The phenotypic expression of the reversion of o2-m(r) alleles to the normal depends on its timing. Reversion of the mutable allele occurring after the first division of the primary endosperm nucleus is manifested by the apparition of variegated kernels which are characterized by alternation of opaque and vitreous sectors in the endosperm. If the reversion takes place before the first division of the primary endosperm nucleus, the phenotypically normal kernel (whole endosperm revertants, WER) appear. In addition, if such a reversion occurs during development of the gametes participating in embryo formation, the embryo revertants develop carrying the dominant allele O2.
Premeiotic reversion of the mutable o2-m(r) alleles should lead to a clustering distribution of WER on ears with the frequency of the embryo revertants in such clusters of 50% (in case of reversion event in one of the homologous chromosomes).
The distribution of WER kernels was analyzed on two selfed ears belonging to earlier obtained strains characterized by a high frequency of WER formation (HFWER strains) which were homozygous for the receptive allele o2-hf and the regulatory element Bg-hf. A high frequency of WER in indicated strains is conditioned by the presence of the o2-hf allele (Maydica 44:195-203, 1999).
The first ear analyzed contained 49 WER and 191 variegated kernels, the second one had 73 WER and 119 variegated. Analysis of ear maps of these ears showed that an overwhelming majority of WER (115 out of 122) were in clusters of contiguous kernels. This was quite unexpected for random WER formation and allowed the suggestion of premeiotic reversion of the mutable allele during ear development. To determine which of the WER were embryo revertants, all WER from indicated ears were planted in field conditions and the resulting plants were selfed or crossed with homozygous recessive o2-m(r) tester lacking the regulatory element Bg. From 80 progenies of WER kernels tested from both ears only 15 were heterozygous for the O2 allele. A low fraction of embryo revertants confirms the earlier conclusion that the major part of WER is formed during the period from the fusion of gametes to the first division of the primary endosperm nucleus (Maydica 44:195-203, 1999).
Most of the embryo revertants (13 out of 15) were found on the first ear. Five embryo revertants in this ear formed two clusters consisting of 2 and 3 contiguous revertant kernels. Since the frequency of gametes carrying reversions to the normal O2 allele on this ear is not less than 0.027 (or 13/(240x2)) the probability of the clusters of 2 and 3 contiguous embryo revertants in this ear should be at least 7.3·10-4 and 2.0·10-5 respectively. Consequently this means that the formation of such clusters through the reversions at the postmeiotic stages is unlikely and indicates the possibility of premeiotic reversion events of the o2-hf allele in at least one of the homologous chromosomes. Taking into account the small size of the clusters mentioned, these reversion events occurred at the late stages of ear premeiotic development.
Another phenomenon to explain is the clustering distribution of WER,
most of which are non-embryo revertants. One of the possible explanations
could be the joint action of two factors at the early stages of endosperm
development: 1) diffusion of the additional quantities of transposase from
the developing endosperm with its level high enough for rbg excision
in adjacent developing kernels; 2) high ratio of kernels with above- and
sub-threshold excision level of transposase in developing endosperm in
HFWER strains. The second cause could be connected both with the possible
lower threshold level needed for the rbg excision from the o2-hf
allele and the higher transposase production by the Bg-hf element.
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