Classes of DNA associated with telomeres in the chironomids C. pallidivittatus and C. tentans

Sammanfattning: Telomeres in Chironomus consist of long complex tandem DNA repeats, which in C. pallidivittatus have been shown to extend to the extreme end of chromosomes. In C. pallidivittatus the telomeric repeats are present in large blocks, 50-200kb in size, most of them belonging to four major subfamilies. The main issue of this thesis has been to elucidate possible functional roles for these different subfamilies of telomeric repeats. Therefore, both intra- and intertelomeric mapping of the subfamilies were performed. One subfamily was found always to be located distally to the others, and have properties that could indicate a role in the protection of the chromosome ends. The subfamily identity of the most proximally situated repeat was identified in five of the seven non-telocentric chromosome ends. The junction between telomeric repeats and subterminal sequences was also characterized. In this region species and junction specific mutations were found, which were highly conserved between different chromosome ends within the same species. The homogenization pattern of these mutations indicated that gene conversion can act also between heterologous telomeres. The telomeric repeats exhibit a remarkable efficient homogenization whether they are located in the same or in different non-homologous chromosomes, an unusual feature since intrachromosomal homogenization usually are more efficient than homogenization between heterologues. Results from several earlier investigations had suggested transcriptional activity in the telomeric repeats in Chironomus. This issue was now addressed both in relation to possible roles for the different subfamilies in this transcription, and if this could be a connection to a process active in the elongation of the telomeres.