A family of complex tandem DNA repeats in the telomeres of Chironomus pallidivittatus

Are the TTAGG and TTAGGG telomeric repeats phylogenetically conserved in aculeate Hymenoptera?

Despite the (TTAGG)_n telomeric repeat supposed being the ancestral DNA motif of telomeres in insects, it was repeatedly lost within some insect orders. Notably, parasitoid hymenopterans and the social wasp Metapolybia decorata (Gribodo) lack the (TTAGG)_n sequence, but in other representatives of Hymenoptera, this motif was noticed, such as different ant species and the honeybee. These findings raise the question of whether the insect telomeric repeat is or not phylogenetically predominant in Hymenoptera. Thus, we evaluated the occurrence of both the (TTAGG)_n sequence and the vertebrate telomere sequence (TTAGGG)_n using dot-blotting hybridization in 25 aculeate species of Hymenoptera. Our results revealed the absence of (TTAGG)_n sequence in all tested species, elevating the number of hymenopteran families lacking this telomeric sequence to 13 out of the 15 tested families so far. The (TTAGGG)_n was not observed in any tested species. Based on our data and compiled information, we suggest that the (TTAGG)_n sequence was putatively lost in the ancestor of Apocrita with at least two subsequent independent regains (in Formicidae and Apidae).

TTAGG-repeat telomeres and characterization of telomerase in the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae)

Telomeres are maintained usually by telomerase, a specialized reverse transcriptase that adds this sequence to chromosome ends. In this study, telomerase activity was detected in the in different somatic tissues, such as midgut and fat bodies, by the telomeric repeat amplification protocol (TRAP) in Spodoptera exigua. The structure of the telomeres of S. exigua was evaluated by sequence analysis of the TRAP products, revealing that the telomerase synthesized a (TTAGG)n repeat. The presence of a telomerase reverse transcriptase (TERT) subunit coding gene has been cloned, sequenced and expressed in vitro successively. Notably, the S. exigua telomerase (SpexTERT) gene structure lacks the N-terminal GQ motif. Telomerase contains a large RNA subunit, TER, and a protein catalytic subunit, TERT. Here we report an in vitro system that was reconstructed by all components of the telomerase complex, a purified recombinant SpexTERT without a N-terminal GQ motif and a mutant human telomerase RNA (TER), showed telomerase activity. Together, these results suggest the GQ motif is not essential for telomerase catalysis.

Karyotype rearrangements and telomere analysis in Myzuspersicae (Hemiptera, Aphididae) strains collected on Lavandula sp. plants

Karyotype analysis of nine strains of the peach-potato aphid Myzuspersicae (Sulzer, 1776), collected on Lavandula sp. plants, evidenced showed that five of them had a standard 2n = 12 karyotype, one possessed a fragmentation of the X chromosome occurring at the telomere opposite to the NOR-bearing one and three strains had a chromosome number 2n = 11 due to a non-reciprocal translocation of an autosome A3 onto an A1 chromosome. Interestingly, the terminal portion of the autosome A1 involved in the translocation was the same in all the three strains, as evidenced by FISH with the histone cluster as a probe. The study of telomeres in the Myzuspersicae strain with the X fission evidenced that telomerase synthesised de novo telomeres at the breakpoints resulting in the stabilization of the chromosomal fragments. Lastly, despite the presence of a conserved telomerase, aphid genome is devoid of genes coding for shelterin, a complex of proteins involved in telomere functioning frequently reported as conserved in eukaryotes. The absence of this complex, also confirmed in the genome of other arthropods, suggests that the shift in the sequence of the telomeric repeats has been accompanied by other changes in the telomere components in arthropods in respect to other metazoans.

Phylogenetic distribution of TTAGG telomeric repeats in insects

We examined the presence of TTAGG telomeric repeats in 22 species from 20 insect orders with no or inconclusive information on the telomere composition by single-primer polymerase chain reaction with (TTAGG)6 primers, Southern hybridization of genomic DNAs, and fluorescence in situ hybridization of chromosomes with (TTAGG)n probes. The (TTAGG)n sequence was present in 15 species and absent in 7 species. In a compilation of new and published data, we combined the distribution of (TTAGG)n telomere motif with the insect phylogenetic tree. The pattern of phylogenetic distribution of the TTAGG repeats clearly supported a hypothesis that the sequence was an ancestral motif of insect telomeres but was lost repeatedly during insect evolution. The motif was conserved in the "primitive" apterous insect orders, the Archaeognatha and Zygentoma, in the "lower" Neoptera (Plecoptera, Phasmida, Orthoptera, Blattaria, Mantodea, and Isoptera) with the exception of Dermaptera, and in Paraneoptera (Psocoptera, Thysanoptera, Auchenorrhyncha, and Sternorrhyncha) with the exception of Heteroptera. Surprisingly, the (TTAGG)n motif was not found in the "primitive" pterygotes, the Palaeoptera (Ephemeroptera and Odonata). The Endopterygota were heterogeneous for the occurrence of TTAGG repeats. The motif was conserved in Hymenoptera, Lepidoptera, and Trichoptera but was lost in one clade formed by Diptera, Siphonaptera, and Mecoptera. It was also lost in Raphidioptera, whereas it was present in Megaloptera. In contrast with previous authors, we did not find the motif in Neuroptera. Finally, both TTAGG-positive and TTAGG-negative species were reported in Coleoptera. The repeated losses of TTAGG in different branches of the insect phylogenetic tree and, in particular, in the most successful lineage of insect evolution, the Endopterygota, suggest a backup mechanism in the genome of insects that enabled them frequent evolutionary changes in telomere composition.

Telomeric localization of the vertebrate-type hexamer repeat, (TTAGGG)n, in the wedgeshell clam Donax trunculus and other marine invertebrate genomes

The hexamer repeat sequence (TTAGGG)(n), found at the ends of all vertebrate chromosomes, was previously identified as the main building element of one member of a HindIII satellite DNA family characterized in the genome of the bivalve mollusc Donax trunculus. It was also found in 22 perfect tandem repeats in a cloned junction region juxtaposed to the proper satellite sequence, from which the DNA tract encompassing the clustered tandem copies was excised and subcloned. Here, the chromosomal distribution of (TTAGGG)(n) sequences in the Donax was studied by the sensitivity to Bal31 exonuclease digestion, fluorescence in situ hybridization (FISH) on metaphase chromosomes and rotating-field gel electrophoresis. To verify the occurrence of the hexamer repeat in the genomes of taxonomically related molluscs and other marine invertebrates, genomic DNA from the mussel Mytilus galloprovincialis and the echinoderm Holothuria tubulosa was also analyzed. The kinetics of Bal31 hydrolysis of high molecular mass DNA from the three marine invertebrates revealed a marked decrease over time of the hybridization with the cloned (TTAGGG)(22) sequence, concomitantly with a progressive shortening of the positively reacting DNA fragments. This revealed a marked susceptibility to exonuclease consistent with terminal positioning on the respective chromosomal DNAs. In full agreement, FISH results with the (TTAGGG)(22) probe showed that the repeat appears located in telomeric regions in all chromosomes of both bivalve molluscs. The presence of (TTAGGG)(n) repeat tracts in marine invertebrate telomeres points to its wider distribution among eukaryotic organisms and suggests an ancestry older than originally presumed from its vertebrate distinctiveness.

Telomerase, immortality and cancer

Replication of eukaryotic linear chromosomes is incomplete and leaves terminal gaps. The evolutionary widely distributed solution to this "end replication" is twofold: chromosome ends are capped with telomeres, bearing multiple copies of redundant telomeric sequences, and the telomerase enzyme can add (lost) telomeric repeats. Telomerase in humans, as in all mammals, is ubiquitous in all embryonic tissues. In adults, telomerase remains active in germs cells, and, although down-regulated in most somatic tissues, telomerase is active in regenerative tissues and notably, in tumor cells. Telomerase activity is linked to cellular proliferation, and its activation seems to be a mandatory step in carcinogenesis. In contrast to mammals, indeterminately growing multicellular organisms, like fish and crustaceae, maintain unlimited growth potential or 'immortality' in all somatic tissues throughout their entire life. Also this cell immortalization is brought about by maintaining telomerase expression. Disease prognosis for human tumors includes evaluation of cell proliferation, based on the detection of proliferation markers with monoclonal antibodies. The significance of the classical marker Ki-67, and of a novel marker repp-86 are compared with semiquantitative telomerase assays. For tumor therapy, telomerase inhibitors are attractive tools. Results with telomerase knock-out mice have revealed promise, but also risk of this approach. On the other side, telomerase stimulation is attractive for expanding the potential of cellular proliferation in vitro, with possible applications for transplantation of in vitro expanded human cells, for immortalizing primary human cells as improved tissue models, and for the isolation of otherwise intractable products, like genuine human monoclonal antibodies.

Detection and distribution patterns of telomerase activity in insects

Telomeres of most insects consist of pentanucleotide (TTAGG)n repeats, although the repeats are absent in Diptera and some other insect species, where the telomere regions are perhaps maintained without telomerase. To understand various and unusual telomere formation in insects, we have studied the characteristic features of a putative insect telomerase that has not been previously described. Using a modified telomeric repeat amplification protocol (TRAP), we first detected the telomerase activity in crickets, cockroaches and two Lepidopteran insects. The telomerase from crickets and cockroaches required dATP, dGTP and dTTP but not dCTP as a substrate and sequence analyses of the products of TRAP revealed that the (TTAGG)n repeats are synthesized by telomerase. The cockroach telomerase was detected both in somatic (fat body, muscle and neural tissues) and germ line (testis) cells, suggesting that expression of this enzyme is not regulated in a tissue-specific manner at an adult stage. While we detected high levels of telomerase activity in crickets and cockroaches, we could not detect activity in all tissues and cell cultures of the silkworm, Bombyx mori and in two Drosophila and one Sarcophaga cell lines. This supports the theory that Dipteran insects maintain their telomeres without telomerase.

TAS49--a dispersed repetitive sequence isolated from subtelomeric regions of Nicotiana tomentosiformis chromosomes

We have isolated and characterized a new repetitive sequence, TAS49, from terminal restriction fragments of Nicotiana tomentosiformis genomic DNA by means of a modified vectorette approach. The TAS49 was found directly attached to telomeres of N. tabacum and one of its ancestors, N. tomentosiformis, and also at inner chromosome locations. No association with telomeres was detected neither in N. otophora nor in the second tobacco ancestor, N. sylvestris. PCR and Southern hybridization reveal similarities in the arrangement of TAS49 on the chromosomes of 9 species of the genus Nicotiana, implying its occurrence as a subunit of a conserved complex DNA repeat. TAS49 belongs to the family of dispersed repetitive sequences without features of transposons. The copy number of TAS49 varies widely in the genomes of 8 species analyzed being lowest in N. sylvestris, with 3300 copies per diploid genome. In N. tomentosiformis, TAS49 forms about 0.56% of the diploid genome, corresponding to 17400 copies. TAS49 units are about 460 bp long and show about 90% of mutual homology, but no significant homology to DNA sequences deposited in GenBank and EMBL. Although genomic clones of TAS49 contain an open reading frame encoding a proline-rich protein similar to plant extensins, no mRNA transcript was detected. TAS49 is extensively methylated at CpG and CpNpG sites and its chromatin forms nucleosomes phased with a 170 +/- 8 bp periodicity.

Interspersed DNA element restricted to a specific group of telomeres in the dipteran Chironomus pallidivittatus

Telomeres in the dipteran Chironomus pallidivittatus terminate with 340bp tandem DNA repeats belonging to different subfamilies with characteristic intertelomeric distribution. We have now found, interspersed between such repeats, a composite element of approx. 1400bp present in two similar size variants, with several components of nontelomeric origin. There were about 50 copies of the element, predominantly or exclusively present in a previously defined group of telomeres, characterized by a unique set of telomeric tandem repeat subfamilies. Elements were integrated at irregular distances from each other, and intervening telomeric tandem repeat DNA was variable in composition. Nevertheless, the flanks immediately surrounding the elements were identical for different elements; in other words, there was a site-specific insertion. We suggest that this selective invasion of a small part of the genome by an interspersed, probably rapidly evolving element is best explained by repeated gene conversions.

Telomeres terminating with long complex tandem repeats

Telomeres of most investigated species terminate with short repeats and are elongated by telomerase. Short repeats have never been detected in dipteran species which have found other solutions to end a chromosome. Whereas in Drosophila melanogaster retroelements are added onto the termini, chironomids have long complex repeats at their chromosome ends. We review evidence that these units are terminal and probably have evolved from short telomeric repeats. In Chironomus pallidivittatus the units have been shown to belong to different subfamilies which have specific inter- and intrachromosomal distribution, the most terminal subfamily of repeats being characterized by pronounced secondary structures for the single strand. The complex repeats are efficiently homogenized both within and between different chromosome ends. Gene conversion is probably an important component in the coordinate evolution of the repeats but it is not known whether it is used for net synthesis of DNA. RNA is used as an intermediate in telomere elongation both by organisms having chromosomes terminating with short repeats and by D. melanogaster. It is therefore interesting that the terminal repeats in chironomids are transcribed.

How do Alliaceae stabilize their chromosome ends in the absence of TTTAGGG sequences?

The Arabidopsis-type telomeric repeats (5'-TTTAGGG-3) are highly conserved. In most families of different plant phyla they represent the basic sequence of telomeres that stabilize and protect the chromosome termini. The results presented here show that Alliaceae and some related liliaceous species have no tandemly repeated TTTAGGG sequences. Instead, their chromosomes reveal highly repetitive satellite and/or rDNA sequences at the very ends. These apparently substitute the original plant telomeric sequences in Alliaceae. Both sequence types are very active in homologous recombination and may contribute to the stabilization of chromosome termini via compensation of replication-mediated shortening.