Telomeres terminating with long complex tandem repeats.

Telomeres of most investigated species terminate with short repeats and are elongated by telomerase. Short repeats have never been detected in dipteran species which have found other solutions to end a chromosome. Whereas in Drosophila melanogaster retroelements are added onto the termini, chironomids have long complex repeats at their chromosome ends. We review evidence that these units are terminal and probably have evolved from short telomeric repeats. In Chironomus pallidivittatus the units have been shown to belong to different subfamilies which have specific inter- and intrachromosomal distribution, the most terminal subfamily of repeats being characterized by pronounced secondary structures for the single strand. The complex repeats are efficiently homogenized both within and between different chromosome ends. Gene conversion is probably an important component in the coordinate evolution of the repeats but it is not known whether it is used for net synthesis of DNA. RNA is used as an intermediate in telomere elongation both by organisms having chromosomes terminating with short repeats and by D. melanogaster. It is therefore interesting that the terminal repeats in chironomids are transcribed.