TELOMERES TERMINATING WITH LONG COMPLEX TANDEM REPEATS

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