Telomere structure in insects: A review

Telomeres are terminal regions of chromosomes, which protect them from fusion with other chromosomes and stabilize their structure. Telomeres usually contain specific DNA repeats (motifs), which are maintained by telomerase, a kind of reverse transcriptase. In this review, we survey the current state of knowledge of telomere motifs in insects. Among Hexapoda, data on telomere composition are available for more than 350 species from 108 families and 25 orders. The telomere motif (TTAGG)n is considered ancestral for the class Insecta. However, certain insects have different and often unknown telomeric sequences. This apparently happens because telomerase-dependent mechanisms usually coexist with various means of alternative lengthening of telomeres as backup mechanisms of telomere maintenance. This coexistence can explain losses and reappearances of the TTAGG repeat and telomerase-dependent telomere replication in insect evolution. For example, a few higher taxa, such as Heteroptera (Hemiptera) and Hymenoptera, show presence of the (TTAGG)n motif in their basal clades as well as a subsequent loss and, at least in the Hymenoptera, independent reappearance of this repeat in some advanced groups. Analogously, most members of Coleoptera also retain the TTAGG repeat, although it is changed to TCAGG in certain families. Furthermore, the (TTAGG)n motif seems to have been irreversibly lost in the order Diptera. In this group, telomeric sequences are represented either by long terminal repeats or by retrotransposons. Retrotransposons are also interspersed with other telomeric sequences in many groups of insects. The accumulating data demonstrate that the class Insecta is substantially diverse in terms of its telomere structure.