The present experiment was planned in order to estimate the back mutation rate and at the same time in order to obtain the plants free from asynaptic effect in common wheat.

The chromosome pairing in the F₁ of Chinese Spring wheat x Imperial rye (n=7) is very poor. The mutation of a gene (or genes) for asynapsis in hexaploid wheats would presumably result in very much increased pairing in the Chinese Spring wheat x Imperial rye F₁.

A dosage of 500 r of X-ray was irradiated on emasculated spikes of Chinese Spring wheat one to three days prior to pollination by rye pollen.

The anthers were fixed in 3 : 1 CARNOY'S fluid. Chromosome pairing in the pollen mother cells was studied by ordinary acetocarmine smear technique.

As is listed in Table 1, the mutation rate of the gene (or genes) for asynapsis to synapsis was 3.43%.

This supports very strongly the gene mutation aspects of the hypotheses of KIHARA and KONDO (1943) and SEARS and OKAMOTO (1956).

Since the above mutation rate is fairly high, it is highly hopeful that common wheat free from the asynaptic effect could be obtained in the not very distant future. As introduction of desirable characteristics of wild relatives of wheats into common wheat was made almost impossible due to lack of chromosome pairing between common wheat chromosomes and those from wild relatives, the loss of asynaptic effect and the consequent good pairing would make it possible to transfer the characteristics of wild relatives into wheat by ordinary crossing over between wheat chromosomes and those from wild relatives.

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