Synaptonemal complex karyotype for maize --Anderson, LK, Stack, SM We are mapping the distribution of recombination nodules (RNs) on spreads of maize (KYS) synaptonemal complexes (SCs = pachytene chromosomes) using electron microscopy. Since RNs occur at sites of crossing over, an RN map will show the frequency and distribution of crossing over on each chromosome. This will be useful for integrating molecular, genetic and cytogenetic maps for maize. As a necessary prerequisite for mapping RNs, it is necessary to identify each of the ten maize SCs. While several different characteristics can be used to identify squashed maize pachytene chromosomes (including relative length, arm ratio, chromomeres, knobs, nucleolar association), only relative lengths and arm ratios can be used to identify maize SCs because the other features are usually lost during the spreading procedure. Nevertheless, we have observed a good correspondence between the SC karyotype and the pachytene karyotype that is based on squashed chromosomes (Freeling, M. and Walbot, V., Eds., The Maize Handbook, 1994; Table 1). The most noticeable difference between the two karyotypes is for chromosome/SC 6. It is possible that the large nucleolus that is present in squashes (but dispersed in SC spreads) may obscure part of the short arm of chromosome 6 and result in a shorter relative length and larger arm ratio for squashes compared to spread SCs. This work was supported by grant MCB-9728673 from the National Science Foundation.

Table 1. Karyotype from pachytene chromosome squashes compared to karyotype from SC spreads.
 
Chromosome or SC rank
Pachytene Chromosome squashes
SC spreads
 
Arm Ratio
Relative Length (%)
Arm ratio
Relative Length (%)
1
1.23
14.5
1.25
14.9
2
1.14
12.4
1.08
11.9
3
2.0
11.3
1.97
11.0
4
1.63
11.1
1.54
10.7
5
1.07
11.1
1.10
10.9
6
3.1
7.7
2.57
8.9
7
2.6
8.9
2.73
8.6
8
3.0
8.9
3.05
8.6
9
2.0
7.7
1.93
7.7
10
2.6
6.3
2.45
6.7

 


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