Necessity for a more comprehensive cytogenetic and phenotypic analysis of the maize-teosinte relationship

The teosinte segments transferred to A158 derivatives developed by Mangelsdorf were selected initially solely on the basis of their dominant effects on induration of the lower glume and reductions in the ear and kernel size and reduced kernel-row-number (MNL 22:19-20, 1948). No selective attention was given to the key traits of teosinte which immediately disappeared. It wasn't until after about five generations of backcrossing of small families to the same maize inbred (first 4R3 and then A158), followed by selfing to make the segments homozygous, that a linkage analysis, using his nine gene marker stock (WMT), was made to identify the segments in terms of number and location. Regardless of how tight the integrity of these segments due to close linkage and cross-over suppressors, their size must have been significantly reduced.

It is clearly necessary that new teosinte derivatives be developed using multiple marker gene stocks for various individual maize chromosomes (at least chromosomes 1, 3, 4, 7 and 9, following those pinpointed by Mangelsdorf) both as the outcross and backcross parent. The relatively intact teosinte chromosome to be captured would be identified from the start by virtue of its dominant alleles masking the recessives on a particular pre-selected maize chromosome. Since only plants showing the dominants for a certain teosinte chromosome are selected for backcrossing to that multi-marker stock and any individuals showing independent teosintoid traits are discarded, probably the extraneous teosinte material would be reduced to much less than 6 percent after three backcrosses. Ideally, both cytologically and morphologically (e.g., for the penetrance of two-ranking), the maize background for the new derivatives should be the knobless Wilbur's Flint. But, expediently, the backgrounds of the available marker stocks may serve, even though the derivatives would not be strictly isogenic. Some attempt has been made by the curators of the Maize Coop collection of tester stocks to give them a common background. The development of Wilbur's Flint tester gene stocks should be started for such comparative studies, but their completion followed by application would probably be beyond the tenure of at least this investigator. More immediate results may be obtained on the now available tester gene stock backgrounds. Without an isogenic background or otherwise known inbred background, the original marker gene stock has to serve as the control. This would prohibit the effective use of certain markers affecting cob morphology such as ramosa on chromosome 7. It is at this point that a more comprehensive cytogenetic and phenotypic analysis of the origin and evolution of maize may commence.

Walton C. Galinat

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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