The telomere repeat motif of basal Metazoa

Abstract

In most eukaryotes the telomeres consist of short DNA tandem repeats and associated proteins. Telomeric repeats are added to the chromosome ends by telomerase, a specialized reverse transcriptase. We examined telomerase activity and telomere repeat sequences in representatives of basal metazoan groups. Our results show that the 'vertebrate' telomere motif (TTAGGG)( n ) is present in all basal metazoan groups, i.e. sponges, Cnidaria, Ctenophora, and Placozoa, and also in the unicellular metazoan sister group, the Choanozoa. Thus it can be considered the ancestral telomere repeat motif of Metazoa. It has been conserved from the metazoan radiation in most animal phylogenetic lineages, and replaced by other motifs-according to our present knowledge-only in two major lineages, Arthropoda and Nematoda.

Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms

“…Longevity experiments to investigate senescence in Hydra that reproduced asexually suggest that they are immortal (4,39), whereas a sexually reproducing species showed clear signs of degeneration and mortality (40). Although immortal Hydra also appears to share the (TTAGGG)n telomere repeat, there is as yet no data on how or whether they avoid chromosome end depletion (41). These data suggest the possibility that senescence or death of asexual individuals and colonies may in part result from a failure to maintain chromosome ends that are restored by going through a sexually reproductive cycle (35).…”

Telomere Maintenance in Organisms without Telomerase

“…Arrows mark the replacement of the metazoan motif with other motifs, as shown. The cladogram is based on Frydrychova et al, 2004;Vitkova et al, 2005;Traut et al, 2007. www.intechopen.com recombination mechanism involving these long blocks of complex repeat units (Biessmann & Mason, 1997;Cohn & Edstrom, 1992;Cohn & Edström, 1992;Nielsen & Edstrom, 1993). A situation has been observed in Anopheles gambiae with a plasmid insertion into the complex satellite telomeric sequences at the tip of chromosome 2L.…”

“…Saccharomyces for example uses (TG 1-3 ) n (Shampay et al, 1984;Wang & Zakian, 1990), while Schizosaccharomyces has (TTACAG 1-8 ) n (Matsumoto et al, 1987). The sequence found at the telomeres of most metazoans is (TTAGGG) n (Meyne et al, 1989;Traut et al, 2007), although arthropods use (TTAGG) n (Okazaki et al, 1993). Lack of the predominant telomeric sequence in a species does not, however, signify that telomerase-generated terminal sequences are missing.…”

“…Conversely, the presence of a canonical telomeric sequence does not necessarily indicate telomerase as a telomere maintenance mechanism. Some species of Calcarea (sponges), Cnidaria (sea anemones and jellyfish) and Placozoa, which keep the metazoan telomeric sequence, display little or no telomerase activity (Traut et al, 2007). Although telomerase may have been the mechanism of telomere maintenance of the last common eukaryotic ancestor, it is not the only mechanism used to maintain chromosome length.…”

Telomere-independent ageing in the longest-lived non-colonial animal, Arctica islandica

“…Shortening of telomeres is discussed as one responsible factor for the replicative senescence of somatic cells, and is proposed to be one of the reasons why animals age and show an age-related increase of mortality (Wright and Shay, 2005). The telomere structure, consisting of DNA repeat sequences (5′-TTAGGG-3′), is highly conserved among vertebrates (Meyne et al, 1989) as well as among invertebrates (Traut et al, 2007). Cells approach senescence if telomere length reaches a certain threshold, the so-called Hayflick limit (Bolzán and Bianchi, 2006;Hayflick, 1965).…”