Small pollen in modern teosinte as a secondary adaptation for self-fertilization

Accepting the evidence that the fossil Zea pollen of 80,000 years ago was large, then its size would seem to be of independent origin from the large pollen now found in long-eared corn. The present-day evidence indicates that large pollen has a selective advantage over small pollen when the competition is within long styles. The selective pressure in wind pollinated plants that are widely dispersed is for a large shed of small pollen. In contrast with a localized dense stand, relaxed selection for small pollen would result in an increase in pollen size. On this basis, one might expect pollen that was larger than average in a perennial diploid of teosinte. We shall soon have an answer on this aspect since such a perennial diploid has recently been discovered in Mexico (Iltis, in press). The perennial tetraploid teosinte does not have unusually large pollen.

If corn came from teosinte, we must then explain how most of the modern teosintes evolved pollen smaller than that of the ancient teosinte. It seems possible that small pollen has a competitive advantage over large pollen in short styles such as occur in teosinte while the reverse situation occurs in the long styles of corn. In short styles, the thinner tubes derived from small pollen may be able to penetrate more rapidly through the stylar tissue and, thereby, win in a short race. But in long styles, there is an accumulative lag in the descent of the organelles in the thin tubes resulting in a delayed fertilization. In contrast, the thick tubes from the large pollen of corn allow the organelles to successfully descend with the growth of the tube.

As corn and teosinte diverged during domestication, teosinte would become threatened with extinction by swamping with the domestic ear traits. There would be a selective advantage to small pollen that could win out in a short stylar race to the teosinte eggs. In certain isolated teosintes that did not either coevolve or coexist with corn as apparently with Jutiapa teosinte, the pollen remained moderately large or about the same size as that of primitive corn (Chapalote).

In testing these hypotheses, mixtures of corn and teosinte pollen are applied to short and long styles in which the fertilizing success of the larger corn pollen is marked by purple aleurone color. For style types it would be preferable to use teosinte where the competition is presumed to have taken place. From a practical point of view, the styles of corn may be shortened to about an inch in length, as is done in making corn-Tripsacum crosses, and the mixture of corn and teosinte pollen applied.

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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