The Duharcourt Lab published a new article in BMC Biology:
Background
Paramecia 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.
Results
We 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 ~ 100-Mb 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.
Arnaiz O, Guérin F, Couloux A, Miró-Pina C, Pellerin G, Nekrasova I, Amselem J, Aury JM, Bhullar S, Frapporti A, Lerat E, Luyten I, Malinsky S, Mathy N, Potekhin A, Régnier V, Sawka-Gądek N, Touzeau A, de Vanssay A, Zangarelli C, Quesneville H, Bétermier M, Labadie K, Duret L, Meyer E, Duharcourt S, Sperling L. The tiny germline chromosomes of Paramecium aurelia have an exceptionally high recombination rate and are capped by a new class of Helitrons. BMC Biol. 2026 Apr 16;24(1):99. doi: 10.1186/s12915-026-02584-w. PMID: 41992225; PMCID: PMC13085455.