Genome sizes of eukaryotic organisms vary substantially, with whole-genome duplications (WGD) and transposable element expansion acting as main drivers for rapid genome size increase. The two North American mudminnows, Umbra limi and Umbra pygmaea, feature genomes about twice the size of their sister lineage Esocidae (e.g., pikes and pickerels). However, it is unknown whether all Umbra species share this genome expansion and which causal mechanisms drive this expansion. Using flow cytometry, we find that the genome of the European mudminnow is expanded similarly to both North American species, ranging between 4.5 and 5.4 pg per diploid nucleus. Observed blocks of interstitially located telomeric repeats in U. limi suggest frequent Robertsonian rearrangements in its history. Comparative analyses of transcriptome and genome assemblies show that the genome expansion in Umbra is driven by the expansion of DNA transposon and unclassified repeat sequences without WGD. Furthermore, we find a substantial ongoing expansion of repeat sequences in the Alaska blackfish Dallia pectoralis, the closest relative to the family Umbridae, which might mark the beginning of a similar genome expansion. Our study suggests that the genome expansion in mudminnows, driven mainly by transposon expansion, but not WGD, occurred before the separation into the American and European lineage.

Center for Desert Agriculture Biological and Environmental Sciences and Engineering Division King Abdullah University of Science and Technology Thuwal Kingdom of Saudi Arabia

Department of Bioinformatics Wissenschaftzentrum Weihenstephan Technische Universität München Freising Germany

Department of Biology Faculty of Biology University of Hradec Kralove Czech Republic

Department of Biology IBIS Université Laval Québec QC Canada

Department of Marine Biology and Ecology Institute of Oceanography Faculty of Oceanography and Geography University of Gdansk Gdansk Poland

Department of Zoology Faculty of Biology and Biotechnology University of Warmia and Mazury Olsztyn Poland

Division of Biological and Environmental Sciences and Engineering Computer Electrical and Mathematical Sciences and Engineering Division King Abdullah University of Science and Technology Thuwal Kingdom of Saudi Arabia

Institute of Life Sciences Scuola Superiore Sant'Anna Pisa Italy

Laboratory of Molecular Epigenetics Institute of Biophysics Czech Academy of Science Brno Czech Republic

Research Department for Limnology Mondsee University of Innsbruck Mondsee Austria

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