| On the genetic mechanism of haploid induction in the
cytoplasm substitution lines of common wheat Mitsue KOBAYASHI and Koichiro TSUNEWAKI Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Kyoto, Japan Triticum aestivum strain Salmon is a hexaploid derivative of the octoploid Triticale (TSUNEWAKI 1964). This strain is different from ordinary common wheat in several respects: It has a 1B/1R translocation, in which the short arm of 1B carrying a nucleolar organizer is replaced by an arm of 1R chromosome of rye (ZELLER 1973), thus having only one pair of satellited chromosomes (6B chromosomes) instead of two pairs in ordinary common wheat (TSUNEWAKI 1964). Also, this strain induces haploids and twin seedlings (mostly diplo-haplo type) at relatively high frequencies when several alien cytoplasms are introduced by repeated backcrosses (TSUNEWAKI et al. 1968). So far, the cytoplasms of eight Aegilops species, including Ae. kotschyi, are known to induce haploids in Salmon (TSUNEWAKI et al. 1976). However, the genetic mechanism of haploid induction in this wheat has not been known yet. The present paper deals with the results of the investigation that was conducted to elucidate this mechanism in Salmon by the cytoplasm of Ae. kotschyi. Materials and Methods An alloplasmic line of Triticum aestivum cv. Chinese Spring (abbreviated as CS) having the cytoplasm of Aegilops kotschyi (2n=28, genome constitution CuCuSvSv) has been produced by MUKAI and TSUNEWAKI (1975). This line was crossed with T. aestivum strain Salmon (abbrev. Slm) or CS, or self-pollinated, according to the breeding scheme shown in Fig. 1. CS is known to carry a dominant fertility-restoring gene Rfv1 against the kotschyi cytoplasm on the short arm of 1B chromosome (MUKAI and TSUNEWAKI 1979). On the contrary, Slm lacks this gene due to replacement of 1BS arm by a raye chromosome arm, and is designated as rfv1. Genotypes of plants obtained in each generation are also indicated in Fig. 1. The selfed seed fertility was estimated from the number of first and second florets in ten normally developed spikelets, and the number of seeds set in them on spikes which were covered with a paper bag before flowering. Chromosome number was observed in root-tip mitosis, using the ordinary aceto-carrmine squash method. Results and Discussion As shown in Fig. 1, (kotschyi)-CS was crossed three times with Slm's pollen. In the B2 generation indicated by [1] in the figure, a partially fertile plant was selected that should carry a single Rfv1 gene. This plant, was self-pollinated, raising 14 offspring, of which five were haploid (Table 1). Among nine diploid offspring, seven fertile plants were selected which were assumed to be the homozygote Rfv1Rfv1 ([2] in Fig. 1). Their selfed seed fertility ranged from 55 to 88%, average being 72% (Table 1). They were crossed with pollen of Slm or some other strain. In total, 353 plants were obtained, but none was haploid (Table 1). The diploids were assumed to be Rfv1rfv1 ([3] in Fig. 1). The selfed seed fertility of 85 plants was examined, their average being 9% with the range of 0 to 41%. Of those, 82 plants were crossed with CS's pollen, and 1,214 offspring were obtained. Detailed classification of those plants is shown in Table 2: 1,154 seedlings were single seedlings, of which 246 were haploid. The remaining 60 seedlings were twins, which were either diplo-diplo, diplo-haplo or haplo-haplo type. In total, 303 seedlings (25%) were haploid, including haploid twins. Plants developed from the diploid seedlings were classified into the fertile and sterile classes, as shown in Table 2, according to their selfed seed fertility observed at their maturity, that is given in the last two rows of Table 1. Two facts must be pointed out from the results: First, the diploid, single seedlings segregated the fertile and sterile in a ratio of 842: 43, greatly deviating from the expected 1:1 ratio. Second, all diploid partners in 56 diplo-haplo twins became sterile, no fully fertile plants being recovered. The karyotype of 56 haploids was analyzed as to the number of satellited chromosomes in root-tip mitosis. The result is summarized in Table 3. All haploids, whose karyotype could be studied, had one or no satellited chromosome, none of them having two sat-chromosomes. A typical karyotype of the haploid is shown in Fig. 2. This fact indicates that haploids were derived only from the egg cells carrying 1B/1R translocation chromosome, but not from those with the normal 1B chromosome. |
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