Comparison of homologous polytene chromosomes in Phaseolus coccineus embryo suspensor cells: morphological, autoradiographic and cytophotometric analyses

The functional behaviour of unpaired homologous polytene chromosomes (2n=22), was investigated in nuclei of Phaseolus coccineus embryo suspensor cells. Observations were carried out on the morphological level and after 3H-thymidine and 3H-uridine autoradiography. Histone and total protein contents in the chromatin were also investigated. It was shown that corresponding regions of homologous chromosomes may show different functional structures. 3H-thymidine incorporation demonstrated differences between homologues in both DNA synthesis leading to chromosome endoreduplication (polytenization) and DNA amplification (extra DNA synthesis). 3H-uridine autoradiography showed that homologous regions in a given chromosome pair may display three labeling patterns: i) both regions labeled; ii) both regions unlabeled; iii) one region labeled and the other unlabeled. These three states are found to occur in different cells of one and the same embryo suspensor. Differences between homologous chromosome regions were also found in the ratios between DNA and protein contents in their chromatin. These results, which show that the functional activity of homologous chromosomes of the same complement may greatly differ, are discussed in relation to the characteristics of the system investigated.