USE OF REPEATED DNA SEQUENCES AS CYTOLOGICAL MARKERS

The majority of DNA that is found in most of the flowering plants appears to be non‐coding DNA. Much of this excess DNA consists of nucleotide sequences which exist as multiple copies throughout the genome and are designated as repetitive sequences. Those sequences which are found in moderately high to high numbers of copies are observed to be of the greatest value as cytological markers. Moderately high copies may exist as sequences which are dispersed throughout the chromosomes of some species and not dispersed in other more distantly related species. By taking advantage of this characteristic and the technique of in situ hybridization with biotinylated probes, breakpoints of chromosomal translocations may be observed between species such as wheat and rye. Many of the high copy number repetitive sequences are organized in a tandem fashion in specific loci in the chromosome. Chromosomal identification may be accomplished by using the in situ hybridization technique. Upon in situ hybridization with a repetitive sequence isolated from Aegilops squarrosa, the patterns of the sites of hybridization allowed the D‐genome chromosomes to be identified. The sequence was also observed only on the D‐genome chromosomes of several polyploid species indicating its usefulness as a genome specific marker. Using this genome specificity, assessment of the orientation of the D‐genome chromosomal segments of hexaploid wheat carrying the sequence during interphase and prophase of mitotic root tip cells was possible. Repetitive DNA sequences, therefore, provide cytological markers necessary for studies of chromosomal identification, genome allocation, and genome orientation. The use of biotin‐labeled DNA probes allows the technique of in situ hybridization to be performed much more rapidly and with a greater degree of safety and reliability.