Among the diploid offspring, 101 plants were selected for observation of their fertility. As shown in Table 1, 63 became fully fertile (average seed fertility 97%), and 38 were partially fertile (average 26%), The former group was assumed to have the genotype Rfv1Rfv1, while the latter Rfv1rfv1 ([4] and [5] in Fig. 1, respectively). Some of them were cross-pollinated with pollen of CS, and the occurrence of haploids was examined. In the offspring of the fully fertile plants, no aphloid was found among 752 plants, while 34 haploids (3.5%) were obtained in the offspring of the partially fertile plants (Table 1).

From the results given in Table 1, it is evident that the presence of rfv1 allele, i.e., 1B/1R translocation chromosome in the female parent is a necessary condition for haploid production, because even a single haploid was not found among 1,105 offspring of Rfv1Rfv1 homozygotes. On the contrary, haploids were obtained at variable frequencies (3.5 to 36%) in the offspring of Rfv1rfv1 heterozygotes.

All these facts point to a fact that only the egg cells carrying the 1B/1R chromosome can develop into the haploid embryos. This directly results in an increase of the relative frequency of 1B (=Rfv1)-carrying egg cells for fertilization over 1B/1R (=rfv1)-carrying egg cells. Thus, much higher frequency of Rfv1Rfv1 homozygotes over Rfv1rfv1 heterozygotes was resulted in the diploid offspring of the heterozygotes crossed with the homozygote (Rfv1Rfv1)'s pollen (rf. the first row of Table 2).

If the rfv1rfv1 homozygotes (=homozygote of 1B/1R translocation) can be produced, we should expect much higher frequency of haploids in its offspring. The diploid partner of the diplo-haplo twins, which are obtained from a cross, (kotschyi)-Rfv1rfv1 x Slm, should have this genotype, because the chromosome complement derived from the female side must have the 1B/1R chromosome. We already obtained this type of plant, getting haploids (including diplo-haplo twins) at a frequency of as high as 75%: This result will be published elsewhere.

The present result also indicates that a second gene is involved in haploid induction, because two classes of the Rfv1rfv1 heterozygotes, i.e., [3] and [5] in Fig. 1 and Table 1, showed a remarkable difference in the frequency of haploids. Undoubtedly, Slm carries the second gene for haploid production, of which chromosomal location must be investigated in a future work.

Literature Cited

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