--Walton C. Galinat
Previously I have described cob differences in cupule and rachilla development as they appear to relate to two different systems for kernel exposure by independent domestications of teosinte (Galinat, 1988). Additional evidence for this double origin of maize appears to come from two pathways in modes of branching and patterns of internode lengths leading to the northern flint and southern dent parents of the Corn Belt dent (as illustrated here in a line drawing adapted from my painting).
Habit as Evidence for a Double Origin of Maize
(A line drawing adapted from a painting by Walton C. Galinat)
The plant habits from the two pathways of corn races based on cob morphology
lend support for the double origin of corn. They form two series of plant
habits that may be termed the basal branching type and the lateral branching
type, both of which lead to the Corn Belt Dent.
1. Teosinte subspecies parviglumis race Balsas, under good growing conditions. Note the proliferation of tillers at the base of the plant.
2. The indigenous flour corn of the upper Missouri area is close to one type of the first corn, pre or proto-Chapalote. Its ear is near the ground where it comes from an elevated and condensed tiller.
3. The Northern Flint (Rhode Island Flint). The upper ear bearing node has elevated to a central position on the plant but a trail of its ascent remains.
4. Teosinte subspecies mexicana race Chalco. Note that the branching is lateral and each branch terminates in a tassel with ears borne below as on the main stalk of corn.
5. Palomero Toluqueno. This multi-eared popcorn has a barren zone both above and below the ear bearing region.
6. Southern Dent (Gourd Seed). The ear is high on the plant and tillers are suppressed.
7. The Corn Belt Dent. This hybrid between the Northern Flint and Southern Dent pathways shows extreme heterosis. It has productivity refinements indicated here as reduced tassels and erect upper leaves.
In each pathway there is a stepwise inhibition of vegetative branching
that is related to increased feminization. In the Balsas or parviglumis
pathway leading to the northern flints, the branching and its feminizing
inhibition is basal. The gt (grassy tiller) gene in maize increases
maleness and immediately restores the plant habit of Balsas teosinte characterized
by basal branching. In the Chalco pathway leading to the southern dents,
the branching and its inhibition by feminizing is lateral. The tb (teosinte
branched) gene in maize increases maleness and immediately restores the
plant habit of Chalco teosinte characterized by lateral branching. In each
pathway, a polygenic trait of increased yield and branch inhibition has
gradually evolved by way of increased levels of feminization only to have
the whole sequence collapse from being undercut by either of two mutant
genes (gt or tb) that increase the level of maleness and,
thereby, promote internode elongation and branch or tiller development.
These two genes differ only in timing of their action, which determines
whether the branching will be basal (gt) or lateral (tb).
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