Worm Breeder's Gazette 10(3): 64
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have isolated and analyzed a repetitive, 18 kb long DNA element ( Pas) which is present in about 50 copies per haploid germ line genome of A. lumbricoides. The copies of this element are not clustered, but rather intermingled with low repetitive DNA. Interestingly, however, all copies are completely expelled from the somatic cell lineage during the process of chromatin diminution. The different chromosomal copies of Pas are very conserved in structure, as indicated by their restriction enzyme pattern. This points towards a possible biological function, which would have to be germ line specific. The following observations suggest that Pas elements may be associated with telomeres: A specific fragment of this element cross- hybridizes even under high stringency conditions to the genome of C. elegans. This sequence contains a block of a tandemly arranged, 6 bp long repeating unit, similar to those found at the telomeres of some lower eukaryotes. In Southern blot experiments, the repeated Pas sequence hybridizes indeed to nuclease Bal 31 sensitive sequences. Hence, this sequence has to be located at the telomeres of A. lumbricoides and most likely also at those of C. elegans. We are currently cloning the telomeres of both nematode species. The telomeres of A. lumbricoides are of particular interest because this species undergoes chromatin diminution during early cleavage stages in presomatic blastomeres. This process, by which the heterochromatic ends of the chromosomes are expelled from somatic cells, most likely also includes rearrangement of the telomeres. Such a hypothesis is supported by some preliminary data showing different hybridization patterns of the telomeric sequences in germ line and somatic DNA. The hope that cloning, followed by detailed analysis of Ascaris telomeres, may enable us to better understand how the elimination process proceeds on the molecular level.