| Finding of a selectively retained chromosome of Aegilops
caudata L. in common wheat* T. Ryu ENDO and Y. KATAYAMA Biological Laboratory, Nara University, Nara, Japan Selective retention of alien chromosomes in common wheat was discovered with certain chromosomes of several Aegilops species, i.e., Ae. triuncialis, synthetic triuncialis, Ae. longissima, and Ae. sharonensis (ENDO and TSUNEWAKI, 1975; MAAN, 1975; ENDO, 1978). In each cases severe fertility reduction occurred in plants having such a monosomic Aegilops chromosome, and those having disomic chromosomes had normal fertility. This phenomenon was proved to be due to a selective gametocidal action of those Aegilops chromosomes on common wheat gametes: When such an Aegilops chromosome was monosomically present in common wheat, only gametes having the Aegilops chromosome could normally develop to take part in fertilization, while those without it almost completely lost their normal function. During an attempt to produce addition lines of common wheat having individual chromosomes of Ae. caudata L. (2n=14; CC), a chromosome of Ae. caudata was found to have been retained selectively in common wheat. Details of this finding will be reported here with regard to its unique breeding behavior in common wheat and effect on the fertility A cultivar of common wheat, Triticum aestivum cv. Jones Fife (2n=42; AABBDD: abbreviated as JF hereafter) was crossed as female with Ae. caudata to obtain a tetraploid hybrid (2n=28; ABDC). This hybrid showing vigorous growth and complete sterility was backcrossed to JF, producing four seeds out of 180 florets pollinated (2.2% seed set). Since one of the four B1 plants was found to have 2n=49 chromosomes with a chromosome configuration of 21"+7' at MI (consequently, the genome constitution was expected to be AABBDDC), it was employed for the further backcross program to produce the addition lines of JF. The second backcross of the 2n=49 plant to JF gave rise to a total of 62 seeds on 21 spikes. About half of the B2 seeds sown germinated (32 in total). Using their root-tips, chromosome numbers of the 30 seedlings were defermined, which ranged from 2n=44 to 49 in the following frequencies: two seedlings with 2n=44; seven with 2n=45; eight with 2n=46; ten with 2n=47; two with 2n=48; and one with 2n=49. No monosomic addition plant (2n=43) was found among them. All the B2 derivatives with 2n=44 to 2n=49 were used for the third bakcross to JF. They all showed severe reduction in crossed seed fertility: eleven plants set no seed; and the highest crossed seed fertility marked in a plant with 2n=44 chromosomes was 15.6%. By selfing only a few plants set seeds very sporadically. B3 seeds thus produced were sown and all the viable seedlings were cytologically studied to screen monosomic addition plants. A total of 22 seedlings having 2n=43 chromosomes were picked out from the progenies of eight B2 plants with various chromosome numbers from 2n=44 to 47. Interestingly, those 2n=43 seedlings without exception had a chromosome with a subterminal centromere which must have been derived from Ae. caudata (Fig. 1), for common wheat does not have this type of chromosome. Although Ae. caudata has three pairs of chromosomes with subterminal centromeres, this fact is not easily explained by a chance occurrence. Some mechanism must have operated to retain this critial chromosome selectively throughout the backcrosses so far made. An 2n=45 plant having the Ae. caudata chromosome, for exmaple, produced eight 2n=43 plants having the critical chromosome among its ten backcrossed progeny. In this respect, all the B2 and B3 plants, whose chromosome constitutions were fully analyzed, turned out to possess one, two, or three Ae. caudata chromosomes with a subterminal centromere. |
| * The work was supported in part by a Grant-in-Aid (No.236002) from the Ministry of Education, Japan. |
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