A miniature fruit-case type of teosinte as the wild ancestor of the first maize

A small rachis segment is expected in the connecting link between Andropogoneae and Maydeae in order to provide a smooth gradation in condensation level from Andropogoneae grasses such as Manisuris toward primitive maize such as the oldest Tehuacan cobs. In Manisuris the rachis segments are about 5 mm long and extremely slender with a cupule-like structure formed by a sterile pedicel positioned along one margin of the internode. Approaching the Tehuacan cob, a rachis segment about 3 mm long is expected, such as occurs in the smallest known fruit cases of teosinte. Any further condensation or reduction in the length of rachis segment results in a pinching off of the plastic primordia into additional ranks located in an area on the rachis with space available for further differentiation (as in Tehuacan cobs). Such 3 mm or miniature fruit cases are known in teosinte both archaeologically and at the low end of variation within present-day teosinte. C. Earle Smith, Jr. has found the carbonized remains of such tiny fruit cases from two sites (San Jose Magote and Tomaltepec) in the Oaxaca Valley that are dated about 3000 to 3400 B.P.

The apparent absence of teosinte in the premaize strata of ancient man's shelters and dump heaps may be explained by the difficulty in recovering and recognizing miniature sized fruit cases. A fine mesh screen such as is used to separate out Setaria seed is necessary for their recovery. The highly fragmented remains of carbonized teosinte fruit cases from the time frame of 8000 to 10,000 years ago could be easily overlooked in the deposited fill or lost in open camp sites. Some of the intermediate products of incipient domestication may have been carried into isolated areas such as the Koster Site where these "missing links" wait to be discovered and, thereby, to document the transformation. Although present-day teosinte does tend to carry the early domestic trait for spike clustering, natural selection rapidly eliminates the other more harmful domestic traits.

Teosinte with tiny 3 mm long fruit cases was found during Beadle's "teosinte-hunt" near the village of Mazatlan in Guerrero in 1971 and then again near El Salada in 1972. In contrast, Chalco teosinte which coexists with maize in the Valley of Mexico has long (8.5 mm) thick fruit cases, partly because of its coevolution with the large-kerneled maize, partly to accommodate introgression from maize that carries factors for large kernels and, thirdly, to promote mimicry of corn during early vegetative growth. Jutiapa teosinte from Guatemala also has a long (8.0 mm) but more slender rachis segment as does Honduras teosinte, with a rachis segment 9.5 mm long.

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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