Chromatin diminution leads to rapid evolutionary changes in the organization of the germ line genomes of the parasitic nematodes A. suum and P. univalens.

Chromatin diminution in the parasitic nematodes Ascaris suum and Parascaris univalens represents a rather complex molecular phenomenon that includes chromosomal breakage, DNA degradation and new telomere formation. At a given elimination site, DNA breakage and new telomere addition does not take place at a single chromosomal locus but at many different places within a several kilobase long chromosomal region, referred to as chromosomal breakage region (CBR). Here we describe the cloning and the characterisation of seven CBRs from A. suum and P. univalens and we show that the process has been conserved between the two species. A detailed sequence comparison provides evidence that the sequences of the CBRs and their flanking regions are not directly important for the specification of the elimination sites. Six out of the seven CBRs are conserved between the two nematode species, suggesting that they have already existed in a common ancestor. We present a hypothesis stating that the elimination process ensures the maintenance of a functional somatic genome and concomitantly allows extremely rapid and profound changes in the germ line genome, thereby allowing the development of new germ line specific functions and thus providing a selective advantage for the chromatin eliminating nematodes during further evolution.

New telomere formation during the process of chromatin diminution in Ascaris suum.

Chromatin diminution in the parasitic nematode Ascaris suum represents an interesting case of developmentally programmed DNA rearrangement in higher eukaryotes. At the molecular level, it is a rather complex event including chromosome breakage, new telomere formation and DNA degradation. Analysis of a cloned somatic telomere (pTel1) revealed that it has been newly created during the process of chromatin diminution by the addition of telomeric repeats (TTAGGC)n to a chromosomal breakage site (Müller et al., 1991). However, telomere addition does not occur at a single chromosomal locus, but at many different sites within a short chromosomal region, termed CBR1 (chromosomal breakage region 1). Here we present the cloning and the analysis of 83 different PCR amplified telomere addition sites from the region of CBR1. The lack of any obvious sequence homology shared among them argues for a telomerase-mediated healing process, rather than for a recombinational event. This hypothesis is strongly supported by the existence of 1-6 nucleotides corresponding to and being in frame with the newly added telomeric repeats at almost all of the telomere addition sites. Furthermore, we show that telomeres are not only added to the ends of the retained chromosomal portions, but also to the eliminated part of the chromosomes, which later on become degraded in the cytoplasm. This result suggests that de novo telomere formation during the process of chromatin diminution represents a non-specific process which can heal any broken DNA end.