The first observation of a factor on the short arm of chromosome 4 that would transform the female spikelet-rachis architecture and increase the glume induration of maize into that of teosinte was made by Mangelsdorf and Reeves (Tex Agric Exp Sta Bull 574:1-315, 1939) in terms of their ill-fated hypothesis of an origin of teosinte by Tripsacum introgression into maize. The position as close to the Su-su locus was confirmed by Rogers (Genetics 35:541-558, 1950). Although a research associate of Mangelsdorf at the time over 33 years ago, Galinat (Econ Bot 17:51-59, 1963) made a unique series of drawings, reproduced here in Figure 1, illustrating this transformation only in the reverse direction starting with Tripsacum and teosinte and terminating in modern maize, with intermediates reconstructed from experimental known introgression. Apparently the significance of the sequence concealed in the beauty of my art work escaped Mangelsdorf at the time. Recently a genetic symbol of Tga (teosinte glume architecture) was assigned to this chromosome 4S factor by Dorweiler et al. (Science 262:233-235, 1993). Although useful, the symbol is inaccurate because the change in architecture is the upward orientation of the whole spikelet - not just the outer glume - except for the degree of outer glume induration. Furthermore, the direction of the evolution is now accepted as from teosinte to maize so that I have coined the symbol of msa (maize spikelet architecture) for the recessive mutant allele leading toward maize because the genetic effect of a recessive gene is the traditional basis for coining a new symbol rather than the original dominant wild type represented here by teosinte.
Since all known races of extant maize carry the msa allele and all teosinte species the Tga allele, this is an important key trait for their separation. From the oldest known archaeological cobs from Tehuacan, Mexico, to modern cobs, the msa allele causes the female spikelet to be reflexed down to a right angle position from the rachis, the outer glume to have reduced induration and the rachilla elongate, all of which combine to expose the grain and make it threshable. Rather than by mutation of teosinte's Tga allele to maize's msa allele, similar phenotypic changes could have been made, in a more unstable manner, by the powerful modifying action upon Tga expression of a different gene, an intermediate tunicate allele down on the long arm of chromosome 4 (Beadle, Field Mus Nat Hist Bull 43:2-11, 1972; Mangelsdorf and Galinat, Proc Nat Acad Sci USA 51:147-150, 1964) but this course of evolution now seems to be ruled out in the origin of maize.
Figure 1. Transitional specimens in longitudinal view extending from Tripsacum (1), teosintes (Florida 2, Nobogame 3), a maize-teosinte hybrid (4) to their homologues in modern maize (8) created by experimental introgression of known teosinte segments into inbred A158 modern maize (5,6,7). The Tga gene is manifest in specimen 5 while its allele msa in specimen 8, as described in the text. Specimen 6 (teosinte chromosome 1) has a reduced rachilla; Specimen 7 (teosinte chromosome 9) an elongate cupule increasing ear length. Solid black indicates teosinte effects. (drawing by W. C. Galinat from Economic Botany 17:51-59, 1963.)
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