Additional NORs and the number of nucleoli per coleoptile cell in maize
--Quinn, C, Maillet, DS, Walden, DB

As part of an ongoing study of the nucleoli of maize we have examined the effect of the presence of additional NORs. In the coleoptile of diploid lines of maize, which have one NOR on chromosome 6, most coleoptile cells (approximately 80 to 90%) Maillet et al., MNL 72:56-58,1998) have one nucleolus, the rest have two nucleoli. In a cultivar which has four NORs per nucleus it should be possible for the cells to have one to four nucleoli. We have counted the number of nucleoli that are present in coleoptile cells from two lines that have four NORs in somatic cells, a tetraploid line of maize and the diploid cultivar 2NOR, in order to determine the effect of additional NORs on the number of nucleoli. The tetraploid has four copies of chromosome 6 and therefore four NORs; the cultivar 2 NOR has two NORs on each of the two copies of chromosome 6, which can be observed in an acetocarmine preparation of cells in early pachytene.

Our observations of 1064 tetraploid (uwo-52509) coleoptile cells confirm that one to four nucleoli can be present . It was expected that the linear arrangement of NORs would alter the ratio of nuclei per cell compared to the tetraploid. Examination of 800 coleoptile cells from the cultivar 2NOR revealed that only one or two nuclei were present per cell. In some cases there was one nucleolus plus one micronucleolus (Table 1).

Table 1. Percentage of nucleoli per coleptile cell of a 2NOR and a tetraploid cultivar.
 
 
# Nucleoli/ cell
 
1
1*
2
3
4
Tetraploid (n =1064)
74.6
0
19.2
5.5
0.7
2NOR (n =800) 
90.8
6.7
2.5
0
0
* = nuclei with 1 nucleolus and one micronucleolus

The observation of one to four nucleoli in the tetraploid demonstrates that it is possible for four NORs on separate chromosomes to form nucleoli. However in the stock 2NOR where the four NORs are in pairs they do not form three or four nucleoli. It is possible that the additional NOR on chromosome six of the 2NOR cultivar is not functional or that one of the NORs can suppress the other, perhaps as a result of their proximity or relative size.
 

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