Meiotic patterns during microsporogenesis of an unstable corngrass macromutation
--M. G. Nikolaeva and V. N. Lysikov
In order to identify mechanisms of genetic instability responsible for intense morphogenetic processes in maize, a cytological study of mutation activity of the Cg2 gene has been undertaken. This allowed the appearance of induced chromosomal rearrangements to be monitored.
The genomic analysis (microsporogenesis analysis) has been performed on genotypes homozygous for mutant and revertant Cg2 alleles and on unstable corngrass macromutation genotypes in maize. Various genotypes exhibit heterogeneous male inflorescence structure: normal panicle, a spikelet (prevailing in homozygotes for the mutant allele) and an accresced spikelet with a small ear. Therefore, the amount of pollen produced by different genotypes depends on the male inflorescence structure.
The most informative stage of meiosis, prophase, was studied in detail. Genotypes differing in the degree of stability have been found to be heterozygous and heteromorphic for heterochromatic knobs. This heterogeneity is more characteristic of unstable genotypes mutating from the corngrass to normal type and back. At pachytene, asynapsis has been observed in some regions of homologous chromosomes, particularly in heterozygous and unstable genotypes. These genotypes exhibited the presence of univalents (M1), irregular assortment of chromosomes during anaphase-telophase of both divisions, and, in some cases, the formation of bridges, and lagging chromosomes. A characteristic feature of meiotic prophase in pollen mother cells (PMC) of the maize corngrass macromutation is adhesion of chromosome centromeric regions and, in some cases, of heterochromatic knobs and chromosome arms, resulting in the appearance of translocation chains and rings at diakinesis-diplotene with possible recombination of genetic material and chromatin redistribution.
The above irregularities result in partial pollen sterility, with the degree of sterility being higher in genotypes homozygous for the mutant allele (Cg2/Cg2). The level of fertility of heterozygous genotypes depends on the genotype of the pollen parent and can range between 60% and 97%. Eventually, these irregularities can result in aneuploidy in a more distant progeny.
Having evaluated the effect of genetic instability on intrachromosomal recombination based on a cytological analysis of chiasma distribution we concluded that the Cg2 macromutation has an inhibitory effect on the overall chiasma frequency, and on the frequency of interstitial and multiple exchanges. Heterozygous genotypes and those undergoing mutations of the type corngrass to normal phenotype are chiasma inducers, i.e. plant genotype affects variation in chiasma frequency and distribution pattern at the microsporocyte level.
The genomic analysis of maize macromutation genotypes homozygous for mutant and revertant Cg2 alleles and of the corresponding unstable genotypes indicates the possibility of exploiting these genotypes to broaden the range of genotypic variation in maize.
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