THE LARGE-SCALE GENOMIC ORGANIZATION OF REPETITIVE DNA FAMILIES AT THE TELOMERES OF RYE CHROMOSOMES

Repetitive DNA sequences in the terminal heterochromatin of rye (Secal e cereale) chromosomes have consequences for the structural and functi onal organization of chromosomes. The large-scale genomic organization of these regions was studied using the telomeric repeat from Arabidop sis and clones of three nonhomologous, tandemly repeated, subtelomeric DNA families with complex but contrasting higher order structural org anizations. Polymerase chain reaction analysis with a single primer sh owed a fraction of the repeat units of one family organized in a ''hea d-to-head'' orientation. Such structures suggest evolution of chromoso mes by chromatid-type breakage-fusion-bridge cycles. In situ hybridiza tion and pulse field gel electrophoresis showed the order of the repea ts and the heterogeneity in the lengths of individual arrays. After Xb al digestion and pulse field gel electrophoresis, the telomeric and tw o subtelomeric clones showed strong hybridization signals from 40 to 1 00 kb, with a maximum at 50 to 60 kb. We suggest that these fragments define a basic higher order structure and DNA loop domains of regions of rye chromosomes consisting of arrays of tandemly organized sequence s.