The 2NOR strain, described by Phillips and co-workers, was used to show that the nucleolus organizer region is the chromosomal site of DNA complementary to rRNA (Chromosoma 36:79-88). When the initial crosses were made to establish W23 near-isogenic stocks of 2NOR and 1NOR, usually no seed was produced whenever the 2NOR stock was used as the female parent. From 55 exact reciprocal crosses, only 33 seeds were obtained when 2NOR was the female in crosses with W23 and 28 of these were from one cross. Normal seed set resulted when the 2NOR stock was used as the male parent in exact reciprocal crosses; from the same 55 exact reciprocal crosses, 20,186 seeds were obtained when W23 was the female parent and 2NOR the male parent. The 2NOR stock could be readily selfed or sibbed. A gametophyte factor present in the 2NOR stock was postulated.
A gametophyte locus (Ga) is located in the short arm of chromosome 4 approximately 35 map units distal to su. There are three alleles at this Ga locus; Ga, Ga-s and ga (J. Hered. 66:5-9). The Ga allele is cross neutral and will accept pollen of any genotype. However, Ga-s Ga-s plants can only be fertilized by Ga-s or Ga pollen; no seed set occurs with ga pollen. Heterozygous Ga-s plants can be fertilized by ga pollen in the absence of Ga-s pollen. If both Ga-s and ga pollen are present, the Ga-s pollen has a competitive advantage over the ga pollen.
Six reciprocal crosses were made between the 2NOR stock and a su su stock. The crosses with 2NOR as the male parent produced normal seed set, but the reciprocal with 2NOR as the female parent gave no seed set. If 2NOR were Ga-s Ga-s and the su stock were ga ga, the above result would be expected. If the proposed genotypes of the two stocks are correct, then (su su x 2NOR) F1 has the genotype su ga/Su Ga-s. If Ga-s pollen has a competitive advantage over ga pollen and su and GA are linked, then selfing the F1 would produce an F2 with less than 25% su kernels. The reduction in su kernels would be a function of the percent recombination between su and Ga-s. Only 17.5% su kernels will occur in the F2 if only Ga-s pollen effects fertilization and su and Ga-s are 35 map units apart. Since the su su F2 would be either su Ga-s/su ga or su Ga-s/su Ga-s, crossing su su F2's as the female with the F1 as the male would result in only 35% sugary kernels instead of 50%.
The results of self-pollinating the (su su x 2NOR) F1 are shown in Table 1, cross 1. Data for the 15 ears were combined because the heterogeneity chi-square was 12.17 with 14 d.f. (non-significant). The expected 3:1 ratio was not observed. The data fit the hypothesis of 35% recombination between su and Ga-s and only Ga-s pollen effecting fertilization. The su su parent, therefore, was su ga/su ga and the 2NOR parent was Su Ga-s/Su Ga-s. The recombination frequency between Ga-s and su is 33.2%. Table 2 shows the results of crossing su su F2 plants by the F1.
The hypothetical genotypes of the su su F2 would be either su ga/su Ga-s or su Ga-s/su Ga-s, and the genotype of the F1 is su ga/Su Ga-s. The data were combined because the heterogeneity chi-square was 10.48 with 8 d.f. (non-significant). The data do not fit the expected 1:1 ratio. However, the data fit the hypothesis of 33% recombination (being derived from F2 data) between Ga-s and su and only Ga-s pollen capable of fertilization. The recombination frequency between su and Ga-s obtained from the data in Table 2 is 31.7% (1198/3780 x 100), which is close to the value calculated from the F2 data. The results suggest that the 2NOR stock is most likely Ga-s Ga-s.
Tests were conducted on the W23 near-isogenic stocks of 2NOR and 1NOR to determine if the Ga-s allele was inadvertently selected in the development of these stocks. The results are presented in Table 1, crosses 2 and 3. The expected 3:1 segregation for su was obtained in the F2's of both su su x W23 (2NOR) and su su x W23 (1NOR) This indicates that the isogenics are probably ga ga at the chromosome 4 gametophyte locus.
Additional crosses will be made to determine if the Ga-s allele in the 2NOR stock is allelic to the previously described Ga-s allele in chromosome 4. The 2NOR Ga-s allele likely is allelic to the previously described Ga-s allele since the map distance from su is approximately the same.
T. J. McCoy, R. L. Phillips and P. J. Buescher
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