Comparative genomics reveals insight into the evolutionary origin of massively scrambled genomes
Ciliates are microbial eukaryotes that undergo extensive programmed genome rearrangement, a natural genome editing process that converts long germline chromosomes into smaller gene-rich somatic chromosomes. Three well-studied ciliates include Oxytricha trifallax, Tetrahymena thermophila and Paramecium tetraurelia, but only the Oxytricha lineage has a massively scrambled genome, whose assembly during development requires hundreds of thousands of precise programmed DNA joining events, representing the most complex genome dynamics of any known organism. Here we study the emergence of such complex genomes by examining the origin and evolution of discontinuous and scrambled genes in the Oxytricha lineage. This study compares six genomes from three species, the germline and somatic genomes for Euplotes woodruffi, Tetmemena sp., and the model ciliate Oxytricha trifallax. To complement existing data, we sequenced, assembled and annotated the germline and somatic genomes of Euplotes woodruffi, which provides an outgroup, and the germline genome of Tetmemena sp.. We find that the germline genome of Tetmemena is as massively scrambled and interrupted as Oxytricha's : 13.6% of its gene loci require programmed translocations and/or inversions, with some genes requiring hundreds of precise gene editing events during development. This study revealed that the earlier-diverged spirotrich, E. woodruffi, also has a scrambled genome, but only roughly half as many loci (7.3%) are scrambled. Furthermore, its scrambled genes are less complex, together supporting the position of Euplotes as a possible evolutionary intermediate in this lineage, in the process of accumulating complex evolutionary genome rearrangements, all of which require extensive repair to assemble functional coding regions. Comparative analysis also reveals that scrambled loci are often associated with local duplications, supporting a gradual model for the origin of complex, scrambled genomes via many small events of DNA duplication and decay.
Custom scripts are public on https://github.com/yifeng-evo/Oxytricha_Tetmemena_Euplotes. DNA-seq reads and genome assemblies are available at GenBank under Bioprojects PRJNA694964 (Tetmemena sp.) and PRJNA781979 (Euplotes woodruffi). Genbank accession numbers for genomes are JAJKFJ000000000 (Tetmemena sp. Micronucleus genome), JAJLLS000000000 (Euplotes woodruffi Micronucleus genome), and JAJLLT000000000 (Euplotes woodruffi Macronucleus genome).Three replicates of RNA-seq reads for vegetative cells are available at GenBank under accession numbers of SRR21815378, SRR21815379, SRR21815380 for E. woodruffi and SRR21817702, SRR21817703 and SRR21817704 for Tetmemena sp..MDSs annotations for three species are available at https://doi.org/10.5061/dryad.5dv41ns96 and https://knot.math.usf.edu/mds_ies_db/2022/downloads.html (please select species from the drop-down menu).
Euplotes woodruffi genome sequencing and assemblyNCBI Bioproject, PRJNA781979.
etmemena sp. micronucleus genome sequencing and assemblyNCBI Bioproject, PRJNA694964.
Euplotes woodruffi RNA-seq for vegetative cellsNCBI Bioproject, PRJNA781602.
Tetmemena sp. RNA-seq for vegetative cellsNCBI Bioproject, PRJNA887426.
MDS and IES annotations for Euplotes woodruff, Tetmemena sp. and Oxytricha trifallaxDryad Digital Repository, doi:10.5061/dryad.5dv41ns96.
Oxytricha trifallax micronucleus genomeGenbank GCA_000711775.1.
Oxytricha trifallax macronucleus genomeGenbank GCA_000295675.1.
Tetmemena sp. macronucleus genomeGenbank GCA_001273295.2.
Oxytricha trifallax RNA-seq for vegetative cellsNCBI SRA SRX5944382, SRX5944383 and SRX5944384.
Article and author information
National Institutes of Health (R35GM122555)
- Yi Feng
National Science Foundation (DMS1764366)
- Yi Feng
Pew Latin American Fellows Program (no)
- Rafik Neme
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
- Detlef Weigel, Max Planck Institute for Biology Tübingen, Germany
- Preprint posted: May 10, 2022 (view preprint)
- Received: August 25, 2022
- Accepted: November 3, 2022
- Accepted Manuscript published: November 24, 2022 (version 1)
- Version of Record published: December 28, 2022 (version 2)
© 2022, Feng et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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