Earwig genomes reveal a brand-new telomere code never seen in insects

Deep inside the genome of one of the world’s most familiar backyard insects, a molecular secret has been hiding in plain sight. Earwigs, the pincer-tailed order Dermaptera, build the protective caps at the ends of their chromosomes from a five-letter DNA word that has never before been recorded in any insect, and never before in Arthropoda as a whole. The discovery, published in the journal Comparative Cytogenetics, rewrites a textbook assumption that has stood for two decades and adds a surprising new branch to the map of how insects safeguard their genetic inheritance.

For most of the class Insecta, and indeed for the phylum Arthropoda at large, the ends of chromosomes are capped by tandem arrays of a short six-base motif, TTAGG, repeated tens, hundreds or even thousands of times. This sequence is considered the ancestral telomeric motif for insects, preserved across the majority of studied orders and families, from beetles to bugs to butterflies. But recent years have chipped away at that tidy picture. Surveys of telomeric DNA across Insecta have uncovered a surprisingly wide range of alternatives, with repeat units ranging from a single nucleotide up to eleven, and in a few Diptera families, such as the hoverflies (Syrphidae) and tachinid flies (Tachinidae), chromosome ends are built from much longer repeats of 173 to 381 base pairs. In many insects, these short telomeric arrays are also interspersed with telomere-specific non-LTR retrotransposons of the SART and TRAS families, adding yet another layer of complexity to what was once considered a simple, uniform structure.

Despite this growing catalogue of variation, the telomeric DNA of many insect orders remains completely uncharacterized, a gap that frustrates attempts to reconstruct the ancestral organization of insect chromosomes and to trace the evolution of the mechanisms that maintain telomere length. Earwigs were one of the most tantalizing blanks on that map. An earlier attempt to probe the telomeres of the common European earwig Forficula auricularia using fluorescence in situ hybridization with a TTAGG probe came up empty, and two separate Southern blot hybridization studies also failed to detect the canonical motif. Those failures were interpreted as evidence that TTAGG is absent from earwig telomeres, but they offered no clue as to what, if anything, was actually sitting at the ends of earwig chromosomes.

The new study, carried out by a single researcher analyzing publicly available genome assemblies, finally cracks the case. The centrepiece of the analysis is a chromosome-level genome assembly of the lesser earwig Labia minor, a small spongiphorid species generated by the Darwin Tree of Life Project and deposited in the European Nucleotide Archive. Because this assembly resolves each of the species’ seven chromosomes as a single contiguous sequence, the terminal regions could be visually inspected directly. In parallel, scaffold-level assemblies of three other earwigs, Forficula auricularia, Euborellia annulipes and Anisolabis maritima, were searched using the GenBank blastn function with the low-complexity filter disabled, probing for extended arrays of both the canonical (TTAGG) and the candidate (TTCGG) motifs.

The results from Labia minor are striking. In the analyzed assembly, no telomeric motifs were found on chromosomes 1, 2, 4 and 6, nor on the left ends of chromosomes 5 and X. That absence is not unexpected. Telomeric sequences at chromosome ends are notoriously the most difficult genome regions to assemble with accuracy and completeness, and many assemblies are simply truncated at the tips or lack telomeric repeats altogether. But where telomeres were successfully assembled, they announced themselves unambiguously. The right ends of chromosomes 3, 5 and X terminate in long arrays of the five-base motif TTCGG, while the left end of chromosome 3 carries the reverse-complement sequence CCGAA, exactly as the orientation of a true telomere would dictate. Each of these terminal arrays contains between 351 and 481 identical repeats, and short TTCGG/CCGAA triplets and quadruplets are vanishingly rare in interstitial positions along the chromosome arms.

The combination of features observed here is the signature that cytogeneticists use to recognize conserved telomeres in genome assemblies. Each telomeric end is represented by a large number of exactly identical short repeats. The same motifs appear on different chromosomes. Where both ends of a chromosome are captured, they show reverse-complement structure and orientation. And short telomere-like motifs are scarce away from the chromosome tips. The probability that the long TTCGG/CCGAA arrays at the ends of chromosomes 3, 5 and X represent a random combination of non-telomeric nucleotides, the study concludes, is vanishingly small. In other words, these are genuine telomeres, and they are built from a motif that has never been reported in insects before.

The analysis did not stop at the telomeres themselves. The subtelomeric regions flanking the terminal arrays turned out to be rich in variant repeats of TTCAGTT and TTCGGTT, seven-base sequences closely related to the telomeric motif but slightly altered. The presence of such near-telomeric variants is a well-known hallmark of subtelomeric regions, previously documented in other organisms, and its recurrence here in earwigs reinforces the identification of the TTCGG arrays as bona fide telomeric DNA rather than some unrelated tandem repeat that happens to sit at chromosome ends.

The scaffold-level assemblies of the three other earwig species tell a consistent story. When these assemblies were blasted with a (TTCGG)30 query, they revealed extended arrays of (TTCGG)n. When they were blasted with a (TTAGG)30 query, no extended arrays of (TTAGG)n appeared. This pattern suggests that the TTCGG telomeric motif is most likely present in Forficula auricularia, Euborellia annulipes and Anisolabis maritima as well, and is therefore widespread across the order Dermaptera, spanning at least three families: Spongiphoridae, Forficulidae and Anisolabididae.

The discovery is significant on several fronts. It is the first record describing the structure of telomeric DNA in Dermaptera, closing a gap that has persisted despite decades of cytogenetic work on the order. It expands the known spectrum of telomeric sequences in insects, since TTCGG has not previously been reported for any insect species. And it raises the possibility that this motif is a synapomorphy, a shared derived character, marking either all the studied earwig families, or all earwigs, or even a phylogenetic lineage of a higher rank. That would be a remarkable finding, because earwigs belong to the monophyletic group of orders called Polyneoptera, and for other orders within this group the canonical TTAGG motif has been confirmed. The presence of TTCGG may therefore be a feature indicating an isolated position of earwigs within Polyneoptera, a molecular fingerprint of their distinct evolutionary trajectory.

There is one more intriguing thread to pull. The order Dermaptera is sister to the enigmatic and poorly studied order Zoraptera, a group so obscure that its very placement in the insect tree was long contested. If TTCGG is indeed a shared derived character of earwigs, the next obvious question is whether Zoraptera shares it, or whether the motif arose uniquely on the earwig branch. Resolving the telomere organization of Zoraptera, the study notes, would be a compelling target for future work. Either way, the humble earwig has now delivered one of the most unexpected telomere stories in Arthropoda, and the ends of insect chromosomes are once again proving to be far more diverse, and far more interesting, than the canonical TTAGG ever suggested.

Subject of Research: Molecular genetics

Article Title: Telomere DNA in the insect order Dermaptera and the first evidence for the non-canonical telomeric motif TTCGG in Arthropoda

Article References: Lukhtanov, V. A. (2025). Telomere DNA in the insect order Dermaptera and the first evidence for the non-canonical telomeric motif TTCGG in Arthropoda. Comparative Cytogenetics, 19, 13-18. https://doi.org/10.3897/compcytogen.19.142613

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.142613

Keywords: telomeres, Dermaptera, earwigs, genomics, TTCGG motif, chromosome ends, Comparative Cytogenetics, Labia minor, Polyneoptera, subtelomeric repeats, Telomere, insect