Genomic GC content can vary locally, and GC-rich regions are usually associated with increased DNA thermostability in thermophilic prokaryotes and warm-blooded eukaryotes. Among vertebrates, fish and amphibians appeared to possess a distinctly less heterogeneous AT/GC organization in their genomes, whereas cytogenetically detectable GC heterogeneity has so far only been documented in mammals and birds. The subject of our study is the gar, an ancient "living fossil" of a basal ray-finned fish lineage, known from the Cretaceous period. We carried out cytogenomic analysis in two gar genera (Atractosteus and Lepisosteus) uncovering a GC chromosomal pattern uncharacteristic for fish. Bioinformatic analysis of the spotted gar (Lepisosteus oculatus) confirmed a GC compartmentalization on GC profiles of linkage groups. This indicates a rather mammalian mode of compositional organization on gar chromosomes. Gars are thus the only analyzed extant ray-finned fishes with a GC compartmentalized genome. Since gars are cold-blooded anamniotes, our results contradict the generally accepted hypothesis that the phylogenomic onset of GC compartmentalization occurred near the origin of amniotes. Ecophysiological findings of other authors indicate a metabolic similarity of gars with mammals. We hypothesize that gars might have undergone convergent evolution with the tetrapod lineages leading to mammals on both metabolic and genomic levels. Their metabolic adaptations might have left footprints in their compositional genome evolution, as proposed by the metabolic rate hypothesis. The genome organization described here in gars sheds new light on the compositional genome evolution in vertebrates generally and contributes to better understanding of the complexities of the mechanisms involved in this process.

Chair in Zoology and Evolutionary Biology Department of Biology University of Konstanz Konstanz Germany

Department of Zoology Faculty of Science Charles University Prague 2 Czech Republic

División Académica de Ciencias Biológicas Universidad Juárez Autónoma de Tabasco Villahermosa Tabasco México

Faculty of Fisheries and Protection of Waters South Bohemian Research Centre of Aquaculture and Biodiversity of Hydrocenoses University of South Bohemia in České Budějovice Vodňany Czech Republic

IBIS Department of Biology University Laval Pavillon Charles Eugène Marchand Avenue de la Médecine Quebec City Canada

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics The Czech Academy of Sciences Liběchov Czech Republic

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics The Czech Academy of Sciences Liběchov Czech Republic Department of Ecology Faculty of Science Charles University Prague 2 Czech Republic

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics The Czech Academy of Sciences Liběchov Czech Republic Department of Zoology Faculty of Science Charles University Prague 2 Czech Republic

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics The Czech Academy of Sciences Liběchov Czech Republic Department of Zoology Faculty of Science Charles University Prague 2 Czech Republic Research Institute for Limnology University of Innsbruck Mondsee Austria

Muséum d'Histoire Naturelle Geneva 6 Switzerland

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