Origin of corn: pollen evidence.
The origin of Indian corn remains controversial. Its closest wild relative is teosinte, with which it hybridizes freely to produce fertile progeny. Teosinte ears are smaller and simpler than those of corn. Searches for a more likely living ancestor have failed, but nine of its assumed pollen grains have been recovered in deep drillcore samples obtained from a stratum of soil under Mexico City, which is believed to antedate man in the Western Hemisphere. These nine largest grains are indistinguishable from pollen of modern corn. It has been assumed to be that of a postulated wild corn other than that of teosinte, but this does not account for the possibility that the pollen grains are those of a tetraploid teosinte-producing pollen with two sets of chromosomes. This likelihood has been examined by treating modern teosinte plants with colchicine, which induces tetraploidy. The result has been many teosinte pollen grains indistinguishable in size from modern corn. In interpreting thrs outcome it is important also to know that heat treatment of corn and other plants induces polyploidy, and that the deep drill-core pollen was recovered in a stratum of volcanic clay indicating the high temperature known to favor doubling of corn pollen volumes.
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