Plant & Animal Genome VI Conference

W79

MOLECULAR CYTOGENETIC ANALYSIS OF CHROMOSOME ORGANISATION IN Arabidopsis thaliana

1. School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K
2. Department of Molecular Biology, Wageningen Agricultural University, Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands
3. Department of Genetics, Wageningen Agricultural University, Dreijenlaan 2, 6703 HA, Wageningen, The Netherlands
4. John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, U.K.

FISH is a powerful technique to study the physical organisation of molecular sequences on chromosomes. We applied FISH to pachytene chromosomes of Arabidopsis thaliana, which are 25 times larger than their mitotic counterparts. This approach allows to fine map DNA clones ranging in size from 1 kb up to 1 Mbp and to resolve targets that are separated by at least 50 kb, depending on the condensation degree of the chromatin. Combined with the availability of a many DNA clones it makes Arabidopsis a model system for molecular cytogenetic studies. We have focussed this FISH study on the chromosomal organisation of the short arm of chromosome 4 using DNA clones that cover the entire short arm. The resulting cytogenetic map was integrated with the molecular map, which allowed to determine the degree of chromatin condensation along the chromosome arm. Differences in chromatin condensation were found between and within euchromatic and heterochromatic regions and could be correlated with differences in recombination frequencies in the genetic map. For example, a proximal heterochromatic knob, which colocalizes with the YAC clone CIC8B1, appears to coincide with a region of suppressed meiotic recombiation. Interestingly, this area contains repeat elements that are abundantly present only in the centromeric and pericentromeric heterochromatin. The chromomere is prominent in the ecotypes Wasselewskija and Columbia, while Landsberg and C24 lack the chromomere in chromosome arm 4S and do not seem to have the repeats outside the heterochromatic area around the centromere. The involvement of the repeats in the formation of the chromomere is discussed.