, (ii) the 18 kDa and the related 10 kDa zein gene are c oordinately regulated, but their products accumulate to different leve ls in a genotype-dependent manner, (iii) the duplication of delta-zein genes probably involved unequal crossing over, (iv) no copy correctio n in either direction has occurred from teosinte to modern corn, and ( v) the duplication of of the 18 kDa zein gene probably occurred before the tetraploidization of a progenitor chromosome. The work shows that important nutritional quality determinants like the high-methionine s eed proteins are abundant in several exotic and wild corn varieties an d low in most of the inbreds screened. The lack of a selectable phenot ype for such quality traits during initial domestication and breeding of corn would have eliminated cis and trans regulatory determinants fr om the germplasm used in modern corn breeding. Examples of the high-me thionine delta-class zeins shown here may be generally applicable in e xplaining the low nutritional quality of most present-day corn grown.