The fourth chromosome of Chironomus tentans Malpighian tubules: an ultrastructural study.

Morphology and banding pattern of the 4th chromosome in Chironomus tentans Malpighian tubules have been investigated by electron microscopy, using the squash and selection technique. The map we composed from our observations shows a remarkable increase (75%) in band numbers as compared to the map previously presented by Beermann for the 4th chromosome from salivary glands. Extrapolation of this increase to the entire genome would result in a total band number of about 3,500. The mean DNA content of bands can thus be calculated to be about 50 kb. Many bands show a complex structure, including the BR2 band. Some bands seem to result from fusion of smaller components. "Minibands" have also been observed. Some interbands contain RNP particles. In our material the interbands appeared to be made up of fibrils with a diameter of about 120 A. On the basis of these results we estimate the DNA in the interbands as amounting to 2% of the entire genome. The results are discussed with respect to the organization of the polytene chromosomes and the functional significance of the banding pattern.