| Midget chromosome found in common wheat cv. Chinese
Spring with rye cytoplasm N. NAKATA, Y. YASUMURO and M. SASAKI Faculty of Agriculture, Tottori University, Tottori, Japan In common wheat, Triticum aestivum cv. Chinese Spring (In abbreviation CS) (2n = 6x = 42) with Secale cereale cytoplasm, we found a diminute chromosome compared with wheat chromosomes and named it "midget chromosome". CS with cereale cytoplasm [(cer)-CS] was developed by means of the successive backcrosses of CS to the rye-wheat amphidiploid (T.Lelley personal communication) and has been maintained in our laboratory by the backcrosses of CS or self-pollination since 1974. The seeds of this line were kindly provided by Dr. T. Lelley, Gottingen University. We report here on the morphology, role, transmission and origin of this chromosome. The midget chromosome was observed at both mitosis of root tip meristematic cell and meiosis of pollen mother cell (PMC) of (cer)-CS. At mitotic metaphase of root tip cells, the size of the midget chromosome was about half of the satellite region of wheat chromosomes 1B and 6B (Fig. 1A). At meiosis of PMC the midget chromosome synchronously behaved together with the normal wheat chromosomes and was attached by the spindle fibers (Fig. 1B). Judging from attaching position of the spindle fibers, the midget chromosome is telocentric or acrocentric. In (cer)-CS having two midget chromosomes, the midget chromosomes paired in 10 out of 35 cells observed. In (cer)-CS, no plant without midget chromosome was obtained and the plants obtained had one or two midget chromosomes. (cer)-CS showed weaker plant vigor than the normal CS, i.e., CS with aestivum cytoplasm and without midget chromosome. The midget chromosome added to CS did not affect plant vigor. The effect of the midget chromosome on plant vigor of (cer)-CS was not apparent. Weak plant vigor of (cer)-CS may be induced by the cereale cytoplasm. Though there was no difference in pollen fertility between cytoplasms and among the number of the midget chromosomes, seed fertility of (cer)-CS was lower than that of CS without or with one midget chromosome (Table 1). Lower seed fertility of (cer)-CS seems to be the result of the partial female sterility caused by weaker plant vigor. The seeds of (cer)- CS segregated fully developed and shriveled ones. The percentage of the shriveled seeds of (cer)- CS with one midget chromosome was higher than that of (cer)-CS with two midget chromosomes (Table 1). No shriveled seed was produced in CS with one midget chromosome as in the normal CS. This indicates that an interaction effect between the midget chromosome and the cereale cytoplasm is critical on the seed development of (cer)-CS. Fully developed seeds of (cer)-CS germinated normally and had one or two midget chromosomes without exception, while shriveled seeds did not germinate and probably did not have the midget chromosome. By the aid of an embryo culture, however, it was possible to raise shoot from excised embryos. This shows that only the endosperm was degenerated in the shriveled seed. |
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