Telomeres in Chironomus - studies on structure and evolution

Sammanfattning: The telomeres, the physical ends of the chromosomes, consist in various species of short simple tandem repeats which are extended by telomerase. The Drosophila melanogaster telomeres, which instead are elongated by retroposons, are an exception. In Chironomus pallidivittatus and Chironomus tentans the telomeres contain 340/350-bp tandem repeats in blocks up to 200 kb in size. The structure of the repeat unit is complex, with subrepeats embedded in linker regions. The questions addressed in this investigation are (i) what is the evolutionary origin of the 340/350-bp repeat units (ii) do these repeats extend to the extreme end of the telomeres, and if so (iii), what is the structure and evolutionary origin of the subtelomeric region. In a comparison of repeat units from C. pallidivittatus and C. tentans, it was shown that mutational differences are predominantly located in linker regions. By characterization of evolutionarily early forms of the 350-bp repeat in C. tentans it could be concluded that linkers are likely to be due to the insertion of non-LTR retroposons, and that the subrepeated regions are built up by arrays of short degenerate repeats, some of which are inverted. It is suggested that 350-bp repeats are formed by the evolution of the short telomeric repeats that are characteristic of the telomeres in most species in a process involving inversions and retrotranspositions. The 340-bp repeats in C. pallidivittatus were demonstrated to indeed comprise the very end of the telomere, and to thus represent a novel type of termini of the chromosomes. The interest in the subtelomeric region was based on the similarity found between the complex 340/350-bp repeats and the subterminal regions in other organisms. In a study of telomere-adjacent sequences in C. pallidivittatus and C. tentans, this region was shown to be conserved and to share features with the subtelomeric regions of many species.