The small pollen of teosinte is coadaptive with the more slender, thin-walled styles of teosinte

There has been a question as to why modern teosinte, that is sympatric with maize, has pollen that is smaller than that of modern maize. Because of this fact, one group of researchers (P. C. Mangelsdorf, E. S. Barghoorn and U. C. Banerjee, 1978) claim that the large size of a few very ancient pollen grains proves that the ancestor of cultivated maize is a wild maize and not teosinte based on this one trait. They overlook the possibility that the large pollen could have come from a raw tetraploid of teosinte, as suggested by G. W. Beadle. In any case, the problem remains as to why the teosintes in contact with maize have small pollen while the teosintes more isolated from maize such as the Guatemala and Honduras races have pollen about the same size as that of primitive corn as represented by the race Chapalote.

It was previously known that at lengths greater than 2.5 cm in maize styles, the maize pollen tubes rapidly outgrow those of teosinte (Aquirre-Gonzalez, 1977). But the problem of teosinte protection from swamping by maize involves tube competition within teosinte styles. Preliminary studies of the teosinte styles by Dr. Chandra Pasupuleti indicate that they have fewer cells with thinner walls than maize styles. Therefore, there was reason to believe that the pollen competition in the two kinds of styles might have different results. Research plans include photographing the pollen tubes from the two sizes of pollen as they penetrate and grow in teosinte styles. A fluorescent stain (Martin's aniline blue) and a fluorescence microscope would be used.

In order to test out the possibility that the thinner pollen tubes from the smaller pollen of teosinte might worm their way more rapidly between the thinner smaller cells of the teosinte styles than the thicker tubes from the larger pollen of maize, we used the standard technique of pollen mixtures with only one stock carrying genes for aleurone color. In this case, a strain of Hopi flour corn with purple aleurone was used as the corn pollen and some of the teosinte's own pollen as the colorless teosinte source. The teosinte has a light brown pericarp which made it necessary to score the kernels for aleurone color on a fluorescent light table. Reserve samples of each mixture of pollen used were saved to eventually plot out the frequency distribution of large vs. small pollen that actually went into the mixture. We tried to get a 50-50 mixture of the two types of pollen but because of their unequal size, this may not have been achieved. In any case, the results obtained with fertilization rates will be compared to the pollen size distribution used. At this writing, we have not had time to plot out the distribution of pollen diameters for the various mixtures used and so only the kernel data are given here as follows:
 

Date

No. Spike Clusters

Aleurone color
None Purple
8/23 4 23 8
8/27 18 154 8
8/30 3 33 2
8/31 7 134 56
9/1 5 35 8
Total 37 379 82
Expected -- 236 236

If the pollen mixture was approximately 50% corn and 50% teosinte, it is seen that there is a great excess of fertilization on teosinte styles by teosinte pollen. This is about the opposite to that which might have occurred had the same pollen mixture been placed on corn styles.

We are postulating that the small pollen and slender styles of teosinte that is sympatric with maize evolved as a protective device within teosinte to reduce the possibility of swamping by maize.

The teosinte kernels were cut from their fruit cases and scored for aleurone color by Mrs. Josephine Starbuck.

W. C. Galinat, J. S. Starbuck and C. V. Pasupuleti


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