Robinson, R. W.
It was noted several years ago that occasional fruits were produced by male sterile plants grown in isolation from male fertile plants and most of these fruits contained seeds. Originally it was assumed that these seeds were the results of pollination by insects with pollen from plants dominant for ms genes. However, open-pollinated seeds produced by ms\10\ms\10 plants grown in isolation were found to segregate for male sterility, so cross polinaton can not account for all seeds produced by male sterile plants.
Seedling marker genes were used in a further study of seed production by male sterile plants. Plants from a backcross generation (Rick's family LA 321) which were recessive for a\1 and ms\17 and heterozygous for d\1, c, and l\1 were grown in isolation and open-polinated seed was harvested. A population grown from seed of one open-pollinated fruit (from approximately 2,000 flowers) gave the following segregations: 1+ : 81 a\1, 63+ : 19 c, 63+ : 19 d\1, 64+ : 18 l\1, and 43+ : 39 ms\17. The one plant dominant for a\1 was also dominant for the other four genes, as would be expected if it were produced by cross pollination. The other plants segregated for the three marker genes the parent was heterozygous for in agreement with a 3:1 ratio, as would be expected for self pollination. The male sterile gene, however, appeared to segregate in a 1:1 ratio. Segregation in agreement with that observed would be expected if all the fertile pollen of the flower these seeds developed from were derived from a single cell which mutated prior to microsporocyte formation. If there were a mutation in an archesporial or sporogenous cell--either the mutation of a dominant gene which restores fertility to plants recessive for the male sterile gene or the reverse mutation of ms\17--the fertile pollen would segregate 1:1 for the mutant gene and would fertilize the female gametes, which would be recessive for the mutant gene, to produce seed which would segregate one fertile: one male sterile. If reverse mutation occurred, the progeny would be expected to exhibit linkage between male sterililty and the lutescent gene; repulsion phase linkage if reverse mutation occurred on the chromosome bearing the lutescent gene while coupling phase linkage would be expected if the reverse mutation occurred on the chromosome with the dominant allele for l. It is unlikely that the hypothetical restorer gene would be on the same chromosome, so linkage would not be expected if this gene mutated. The following data reveal that good fits to the expected 3:1:3:1 ratio were obtained between ms\17 and d and c, but linkage occurred between ms\17 and l\1 indicating that Thte seeds-were produced by reverse mutation of the male sterile gene.
Contingency
Combination + + + tester ms\17 + ms\17 tester chi-square
ms\17-d\1 31 11 31 8 0.4
ms\17-c 33 9 29 10 0.2
ms\17-l\1 39 3 24 15 11.5**