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Eukaryotic Cell, October 2003, p. 1076-1090, Vol. 2, No. 5
1535-9778/03/$08.00+0     DOI: 10.1128/EC.2.5.1076-1090.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Developmentally Regulated Chromosome Fragmentation Linked to Imprecise Elimination of Repeated Sequences in Paramecia

Anne Le Mouël, Alain Butler, François Caron,{dagger} and Eric Meyer*

Laboratoire de Génétique Moléculaire, CNRS UMR 8541, Ecole Normale Supérieure, 75005 Paris, France

Received 12 March 2003/ Accepted 1 July 2003

The chromosomes of ciliates are fragmented at reproducible sites during the development of the polyploid somatic macronucleus, but the mechanisms involved appear to be quite diverse in different species. In Paramecium aurelia, the process is imprecise and results in de novo telomere addition at locally heterogeneous positions. To search for possible determinants of chromosome fragmentation, we have studied an ~21-kb fragmentation region from the germ line genome of P. primaurelia. The mapping and sequencing of alternative macronuclear versions of the region show that two distinct multicopy elements, a minisatellite and a degenerate transposon copy, are eliminated by an imprecise mechanism leading either to chromosome fragmentation and the formation of new telomeres or to the rejoining of flanking sequences. Heterogeneous internal deletions occur between short direct repeats containing TA dinucleotides. The complex rearrangement patterns produced vary slightly among genetically identical cell lines, show non-Mendelian inheritance during sexual reproduction, and can be experimentally modified by transformation of the maternal macronucleus with homologous sequences. These results suggest that chromosome fragmentation in Paramecium is the consequence of imprecise DNA elimination events that are distinct from the precise excision of single-copy internal eliminated sequences and that target multicopy germ line sequences by homology-dependent epigenetic mechanisms.

* Corresponding author. Mailing address: Laboratoire de Génétique Moléculaire, CNRS UMR 8541, Ecole Normale Supérieure, 46 Rue d'Ulm, 75005 Paris, France. Phone: 33 1 44 32 39 48. Fax: 33 1 44 32 39 41. E-mail: emeyer{at}wotan.ens.fr.

{dagger} Deceased.

Eukaryotic Cell, October 2003, p. 1076-1090, Vol. 2, No. 5
1535-9778/03/$08.00+0     DOI: 10.1128/EC.2.5.1076-1090.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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