Temperature-dependent functional structures in the polytene chromosomes of Phaseolus, with special reference to the nucleolus organizers

ABSTRACT During the endopolyploidization of the suspensor cells of Phaseolus coccineus and P. vulgaris, polytene chromosomes and an important nucleolar apparatus develop within the nuclei. The nucleolus-associated regions of the four nucleolus-organizing chromosomes and the regions of the secondary constrictions (intranucleolar chromatin threads) show a temperature-dependent, reversible variability in their structure. They split up and are dispersed at optimal temperatures, and they are compact and partly banded at too low or too high ones. Many nucleolar extrusions, and additional nucleoli which originate on distended elements of the heterochromatic parts of different giant chromosomes, occur at temperatures higher than 15° C. The polytene chromosomes of P. coccineus are able to form lateral loops projecting at right angles to their axes, shortly after temperature elevation. This occurs by the uncoiling of the superficial chromomeres. Several loops synthesize droplets and, moreover, they form nucleolus-like bodies in a hitherto unknown manner. The duration of the lampbrush state is about 3–4 h. Cytochemical staining methods indicate the presence of DNA in all parts of the polytene chromosomes, including the intranucleolar secondary constriction and the lateral loops, and the presence of RNA in all types of nucleoli and nucleolus-like bodies. The structural modifications described are compared with the structure of salivary gland chromosomes of Diptera and lampbrush chromosomes in oocytes and spermatocytes. It is suggested that the structural changes are an expression of temperature-dependent changes in gene activity.