Identification of the chromosomes of rye by distribution patterns of DNA.

Ultramicrospectrophotometric measurements were made on unstained and Feulgen-stained rye chromosomes at 265 mμ and 546 mμ, respectively. The total extinction values of the individual chromosomes corresponded to their relative lengths. Similarly, the long/short arm extinction values agreed well with the index ratios, and were thus useful for the classification of the chromosomes into groups. The distribution of the nucleic acids along the chromosomes was determined by scanning individual chromosomes transversally and in a few instances longitudinally. More details were found in the pattern curves obtained from measurements made on unstained than stained chromosomes. Also the greater the length of the chromosome, the greater was the detail that could be seen. The distribution pattern of DNA was in accordance with the heteropycnotic differentiation along late prophase chromosomes. A specific pattern of nucleic acid distribution along each of the seven chromosome types was useful for chromosome identification. In the pattern curves, sites indicating a markedly low nucleic acid content coincided with the positions of primary and secondary constrictions. Sites with slightly lower nucleic acid content than adjacent areas most likely indicate positions of potential secondary constrictions, since most of these sites coincided with locations of faint constrictions that were detectable in the rye chromosomes when these were present in the intergeneric hybrid Elymus arenarius X Secale cereale. Factors such as the technique for chromosome preparations, degree of chromosome contraction, intensity of Feulgen coloration, chromosomal RNA cycle, nucleolar behavior during mitosis, and background effects may lead to inconsistencies in the photometric measurements.