Failure to verify the Gag locus on chromosome 4

J. Jimenez and O. E. Nelson (J. Hered 56:259-263, 1965) described a new gametophyte factor isolated from the popcorn variety White Rice. Their data showed that the factor was located on chromosome 4 but was not an allele of Ga1, which has been placed at position 71 in the short arm, 28 crossover units to the left of su1. The new Ga factor showed 30% recombination with su1 but assorted independently of Ga1. The locus was designated Ga9 and placed 30 units to the right of su1. The Gag allele was described as interacting with Ga1 alleles--Ga9 pollen had a competitive advantage over ga9 pollen on silks carrying the Ga1-s allele.

In conjunction with studies on cross-sterility in popcorn, we made several tests designed to place Ga9 relative to other chromosome 4 loci. su is in the short arm but very close to the centromere, and 30 units to the right of su would place Ga9 near position 101 on the long arm. C2 has been placed at position 123 and might reasonably be expected to show linkage with Gag. Pollen from ga1 Su Ga9 C2/ ga1 su ga9 c2 plants was used to pollinate ears homozygous Ga1-s su ga9 c2 and ears homozygous ga1 su ga9 c2. Since Gag pollen has a competitive advantage over ga9 on Ga1-s Ga1-s silks, crosses to this tester gave a reduced frequency of sugary kernels, 5504 Su:1647 su (23.0% su). Assuming no ga9 gametes functioned, 23.0 is a measure of the distance between su and Ga9. Ga9 pollen does not have a competitive advantage on ga1 ga1 silks, and the crosses to this tester did not give a reduced frequency of sugary kernels, 753 Su:956 su (55.9% su). The difference in frequency of sugary kernels on the two testers verifies the presence of a segregating Ga factor in the test stock.

The aleurone pigmentation intensity was quite variable in the above crosses, and only the non-sugary kernels could be classified for C2. The frequency of c2 kernels in crosses to the Ga1-s Ga1-s tester was 46.9% (2534/5404) and in crosses to the ga1 ga1 tester was 53.0% (399/753). The frequency of c2 kernels was slightly less on the Ga1-s Ga1-s tester, but the Ga9 and C2 loci assorted nearly independently.

Three chromosome interchange stocks with breakpoints at 4 cent, 4L.30, and 4L.63 were crossed to a Gag stock, and the plants heterozygous for the interchange were used as males in crosses to plants homozygous Ga1-s ga9: Ga1-s N ga9 x ga1 T ga9/ga1 N Ga9. The ears on plants from these crosses were scored for ovule abortion (semi-sterility). The percent semi-sterile ears (T ga9 crossovers) for each interchange was: 4 cent - 23.1%, 4L.30 - 26.6%, and 4L.63 - 45.7%. The distal breakpoints were less closely linked with the Ga factor than the centromere breakpoint.

Jimenez and Nelson hypothesized Ga9 on the basis of isolation of ga1 ga9 gametes from Ga1-s ga9/ga1 Gag heterozygotes. Because of our failure to place Ga9 on the long arm of chromosome 4, a retest was made for the occurrence of ga1 ga9 gametes in the above heterozygote. Plants from the cross Ga1-s Su ga9/ga1 Su Ga9 x ga1 su ga9 were selfed and test crossed on Ga1-s su/ga1 su ears, essentially the same test made by Jimenez and Nelson. Since ga1 and ga9 assort independently, 1/4 of the gametes from the heterozygote in the above cross would be expected to be ga1 Su ga9 crossovers, which when selfed would give ears with 25% sugary kernels and when test crossed would give ears with 50% sugary kernels. The parental and all other crossover classes would give a reduced frequency of sugary kernels on either or both the selfs and test crosses. Forty-four plants were selfed and test crossed and none gave the sugary segregations expected from a ga1 ga9 gamete.

The data from our tests have failed to verify the Ga9 locus on the long arm of chromosome 4. All our data are consistent with the interpretation that the Ga action in the variety White Rice is due to a Ga1 locus allele that has action in the pollen but not in the silks. Such action is essentially that of the cross-neutral allele, Ga1. Ga1 Ga1 silks do not give a competitive advantage to either Ga1-s or Ga1 pollen over ga1 pollen, but Ga1 pollen has a competitive advantage over ga1 pollen on Ga1-s Ga1-s silks.

R. B. Ashman

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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