Regnier, V. | bioRxiv Science

bioRxiv2025

The tiny germline chromosomes of Paramecium aurelia have anexceptionally high recombination rate and are capped by a new class of Helitrons

BackgroundParamecia belong to the ciliate phylum of unicellular eukaryotes characterized by nuclear dimorphism. A diploid germline micronucleus (MIC) transmits genetic information across sexual generations. A polyploid transcriptionally active somatic macronucleus (MAC) develops at each sexual generation from a copy of the MIC through programmed DNA elimination (PDE) of > 30% of germline DNA. PDE requires the domesticated PiggyMac (Pgm) transposase. Assembly of Paramecium germline genomes has presented an enormous challenge owing to the difficulty of MIC isolation. ResultsWe report chromosome-scale short-read MIC assemblies for 7 species from the P. aurelia species complex. We discovered a novel clade of Helitrons, with 9-10 kb transposase ORFs under purifying selection, that have remained active in all P. aurelia lineages. A long-read assembly for P. tetraurelia together with a genetic linkage map provided a nearly telomere-to-telomere assembly. Conclusions.The genome consists of tiny (300 kb - 1.2 Mb) and numerous ([~]160) germline chromosomes with the highest recombination rate ever reported for a eukaryote (420 cM/Mb). The ends of the chromosomes consist of Helitrons inserted in telomeric C4A2 repeats, forming a distinct genomic compartment that is eliminated very early during MAC development in a Pgm-independent manner.

genomics↗

bioRxiv2022

Developmental timing of programmed DNA elimination in Paramecium tetraurelia recapitulates germline transposon evolutionary dynamics

With its nuclear dualism, the ciliate Paramecium constitutes an original model to study how host genomes cope with transposable elements (TEs). P. tetraurelia harbors 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 non-coding 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 next-generation 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 time, rather than replication, controls the progression of DNA elimination. 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.

genomics↗