Terminal Long Tandem Repeats in Chromosomes from Chironomus pallidivittatus

Abstract

We provide evidence that a chromosome end in the dipteran Chironomus pallidivittatus contains 340-bp tandem repeats reaching the extreme terminus of the chromosome. After adding synthetic oligonucleotide tails to DNA extracted from the microdissected right end of the fourth chromosome, we could demonstrate that the blocks of repeats were tailed at only one end, the chromosome terminus, the interior of the arrays being unavailable for tailing. Using PCR, we furthermore showed that the added tails were connected to 340-bp repeat DNA directly, i.e., without intervening DNA of any other kind. The tailed repeats belong to a subfamily previously known to be the most peripheral one of the different types of 340-bp units. Using plasmid controls, we could also make certain that we did not amplify rare or nonrepresentative DNA termini.Telomeres of most investigated eukaryotic species terminate with short repeats, usually with guanine excess in the 3Ј-terminating strand (4). Such repeats, whether regular, as in most cases (5,17,21,25), or irregular, as in Saccharomyces cerevisiae (29) among others, are likely to result from telomerase activities (6,11,16,31).Short repeats are also found in several insect orders (23), but for the dipteran Drosophila melanogaster a different mode of chromosome termination has been established. Here terminally deleted chromosomes can be capped by non-long-terminal-repeat retrotransposons (2,3,30,33), which can also be recovered at the ends of normal chromosomes (15).No other type of ending is known for nuclear chromosomes, but the linear mitochondrial DNA from Tetrahymena thermophila ends with Ͼ50-bp-long repeat units (19,20). We present evidence that this alternative, here in the form of 340-bp tandem repeats (27), is used by nuclear chromosomes in the dipteran Chironomus pallidivittatus.The 340-bp tandem repeat family is localized at seven of the eight (pairs of) chromosome ends in Ͻ200-kb blocks. Although the blocks are close to the chromosome ends as revealed by Bal 31 digestions (34), it has not been known whether they reach the extreme termini. The repeat family contains four different subfamilies, three of which, D1, D2, and D3, are derived from the fourth one, M1, each by a cluster of mutations in short regions designated ds1, ds2, and ds3, respectively (7). In the case of D3 there is a second difference, i.e., one large region has become substituted for another one. Telomeres differ in subfamily composition (8,34). When D3 is present together with other kinds of repeats, it is always the most peripheral unit (34).Briefly, our strategy was to gently release DNA from single microdissected right ends of the fourth chromosome and provide it with tails. Anchors added to the tails and primers for the 340-bp repeat were used for PCR amplifications. We could demonstrate that no internal breaks were tailed within the repeat blocks and that the tails were added directly to the 340-bp repeat DNA. We also showed that only DNA from a specific subfamily, previously shown to be distal (34), was amp...

Unusual features of theDrosophila melanogastertelomere transposable elementHeT-Aare conserved inDrosophila yakubatelomere elements

“…Species that had no cross-hybridization included the five Diptera and three of the eight Coleoptera studied. Lack of cross-hybridization does not prove that a species lacks telomerase; however, at least two Diptera, D. melanogaster and Chironomous pallidivittatus, are known to have chromosome end sequences that differ significantly from the simple repeats generated by telomerase (2,23,24). Thus these species have either lost telomerase or, as we have argued (3,25), their telomerase has undergone significant evolution.…”

“…Telomeres in D. melanogaster are composed of the retrotranposons, HeT-A and TART, whereas telomeres in C. pallidivittatus and Chironomous thummi are composed of large complex repeats (23,24). The mechanism by which the Chironomous repeats are generated is not known.…”

Telomere maintenance in Drosophila: Rapid transposon evolution at chromosome ends

“…1C), suggesting that telomere elongation takes place by homologous recombination. 21 In Drosophila, however, telomeres are primarily maintained by transposition of telomeric retrotransposons to receding chromosome ends (Fig. 1D), and, as in any eukaryote, by recombination (reviewed in refs.…”

Telomere-Telomere Recombination Is an Efficient Bypass Pathway for Telomere Maintenance in Saccharomyces cerevisiae

“…Both telomerase and transposition contribute to telomere maintenance in the green alga Chlorella (17). Telomere-telomere recombination is thought to be the sole mechanism for maintaining the repeats at chromosome ends in some insects, such as the mosquito Anopheles (40) and the dipteran Chironomus (23).…”