Figure 1 shows a pachytene nucleus displayed as projections of 3D images from separate wavelengths (A, DAPI, total DNA; B, rhodamine, maize 9 FISH signals; C, Cy5, MCCY centromere FISH signals). The computer-straightened chromosome (D) is shown at the bottom (9S, short arm; C; centromere; 9L; long arm) and the FISH probes are indicated at the right. We have found that the arm length ratios are similar to those for the endogenous maize chromosome 9 (in maize) from either meiotic prophase or somatic metaphase cells (Table 1). The maize 9 chromosome in OMAd9.2 maintained a pachytene arm ratio of about 1.8, despite large variations in total length. Previously published arm ratios for maize 9 range from 1.7 to 2.0.
These numbers suggest that the higher order packaging of meiotic chromosomes, at least with respect to the constancy of arm ratio, may be governed by some intrinsic properties of the chromosome itself. Such properties may involve matrix attachment region sequences, genes, aspects of the chromatin-folding pathway, or the distribution of trans-acting factors. It is interesting to consider that the packing and segregation of maize 9 chromosome in OMAd9.2 probably involves specific and conserved molecular interactions between Avena proteins and Zea DNA or chromatin elements. Analysis of this material may help unravel some of the complexities of the structure-function relationships operating on meiotic chromosomes. This material should also be suitable for cytogenetic mapping of maize loci by FISH.
Table 1. Maize 9 arm length ratios (long/short).
|Ratio L/S||maize line||cell type||Reference or source of information|
|1.8||KYS||mei||Rhoades (1950) J Hered. 61:59.|
|2.0||KYS||mei||Neuffer et al. (1997) in "Mutants of maize" CSHL Press.|
|1.93||KYS||mei||Chen et al. (2000) Theor. Appl. Genet. 101:30.|
|2||KYS||som||Sadder and Weber (2001) Plant Mol. Bio. Reporter 19:117-123.|
|1.7||Se60||som||Chen, CC, (1969) Can. J. Genet. Cytol. 11:752.|
|1.8||OM9||mei||this report, total length 22.8 micron.|
|1.8||OM9||mei||this report, total length 25.2 micron.|
|1.7||OM9||mei||this report, total length 35.2 micron.|
Table Notes: L/S long to short arm ratio; mei meiotic; som somatic; Se60 Seneca60; OM9 OMAd9.2 maize 9 from oat x maize addition line OMAd9.2 (from pollen of Seneca 60).
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