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Germ line — soma differentiation inAscaris: A molecular approach

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Summary

The early ontogenetic segregation of germ line and somatic cells in the animal kingdom is phylogenetically very old and represents probably the first step in differentiation. While this phenomenon has been shown to occur in various animal phyla, it seems to be completely msssing in the plant kingdom. In several animal species, the segregation of the germ and somatic cell lines is accompanied by the loss of whole or parts of chromosomes in the presumptive somatic cells. The cause of the chromatin diminution process as well as the significance of the germ line limited DNA sequences in species undergoing chromatin or chromosome loss still remain unknown. However, using modern biochemical and molecular biological techniques, it has become possible to analyze the process of chromatin diminution and the composition of the germ line specific DNA sequences at the molecular level.

InAscaris lumbricoides, about a quarter of the total amount of germ line DNA is eliminated from the presumptive somatic cells during chromatin diminution. Hybridization experiments revealed that germ line and somatic DNA contain the same percentage of 18S+28S rRNA genes. Therefore, chromatin diminution does not serve to discard large amounts of rRNA-coding genes from the germ line cells. On the other hand, over 99%, but not all satellite DNA sequences present in the germ line genome, are expelled from the presumptive somatic cells by chromatin diminution. Molecular cloning and sequence analysis of different restriction endonuclease fragments isolated from the germ line satellite DNA indicated that this eliminated satellite is composed of a whole set of related variant sequences, which differ by small deletions, insertions and single base substitutions. Members of the same variant class are tandemly linked and therefore physically separated from other variant classes. By comparing all the determined sequences, it was possible to establish a 121 bp long and AT rich consensus sequence which itself exhibits an 11 bp long internal short range periodicity. We have no indication for transcriptional activity of the satellite DNA sequences at any stage or tissue tested. Evidence is accumulating that the eliminated DNA contains also other DNA sequences apart from the class of highly repetitive satellite DNA.

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Author notes

  1. E. Back

    Present address: Ciba-Geigy Corp., P.O. Box 12257, NC 27709, Research Triangle Park, USA

Authors and Affiliations

  1. Institute of Zoology, University of Freiburg, Pérolles, CH-1700, Freiburg, (Switzerland)

    H. Tobler, F. Müller, E. Back & P. Aeby

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Tobler, H., Müller, F., Back, E. et al. Germ line — soma differentiation inAscaris: A molecular approach. Experientia 41, 1311–1319 (1985). https://doi.org/10.1007/BF01952073

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