In 1979 diploid and tetraploid perennial teosintes were grown from seed collected in the state of Jalisco and established in the field nearby Chapingo, state of Mexico. Pollen was collected on an individual plant basis and the size of 100 individual grains per plant was measured. For each grain two measurements were obtained, the largest diameter and that of the diameter perpendicular to the middle point of the former. The average value of these two diameters, in micrometers, was considered as the size of a particular pollen grain.
The results given in Table 1 show some differences in average grain size among plants, within both types of teosinte. However, it seems that there is practically no difference in grain size between the two populations. The overall average difference is only of 1 µm and there is a great deal of overlapping in the size frequency distribution. Furthermore, two tetraploid plants produced smaller average grain size than the smallest average found among diploid plants.
These results are one more exception to the general rule that doubling of the diploid chromosome number increases pollen grain size. It is known that maize and annual teosinte, when artificially tetraploidized, form much larger pollen grains than the parental plants. Therefore, it would seem as if, in the case of the perennial teosinte, once the autotetraploid was originated natural selection had favored the smaller size, until the original size of the pollen grains of the ancestral diploid plants had been reached. At present it would be difficult to explain, with certainty, what kind of advantage was conferred to the tetraploid populations by this change in pollen grain size. Some experiments trying to answer these questions have already been started in our laboratory at Chapingo.
T. Angel Kato Y.
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