Studies of pollen and somatic instability in Mu-induced waxy mutants

In the 1984 Newsletter, we reported on studies of several Mu-induced waxy (wx) mutants. One mutable wx allele (wx-Mum1) had a high reversion rate as measured by the frequency of pollen grains that stained purple with KI/I2 staining. In the 1984 tests and those reported here, procedures modified from O.E. Nelson (Genetics 60:507-524, 1968) were used for pollen staining.

In 1984 we grew 5 new Mu-induced waxy mutants, three of which were mutable. One of these, wx-Mum2, occurred on a selfed ear of a Mu6 per se plant in 1983. Among the waxy seeds on this ear were both mutable and stable seeds. In 1984, shedding central spikes were collected and preserved in 70% alcohol. Eleven plants from mutable seeds and three plants from stable waxy seeds from this original ear were sampled. The pollen was stained and the frequency of phenotypically revertant pollen grains was determined (Table 1).

As with wx-Mum1, the reversion sectors in the endosperm of wx-Mum2 are quite late and consist predominantly of single cells. This mutant has an extremely high frequency of phenotypically Wx pollen grains. It is of interest to note that stable derivatives of this mutant (Family 1282) do not show any reversion in the pollen. Plant 1281-3 is interesting. It came from a seed with a mutable endosperm but has lost instability in the pollen and also in the female germ line, since only stable seeds are observed on the selfed ear of this plant. In the production of the seed that gave rise to plant 1281-3, perhaps the allele in the sperm fertilizing the egg carried a stable derivative wx allele while the other sperm still had an unstable allele. Heterofertilization could account for this seed, but it would require the simultaneous occurrence of rare events (i.e., mutation and heterofertilization).

Another mutable waxy mutant (wx-Mum3) occurred as a five seed sector on the ear of a Wx Wx Mu female plant pollinated by a y1 wx gl1 stock. In 1984, these seeds, which were heterozygous for the wxMum3 allele and the standard wx allele, were planted and the central spikes of the resulting plants sampled. All plants were self-pollinated. The results of these tests are given in Table 2. Selfs of all these plants segregated for mutable seeds, yet only one of them showed instability in the pollen. The reversion frequency in the pollen of the latter plant was high, considering that half of the pollen grains carry the standard wx allele. Plant numbers 2, 4 and 5 of this family seem to have lost instability in the cell lineage giving rise to the tassel, but not in the ear cell lineage. If this is true, the outcross plants resulting from the cross of these plants as males to standard should segregate for only stable waxy seeds on their selfed ears.

In 1983, some putative Mu-induced waxy mutants were obtained in a cross of Wx Wx Mu as a male with a y1 wx stock. Results of selfs of three of the plants from these seeds revealed that two segregated for stable seeds while one had only mutable seeds. These results and the results of the pollen examination of these plants are given in Table 3.

These three mutants are all independent in origin since they come from pollen of different plants. One (1145-3) shows instability in both tassel and ear. Again the plants are heterozygous for the standard wx allele, and thus actual frequencies of wx-Mum reversions would be expected to be twice the observed values. The 1145-3 mutant has been assigned the symbol wx-Mum4. Plant 1145-1 shows an unexpected discordance between tassel and ear. The pollen has a high level of instability yet no mutable seeds are seen on the selfed ear. This allele may only be unstable in the germ line and not in the somatic tissue. The mutant carried by plant 1145-2 seems to be a stable wx mutant.

All of these wx-Mum alleles are being put in a multiply marked stock so that germinal reversion rates, as measured by revertant seeds, can be determined.

Table 1. The frequency of phenotypic reversions to Wx in pollen from homozygous wx-Mum2.

Table 2. The frequency of phenotypic reversions to Wx in pollen grains from wx-Mum3/wx plants.

Table 3. The frequency of phenotypic reversions to Wx in pollen grains from three new Mu- induced waxy mutants which are hetero-zygous for the standard wx allele.

Solomon Sackitey and Donald S. Robertson
 
 


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