Tallahassee, Florida
The Florida State University Pachytene arm ratios for maize chromosome 9 in OMAd9.2, a maize chromosome addition line of oat --Koumbaris, G, and Bass, HW We are developing a cytogenetic mapping system using a maize disomic chromosome addition line of oat designated OMAd9.2 (origin and use of OMA lines reviewed by Kynast et al., Plant Physiol. 125:1216, 2001). Cytogenetic maps are experimentally useful and biologically informative (reviewed by Harper and Cande, Funct. Integr. Genomics, 1:89, 2000). We have used a newly designed centromeric FISH probe and whole chromosome painting to examine the cytogenetic structure of maize chromosome 9 from OMAd9.2 anthers fixed at the pachytene stage of meiotic prophase. The probe, MCCY, was designed from the junction between two CentC sequence repeats (Ananiev et al., Proc. Nat. Acad. Sci. USA 95:13073, 1998) and used with the 3D acrylamide FISH method as previously described (Bass et al., J. Cell Biol. 137:5, 1997). The MCCY probe is 5'-Cy5-GAAAAACGAAGAAATGGTTCTGGTG-3'. The MCCY probe hybridized to a small discrete spot that appears to co-localize with the primary constriction of the maize 9 centromere. The FISH paint signals from the maize 9 pachytene fibers were imaged, traced, and straightened as previously described (Bass et al., J. Cell Sci. 113:1033, 2000). Total DNA from knobless lines of maize provided good whole-chromosome paints. Knobless Tama Flint (KTF, MGCSC stock 11074) DNA painted the entire fiber with fairly uniformly FISH signal intensity along its length. Knobless Wilbur�s Flint (KWF, MGCSC stock 11075) DNA provided a good baseline fiber image for cytogenetic mapping because it painted the entire fiber and in the same wavelength revealed two important bonus loci, the 9S telomeric knob and the centromere.

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).

Figure 1.
 
 

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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