Accumulation of telomeric-repeat-specific retrotransposons in subtelomeres of bombyx mori and tribolium castaneum

Abstract

Arthropod telomeres are generally constituted by TTAGG pentanucleotide repeats, which are synthesized by telomerase. However, all species in Diptera examined to date have lost TTAGG repeats and are suggested to recruit telomerase-independent telomere maintenance. In contrast, the silkworm Bombyx mori retains TTAGG telomeric repeats, but the telomerase activity is repressed in quite a low level in all investigated tissues. In addition, the flour beetle Tribolium castaneum, which contains unconventional TCAGG telomeric repeats, also shows a weak telomerase activity. Telomerase reverse transcriptase (TERT) genes for B. mori (BmoTERT) and T. castaneum (TcTERT) have several unusual features; both TERT genes without introns have upstream ATG codons and no N-terminal GQ motifs, which possibly explain their repressed telomerase activity. In subtelomeres of Bombyx and Tribolium, telomeric-repeat-specific non-long terminal repeat (LTR) retrotransposons (or long interspersed nuclear elements (LINEs)), SARTBm and SARTTc, are accumulated. Respective retrotransposons prefer telomeric repeats of their hosts. This chapter focuses on subtelomere, TERT, and telomeric-repeat-specific LINEs in Bombyx and Tribolium and discusses mechanisms and evolution for telomere maintenance in higher insects.