Pollen competition and heterosis

Preliminary studies of the relationship between pollen competition and heterotic patterns were carried out in 1985. White endosperm versions of A632 (stiff stalk) and A619 (Lancaster), designated K304 and H104 respectively, were pollinated with a mixture of pollen from a white version of Mo17 (Lancaster) designated K303 and a yellow B73 (stiff stalk). Tassels were bagged before shedding began in the morning and taken down at 11:00 a.m. - about 2 hours after shedding began. The mixture was made on a volume basis with 2 ml of each type. No attempt was made to examine pollen size or number of grains per volume. The same mixture was used to pollinate 5 ears of K304 and H104. Mature ears were measured and divided into 4 sectors based on the total length. The total number of kernels was counted in each sector as was the number of yellow kernels. Data are presented as percentage of yellow kernels. Assuming a one to one mixture of viable pollen X2 values were calculated to estimate deviation from expected for each sector and the total for each ear.

The percentage of yellow kernels in an entire ear was not significantly different from 50% for any of the H104 ears (Table 1). Only one sector of the 20 H104 sectors was significantly different from 50% yellow. The tip sectors of H104 were very close to 50% yellow while the butt sectors appeared to have fewer yellow kernels. However, the percentages of yellow kernels in the butt sectors were not significantly different from the expected. When H104 was the ear parent no clear competitive differences between the two types of pollen were observed despite the slightly higher frequency of white kernels at the butt end of the ears.

Pollen competition is indicated by the data from the crosses when K304 was used as the ear parent. Three of the five totals deviate significantly from the expected one to one ratio. Another has a high but nonsignificant X2 value. The fifth total is close to a one to one ratio due to unusually high numbers of yellow kernels in sectors III and IV. As we do not know the actual ratio of pollen grains from the white and yellow parents the totals alone do not indicate competition between the two types of pollen. If the mixture was a one to one ratio numerically and pollen competition was occurring, we would expect to see one to one ratios of yellow and white kernels at the tips of the ear with progressively more of the competitive type (in this case white) toward the butt. Such a pattern is obvious in the data from the K304 ears. The tip sectors while variable in ratio are not significantly different from a one to one. In contrast, all five of the butt sectors have an excess of white kernels. Four of the five butt sectors are significantly different from expected. The fifth X value was not significant due to a low number of kernels (33) in that sector. Sector II consistently has more white kernels than sector III and IV, while sector III is not very different from sector IV, thus indicating that competition by the white-endosperm parent is enhanced on K304 silk beyond the half way point of the ear. This observation was made on mature dry ears, and how it relates to silk length at pollination is not known.

When H104 (Lancaster) was the ear parent no competitive advantages were evident for pollen from either the stiff stalk (B73) or Lancaster (K303) pollen parent. However, the Lancaster pollen parent had a clear advantage when K304 (stiff stalk) was used as the ear parent. Based on the pedigrees it is expected that plants from the union of K304 and K303 will be more productive than those from the union of K304 and B73.

These studies were limited due to inadequate seed supplies. They will be expanded to include all possible combinations of pollen and ear parents including self pollen. Investigations have begun on factors that may be involved in these competitive differences.

White endosperm inbreds were supplied by Clyde Wassom of Kansas State and Paul Crane of Purdue.

Table 1. Percentage of yellow kernels on ears of H104 and K304 pollinated by a mixture of B73 (yellow) and K303 (Mo17 white) pollen.

W. F. Tracy
 
 


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