An international team of researchers led by Dmitrii I. Ostromyshenskii and Aleksandra O. Travina of the Institute of Cytology of the Russian Academy of Sciences, together with colleagues from The Center for Bio- and Medical Technologies in Moscow, carried out a genome-wide analysis of tandem repeats in nine species of ranid frogs. The species surveyed span a broad evolutionary range within the family, including the European common frog Rana temporaria, the Cascades frog Amerana muscosa, the Siberianewood frog relative Rana kukunoris, the moor frog relative Pelophylax lessonae, the northern leopard frog Lithobates pipiens, the American bullfrog Aquarana catesbeiana, Aquarana septentrionalis, the wood frog Boreorana sylvatica, and the torrent frog Staurois parvus. This choice of species allowed the team to examine repeat repertoires across multiple genera and lineages, giving the analysis genuine comparative power.

The technical foundation of the study was a systematic computational search for tandem repeats, sequences in which a repeating unit, or monomer, is arranged head to tail in long arrays. Using the Tandem Repeats Finder algorithm on genome assemblies of all nine species, the researchers catalogued thousands of repeat families, characterized their monomer lengths, GC content, array lengths, and copy numbers, and grouped them into families based on sequence similarity. The scale of the result was remarkable. Tandem repeat content in these frog genomes proved to be exceptionally high, and, crucially, it correlated positively with overall genome size. In other words, the frogs with the largest genomes also carried the largest loads of satellite DNA, a finding that supports the long-standing hypothesis that non-coding repetitive sequence accumulation, rather than gene duplication alone, is a major driver of genome size expansion in amphibians.

Perhaps the most eye-catching figure in the study concerns the origin of these repeats. When the researchers searched the repeat monomers against databases of known transposable elements, they found that a very large proportion of the tandem repeats shared sequence homology with transposable element families. Depending on the species, repeats with recognizable transposable element ancestry accounted for between 43 and 91 percent of the total tandem repeat length in the genome. These TE-based tandem repeats, the authors note, correlated not only with genome size but also with overall tandem repeat content, suggesting that the flux of transposable element sequence into satellite DNA is a continuous and quantitatively significant process in ranid evolution rather than a rare curiosity.

The transposable element classes implicated include some of the most widespread mobile genetic elements in eukaryotes: LINEs, or long interspersed nuclear elements; SINEs, or short interspersed nuclear elements; LTR retrotransposons, which move via an RNA intermediate; and MITEs, miniature inverted-repeat transposable elements, along with DNA transposons of the Tc1-Mariner superfamily. Sequence alignments revealed fragments of these elements embedded within repeat monomers and arrays, providing a molecular fingerprint of their conversion into tandemly repeated satellite DNA. The mechanistic picture that emerges is one in which insertions of transposable elements, or fragments thereof, occasionally seed new tandem repeat arrays, which can then expand through processes such as replication slippage, unequal crossing over, and rolling circle-like amplification into large satellite families.

Beyond their sheer abundance, the repeat families displayed a hierarchical pattern of conservation across the nine species. At one end of the spectrum, the team identified widely conserved repeat families whose sequences could be detected by similarity searches in many or even all of the sampled frogs, hinting at ancient repeat lineages that predate the diversification of the family. At the other end, many repeat families were lineage-restricted or entirely species-specific, detectable in only one of the nine genomes. This layered organization, from ancient shared repeats to young species-private satellites, mirrors patterns seen in other organisms and underscores the rapid turnover of satellite DNA, which can expand, degrade, and be replaced over relatively short evolutionary timescales.

Computational mapping of the repeats onto genome assemblies suggested that the tandem repeats were broadly distributed across chromosomes rather than confined to a single genomic compartment. To test this at the cytogenetic level, the researchers performed fluorescence in situ hybridization, or FISH, using probes derived from representative repeat families. The FISH experiments confirmed the in silico predictions, showing that both the transposable element-derived repeats and the repeats without detectable TE homology occupy pericentromeric regions near the centromeres, subtelomeric regions near chromosome ends, and interstitial positions along chromosome arms. This widespread chromosomal distribution distinguishes the frog repeats from the classic model of satellite DNA as material concentrated exclusively at centromeres.

The comparative presence-absence analysis across the nine species, compiled in a BLAST score-based matrix in the supplementary materials, allowed the team to quantify how repeat families diverge in sequence similarity between relatives and how quickly new families appear in individual lineages. Species such as Rana temporaria, Amerana muscosa, and Staurois parvus served as reference points for characterizing the transposable element composition of their repeat arrays, and the patterns consistently reinforced the conclusion that TE-derived satellites are a pervasive feature of ranid genomes. The coexistence of conserved and lineage-restricted repeat families, many of which carry transposable element signatures, indicates that transposable elements have been recruited repeatedly, and independently, in the emergence of new satellite repeats throughout the evolutionary history of the family.

The findings carry broader implications for understanding amphibian genome evolution and genome biology in general. Amphibian genomes can reach sizes many times larger than the human genome, and the sustained accumulation and expansion of tandem repeats documented in this study offers a compelling explanation for much of that bulk. More broadly, the study adds frogs to the growing list of organisms in which transposable elements act as raw material for the birth of satellite DNA, contributing to centromere structure, heterochromatin formation, and potentially to the reproductive isolation of lineages through rapid satellite divergence. The work was supported by the Russian Science Foundation under grant 25-24-01071, and all data and supplementary tables, including family-by-family repeat catalogues and comparative matrices, are openly available alongside the open-access publication, providing a valuable resource for anyone seeking to decode the repetitive architecture of these extraordinary genomes.

Subject of Research: Genome-wide analysis of tandem repeat repertoires and transposable element contributions in ranid frog genomes

Article Title: Extensive tandem repeat repertoires in ranid frogs and the role of transposable elements in their evolution

Article References: Ostromyshenskii, D. I., Ivanova, N. G., Popova, M. A., Pasynkova, R. A., Podgornaya, O. I., Litvinchuk, S. N., & Travina, A. O. (2026). Extensive tandem repeat repertoires in ranid frogs and the role of transposable elements in their evolution. BMC Biology. https://doi.org/10.1186/s12915-026-02718-0

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02718-0

Keywords: tandem repeats, satellite DNA, transposable elements, Ranidae, amphibian genomes, genome evolution, Anura, FISH, genome size, LINE, SINE, LTR retrotransposons