| In order to determine the chromosomal location of a fertility-restoring
gene(s) of Chinese Spring for the MAAN's mutica cytoplasm, male sterile
(mutica M)-Salmon was crossed as female to ditelo-1BL and 1BS of
Chinese Spring. In the F1 generation, the following three types
of plants were produced ; monotelo-disomics, haplo-diplo twins and haploids.
The mechanism of the haploid induction in (kotschyi)-Salmon was clarified by KOBAYASHI & TSUNEWAKI (1980) : Salmon possesses a chromosome consisting of an arm of rye's 1R chromosome and the short arm of chromosome 1B of common wheat (ZELLER 1973). This translocation chromosome is written as "1B/1R". All female gametes of (kotschyi)-Salmon carrying the 1B/1R chromosome form the haploid embryo, giving rise to the haploid or haplo-diplo twin pair. Thus, this translocation chromosome is not transmitted from the female parent to the offspring as a rule. Accordingly, (kotschyi)-Salmon always remains heterozygous for normal 1B and translocation 1B/1R chromosome through many generations of successive backcrosses with the pollen of Salmon. If the same mechanism is operating for the haploid induction in (mutica M)-Salmon, the F1 progeny between (mutica M)-Salmon and the ditelo-1BL or 1BS of Chinese Spring are expected to segregate the various plant types shown in Fig. 2. Selfed seed and pollen fertilities observed in each plant type are given in Table 1 : Diploid partner of the n-2n twins is presumed to have the 1B/1R translocation chromosome, because it derives from a synergid having a karyotype identical with that of the egg cell. Consequently, the diploid partner of the n-2n twins from the cross, (mutica M)-Salmon x Chinese Spring ditelo-1BL (Plant B in Fig. 2) has no 1BS arms carrying a satellite. Complete male sterility of the 2n partner of the twin, in contrast with partial male fertility of the type A plants indicates that a fertility-restoring gene for MAAN's mutica cytoplasm is located on the short arm of the 1B chromosome of Chinese Spring. The same conclusion is drawn from normal fertility of the type G plants, contrasting with low fertility level of the type A plants. To this fertility-restoring gene, a symbol, Rfm1, will be tentatively given. This gene restores little fertility under its hemizygous condition because diploid F1's between (mutica M)-Salmon and Chinese Spring ditelo-1BL (Plant A) had very low fertility. The fertility of the diploid plants (Plant D) from the cross, (mutica M)-Salmon x Chinese Spring ditelo-1BS is lower than that of the diploid F1's between (mutica M)-Salmon and normal Chinese Spring (Plant G). This indicates that the long arm of the chromosome 1B of Chinese Spring also concerns with fertility restoration, i.e., it carries a promoter. However, this gene itself can not restore any fertility because the plant carrying two chromosomes, 1B/ 1R and 1BL (Plant B) is completely sterile. The diploid partner of the n-2n twins (Plant E) from the cross, (mutica M)-Salmon x Chinese Spring ditelo-1BS was completely male sterile, probably due to an additive effect between the hemizygous conditions for both Rfm1 gene and a promoter on 1BL arm. Both n-2n twins and single haploid showed somewhat weak vigor and complate sterility. |
| <-- Back | | | --> Next |