The biphyletic hybrid vigor of the Corn Belt Dent

--Walton C. Galinat

I have described in last year's News Letter how the races of maize may be divided up into extreme differences in pronounced cupule with elongate rachilla development on the one hand and reduced cupule with short rachilla development on the other and how these cupule-rachilla associations probably stem from two systems for kernel exposure during two independent domestications from teosinte.

Hybridizations between members from the two domestic pathways have occurred repeatedly over time yielding productive new races such as Tuxpeno and the Corn Belt Dent. On inbreeding and/or backcrossing to one parent or the other, the hybrids yield varieties and inbreds which may be used to reconstruct the heterosis of the hybrid, such as Reids Yellow Dent/Southern Dent (inbreds Wf9, B73) crossed by Lancaster/Northern Flint (inbreds C103, Mo17).

Since my hypothesis was presented, I have rendered in a drawing of ear cross-sections, the highlights of 8000 years of evolution leading to biphyletic hybrid vigor in the Corn Belt Dent. This is presented here in 11 figures described as follows:

Fig. 1. On the lower left, there is kernel exposure from Guerrero (Balsas) teosinte by rachilla elongation with teosinte's cupule still unchanged.

Fig. 2. With the addition of two more traits, a pairing of female spikelets and a switch to multiranking, the first cob of Tehuacan corn was evolved.

Fig. 3. There was an increase in cob diameter together with an increase in the level of ranking represented here by a 12-rowed ear of Chapalote.

Fig. 4. When the larger supply system of the Chapalote ear is combined with the less demanding low-level ranking, the surplus photosynthate may be either deposited in the form of cob induration or kernel enlargement.

Fig. 5. Large kerneled eight-rowed corn arose independently in wide-spread areas from the Northern Flints of New England (A.D. 1300) to the giant Cuzco corn of Peru (A.D. 1500).

Fig. 6. Meanwhile a parallel sequence starting with the domestication of Chalco (Central Plateau) teosinte was based on a different system of kernel exposure by cupule reduction.

Fig. 7. A hypothetical race of eight-rowed corn, almost without cupules and with short rachillae, that is similar to the Peruvian race, Confite Morocho, known from archaeological remains about 4000 B.P. (before present) at Ayacucho (see Fig. 11).

Fig. 8. Palomero Toluqueno is an ancient indigenous race from the Mexico City area that is considered here to have evolved soon after domestication of Chalco Teosinte. It has reduced cupules, short rachillas and elongate kernels.

Fig. 9. Pepitilla is clearly related to Palomero Toluqueno. It is ancestral in varying degrees to the Southern Dents, especially to Shoe-Peg with its narrow deep kernels and to Gourd-Seed with its broad, deep kernels. These races all have reduced cupules and short rachillae.

Fig. 10. The Corn-Belt Dent is known to be a hybrid between the Northern Flint (5) and Southern Dent (9) pathways.

Nothing in the evidence from isozyme variation or DNA sequencing presented so far rules out my hypothesis of a biphyletic origin of maize by two systems for kernel exposure. The patterns of isozyme variation between Andean maize and the Mexican races of Pepetilla and Palomero Toluqueno as well as Chalco teosinte should be compared.

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