A CONCERTEDLY EVOLVING REGION IN CHIRONOMUS, UNIQUE WITHIN THE TELOMERE

Chromosome terminal, complex repeats in the dipteran Chironomus pallid ivittatus show rapid concerted evolution during which there is remarka bly efficient homogenization of the repeat units within and between ch romosome ends. It has been shown previously that gene conversion is li kely to be an important component during these changes. The sequence e volution could be a result of different processes-exchanges between re peats in the tandem array as well as information transfer between unit s in different chromosomes-and is therefore difficult to analyze in de tail. In this study the concerted evolution of a region present only o nce per chromosome, at the junction between the telomeric complex repe ats and the subtelomeric DNA was therefore investigated in the two sib ling species C. pallidivittatus and C. tentans. Material from individu al microdissected chromosome ends was used, as well as clones from bul k genomic DNA. On the telomeric side of the border pronounced species- specific sequence differences were observed, the patterns being simila r for clones of different origin within each species. Mutations had be en transmitted efficiently between chromosomes also when adjoining, mo re distally localized DNA showed great differences in sequence, sugges ting that gene conversion had taken place. The evolving telomeric regi on bordered proximally to subtelomeric DNA with high evolutionary cons tancy. More proximally localized, subtelomeric DNA evolved more rapidl y and showed heterogeneity between species and chromosomes.