Endosperms with three doses of the bz2-m responding allele and two or three doses of the Ac2 controlling element display dots of Bz tissue. With rare exceptions, the dots are uniformly small indicating the bz2-m to Bz mutation occurs late in development of the endosperm. The bz2-m to Bz mutations also occur in plant tissue when high activity Ac elements (AcAc) are present but with a reduced frequency. Again, the Bz sectors consist of a small number of cells and have been observed only in the anther wall. The question arose about the timing of bz2-m to Bz germinal mutations: is the opportunity for germinal mutation also restricted to a specific time and place?
A number of bz2-m plants of AcAc/Ac and AcAc/ac genotypes were used as male and female parents in crosses to bz2-m ac testers. A population of 21,892 kernels from female testcrosses showed only bz2-m mutable or bz2-m stable phenotypes, i.e., no Bz revertants were obtained. Reciprocal crosses produced 36 Bz kernels in a population of 8744, a frequency of 0.4% reversions in the male inflorescence. Not only does it appear that germinal mutation is confined to the staminate inflorescence but it could be localized even more precisely to a specific mitosis in the microspore. A number of Bz kernels from testcrosses of AcAc/ac or Ac/ac male parents were selected for further study. All thirteen cases analyzed proved to have the Bz allele in the embryo as well as in the endosperm and possessed either the AcAc or Ac allele from the heterozygous parent. If the bz2-m to Bz event took place before meiosis, one half of these cases should possess ac because of independent assortment of Bz on chromosome 1 and Ac2 on chromosome 8. All Bz reversions possessed an Ac allele. If the bz2-m to Bz event occurred at the second microspore division, noncorrespondence of the two sperms would result and Bz endosperms should be associated with bz2-m embryos or vice versa. The 13 Bz cases all showed concordance of embryo and endosperm and, in a comparable test of Bz reversions from Ac/Ac and AcAc/AcAcAc male parents, a total of 22 Bz individuals showed concordance in 21 and noncorrespondence in only one. The data clearly indicate that the first microspore division is the time when bz2-m to Bz mutations occur. Mutations after meiosis and before the second microspore division would lead to concordance of endosperm and embryo and the newly arisen Bz allele would be associated with Ac; this was observed.
It is not surprising that a somewhat higher rate of Bz germinal reversions was found in testcrosses of V AcAc ms/v AcAcAc Ms male parents. In a population of 1189, twelve reversions occurred, a frequency of 1.0%. All but one of the Bz plants carried the v marker of the more potent AcAcAc allele.
M.M. Rhoades and Ellen Dempsey
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