| The above-mentioned results on fertility and chromosome constitution indicated
a mechanism for the selective retention of the cylindrica chromosome
in the cytoplasm substitution line of Jones Fife having the cylindrica
cytoplasm. When the clyindrica chromosome exists in a monosomic condition
in Jones Fife, female gametes to which the cylindrica chromosome
is not distributed will not be successfully fertilized, and only those carrying
that chromosome will be fertilized. This selective fertilization (or sterilization)
was proved to be caused by the cylilrdrica chromosome by itself,
not by an interaction between the cyliudrica chromosome and the cylindrica
cytoplasm. Although the aborted seeds produced in the monosomic addition
line were supposed to originate mostly from egg cells lacking the cylindrica
chromosome, the presence of the cylindrica chromosome in zygotes
would not offer a sufficient condition for their normal development ; because
among the offspring obtained from self-pollination of the monosomic addition
plants were found no viable 2n=43 Plants that would have received the cylindrica
chromosome through pollen. Therefore, it follows that egg cells lacking
the cylindrica chromosome were defective ; they were not fertilized
at all or unsuccessfully fertilized to develop into abortive seeds. Selective fertilization was also seen on the male side. It is evedent from the chromsome constitutions of the selfed-offspring of the monosomoic addition line (see Table 2) that the pollen carrying the cylindrica chromosome exclusively took plart in the fertilization. It could not be ascertained whether or not the pollen grains lacking the cylindrica chromosome had lost its function ; they may have been functioning, but just unable to compete in certation with those carrying the cylindrica chromosome. Thus the cylindrica chromosome was demonstrated to be selectively transmitted to the next generations in Jones Fife through both egg cells and pollen, and, at least on the female side, it had selective gametocidal action which is independent of the cylindrica cytoplasm. The selective gametocidal action of the cylindrica chromosome, however, turned out to be dependent on the kind of common wheat into which the chromosome was introduced. The cylindrica chromosome in hybrids between Jones Fife and some kinds of common wheat (e. g., cv. Chinese Spring) lost its function and was quickly eliminated in the following generations. Genome analysis demonstrated that Ae. cylindrica is an amphidiploid with C genome from Ae. caudata and D genome from Ae. squarrosa (2n=14, DD) (KIHARA 1949). As the D genome of Ae. squarrosa has no chromosome with a subterminal centromere, the cylindrica chromosome is supposed to have derived from the C genome of Ae. caudata in which three pairs of chromosomes with subterminal centromeres are included, one of which is very similar to the cylindrica chromosome (CHENNAVEERAIAH 1960). |
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