Telomere-specific non-LTR retrotransposons and telomere maintenance in the silkworm, Bombyx mori

Most insects have telomeres that consist of pentanucleotide (TTAGG) telomeric repeats, which are synthesized by telomerase. However, all species in Diptera so far examined and several species in other orders of insect have lost the (TTAGG)_n repeats, suggesting that some of them recruit telomerase-independent telomere maintenance. The silkworm, Bombyx mori, retains the TTAGG motifs in the chromosomal ends but expresses quite a low level of telomerase activity in all stages of various tissues. Just proximal to a 6-8-kb stretch of the TTAGG repeats in B. mori, more than 1000 copies of non-LTR retrotransposons, designated TRAS and SART families, occur among the telomeric repeats and accumulate. TRAS and SART are abundantly transcribed and actively retrotransposed into TTAGG telomeric repeats in a highly sequence-specific manner. They have three possible mechanisms to ensure specific integration into the telomeric repeats. This article focuses on the telomere structure and telomere-specific non-LTR retrotransposons in B. mori and discusses the mechanisms for telomere maintenance in this insect.