H3-Thymidin-Markierung einzelner Chromatiden in Riesenchromosomen

Summary Tritiated thymidine was administered to fertilized eggs of Chironomus tentans at the time of oviposition. Larvae hatching from these eggs were raised until the end of the 4th instar when they were dissected and their salivary glands squashed for autoradiography of the giant chromosomes. Good autoradiographs were obtained after 2 years exposure from preparations stored in plastic boxes.Three principal patterns of labelling were found: (1) single-strand labelling, where one or two chromosome pairs per nucleus show one single helical track of silver grains typically running from one end of the chromosome to the other; (2) two or four-strand labelling, where all chromosome pairs of a nucleus show 2 or 4 densely labelled tracks; and, (3), diffuse strand labelling where the level of labelling is generally low and the number of labelled strands per chromosome pair seems to be higher than 8. Approximately one half of all nuclei were found unlabelled. In 9 out of 70 chromosome pairs with single strand labelling the labelled strand begins at one end of the chromosome but ends interstitially.The labelled single strands must be intact mitotic half-chromatids (or crossover products of these) which received their label during DNA synthesis early in embryonic development, probably before blastoderm formation. A model of cell lineage involving selection against labelled nuclei accounts for the observed distribution of labelled single strands in our material. The occurrence of 2- and 4-strand labelling points to a smaller number of mitotic divisions preceding the formation of some portions of the salivary glands as contrasted with others. Cells showing this type of labelling have either not divided at all, or they are descendants of just one division, after the supply of tritiated thymidine was exhausted in early development. Our findings confirm the classical concept of polyteny.

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Herrn Professor Dr. Hans Bauer zum 60. Geburtstag.