Complex and tandem repeat structure of subtelomeric regions in the Taiwan cricket, Teleogryllus taiwanemma.

Telomeres of most insects are composed of simple (TTAGG) (n) repeats that are synthesized by telomerase. However, in some dipteran insects such as Drosophila melanogaster, (TTAGG) (n) repeats or telomerase activity has not been detected. Although telomere structure is well documented in Diptera and Lepidoptera, very limited information is available on lower insect groups. To understand general aspects of telomere function and evolution in insects, we endeavored to characterize structures of the telomeric and subtelomeric regions in a lower insect, the Taiwan cricket, Teleogryllus taiwanemma. FISH analysis of this insect's chromosomes demonstrated (TTAGG) (n) repeat elements in all distal ends. Just proximal to the telomeric repeats, the highly conserved 9-kb long terminal unit (LTU) sequences are tandemly repeated. These were observed in four of six chromosomes, three autosomal ends, and one X-chromosomal end. LTU sequences represent about 0.2% of the T. taiwanemma genome. Each LTU contains a core (TTAGG)(8)-like sequence (TRLS) and five types of conserved sequences-ST (short telomere associated), J (joint), X, SR (satellite sequence rich), and Y-which vary in length from about 150 bp to 2.7 kb. The LTU sequence is defined as ST-J-TRLS-SR-X-Y-X-Y-X. Most LTU regions may be derived from the ancestral common sequence, which is observed in ST regions six times and at many other LTU sites. We could not find the LTU-like sequence in three other crickets including the closest species, T. emma, suggesting that the LTU in T. taiwanemma has been rapidly amplified in subtelomeric regions through recent evolutional events. It is also suggested that the highly conserved structure of the LTU is maintained by recombination and may contribute to telomere elongation, as seen in dipteran insects.