Three haploid plants (stock-6 derived) were analyzed meiotically. Grouping of chromosomes in diakinesis and metaphase I was very pronounced. About 60% of the cells had 5:5 grouping, 15% of the cells had 6:4 grouping (which may be potentially 5:5), five groups of two bivalents were noticed in 15% and in the rest of the cells, no grouping was observed. This raises some doubts about the exact basic chromosome number in maize, indicating that it might be 5 rather than 10.
Besides grouping of chromosomes, it was also noted that three chromosomes
were attached to the nucleolus in more than 75% of the cells in diplotene
and diakinesis (Fig. 1). The presence of 1-4 micronucleoli was found in
more than 50% of the cells. Two diploid plants of the same stock-6 background
grown in moisture stress and high temperature had a high frequency
of cells with 3 bivalents attached to the nucleolus, and the presence of
micronucleoli was very common. But the diploid plants of the same genotype
grown under normal conditions had no cells with more than one bivalent
attached to the nucleolus. Adverse microenvironment of the cells in the
plants grown in moisture and temperature stress or genomic imbalance in
the haploids may be responsible for the occurrence of micronucleoli. These
observations also indicate that there might be two other chromosomes, other
than chromosome 6, in the maize genome which have nucleolar organiser regions
(NOR) expressed only under adverse conditions.
Figure 1. Diakinesis of a stock-6 derived haploid plant showing three chromosomes attached to the nucleolus and presence of three micronucleoli.
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