A new mutator system?
--Krivov, NV

Test-crossing of sh wx marker stock (as ear parent) with stocks isolated in the progeny of the heterochronic mutant Cg2 resulted in Wx-m mosaics in the F1 among the Wx/wx/wx kernels. The Wx-m kernels had single brick-red specks in a dark blue background. The frequency of Wx-m kernel origination in two experiments was 13.45 x 10-2 and 9.0 x 10-2 on average for families (MNL 1992). In F2 Wx-m kernels appeared with a frequency of 5.25 x 10-2. Reliably significant deviations from the theoretically expected ratio of 3:1 were observed in some families. In total 64,168 kernels with the Wx/wx/wx genotype were examined, Wx-m kernels being revealed in 4099 of them, i.e. the frequency of appearance of these kernels averaged 6.4 x 10-2. However, in individual families, the mutation frequency reached 29.0 x 10-2 and even higher.

The origination of Wx-m kernels in such a frequency and the pattern of the distribution of the mutant wx sectors is, as a rule, associated with the excision of the transposable element (TE), located proximally to the Wx locus in the chromosomes introduced from the plants tested. Therefore, the segregation of the Wx/wx heterozygotes is accompanied not only by the origination of Wx-m kernels but significant deviation from the ratio of 3Wx :1wx.

Saturation crossings, in which the wx sh marker stock was used as an ear parent, were carried out during recent years. As a result, a segment of the 9th chromosome, located in the nearest proximity to the Wx responsible for the appearance of Wx-m kernels, was introduced into the genetic background of the marker stock that did not contain complete copies of the regulatory elements (MNL 1995). Crossings with the test stocks a-dt(Dt), c-m(Ac), a-m(En), a-ruq(Uq), and a-mrh(Mrh) have been performed this year. The test results appeared to be negative for all five mutator systems Dt, Ac, En, Uq and Mrh. In other words, the sub-stock carrier of the Wx allele does not contain complete copies of the regulatory elements Dt, Ac, En, Uq and Mrh any more, but the Wx-m kernels continue to appear with a frequency of 10-2 in most families.

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