With its nuclear dualism, the ciliateParameciumconstitutes an original model to study how host genomes cope with transposable elements (TEs).P. tetraureliaharbors two germline micronuclei (MIC) and a polyploid somatic macronucleus (MAC) that develops from the MIC at each sexual cycle. Throughout evolution, the MIC genome has been continuously colonized by TEs and related sequences that are removed from the somatic genome during MAC development. Whereas TE elimination is generally imprecise, excision of ~45,000 TE-derived Internal Eliminated Sequences (IESs) is precise, allowing for functional gene assembly. Programmed DNA elimination is concomitant with genome amplification. It is guided by noncoding RNAs and repressive chromatin marks. A subset of IESs is excised independently of this epigenetic control, raising the question of how IESs are targeted for elimination. To gain insight into the determinants of IES excision, we established the developmental timing of DNA elimination genome-wide by combining fluorescence-assisted nuclear sorting with high-throughput sequencing. Essentially all IESs are excised within only one endoreplication round (32C to 64C), while TEs are eliminated at a later stage. We show that DNA elimination proceeds independently of replication. We defined four IES classes according to excision timing. The earliest excised IESs tend to be independent of epigenetic factors, display strong sequence signals at their ends and originate from the most ancient integration events. We conclude that old IESs have been optimized during evolution for early and accurate excision, by acquiring stronger sequence determinants and escaping epigenetic control.

Multinucleate cells are found in many eukaryotes, but how multiple nuclei coordinate their functions is still poorly understood. In the cytoplasm of the ciliateParamecium tetraurelia, two micronuclei (MIC) serving sexual reproduction coexist with a somatic macronucleus (MAC) dedicated to gene expression. During sexual processes, the MAC is progressively destroyed while still ensuring transcription and new MACs develop from copies of the zygotic MIC. Several gene clusters are successively induced and switched off before vegetative growth resumes. Concomitantly, programmed genome rearrangements (PGR) remove transposons and their relics from the new MACs. Development of the new MACs is controlled by the old MAC, since the latter expresses genes involved in PGR, including thePGMgene encoding the essential PiggyMac endonuclease that cleaves the ends of eliminated sequences. Using RNA deep sequencing and transcriptome analysis, we show that impairing PGR up-deregulates key PGR genes, together with ~600 other genes possibly also involved in PGR. Among these genes, 42% are no longer induced when no new MACs are formed, including 180 genes that are co-expressed withPGMunder all tested conditions. We propose that bi-directional crosstalk between the two coexisting generations of MACs links gene expression to the progression of MAC development.