Diversification of U.S. grain hybrids away from B73-Mo17 with some new hybrids derived from adapted exotic races
--Galinat, WC

If we are to be confined by the GEM (germplasm enhanced maize) objective of increasing diversity within the Northern Flint-Southern Dent pattern of heterosis by using Mo17 and B73 related inbreds as the recurrent parents for introgressing unknown genes from their list, LAMP (Latin American Maize) alien germplasm, there are tactical problems.

If we identified ahead of time what we are going to transfer we could do it and maintain the prescribed pattern of heterosis, especially now with the tool of biotechnology. But this would usually involve just single gene transfers and not really increase the diversity much in modern hybrid feed grain, in contrast to silage hybrids grown mostly for vegetal material and sometimes of diverse tropical pedigrees.

If the genetic diversity and different heterotic patterns represented by certain obsolete races of gigantic maize still available in germplasm banks is adapted to modern agriculture and made competitive with B73-Mo17 related hybrids, the U.S. grain hybrids will have gained the diversification that authorities claim we need to cope with sudden changes in the environment, including problems with water, weeds, insects and diseases.

The important proposal here is to adapt certain of the ungainly giant, now obsolete races of Latin American maize such as Jala, Oloton and Montana for use in modern U.S. maize agriculture by the transfer to them of a new semi-dwarf gene designated here as rd3. The rd (reduced) symbol is used because of rd3 similarity to the phenotype of rd1 and rd2 (peewee) genes discovered by Singleton in C30 inbred sweet corn but non-allelic and not as potent. While rd1 reduces plant height to about 1/3 of normal, rd3 has a weaker reduction to only about 1/2 of normal. The rd3 gene appeared in a 10 foot tall line of Havel's Dent (JHLE) reducing the plant down to a reasonable height of 5 feet. The height reduction is due to shorter internodes at the base of the plant. During the early growth period of reduced elongation of internodes, root development is enhanced. This increased root development may provide a degree of drought tolerance and even some Roundup herbicide tolerance.

The rd3 gene does reduce leaf and tassel size, both of which help in adapting to high population densities. It may be helpful to also modify the tassel with the ub (unbranched) gene and then restore some of the tassel with a recessive tassel ramosa, ra-D gene. The first maize, like its wild ancestor, teosinte of now and then, must have been adapted to high population competition. It should not be all that difficult to return this trait to any maize. 

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