Telomeres in Chironomus thummi are characterized by different subfamilies of complex DNA repeats

Members of the genus Chironomus, as well as other Diptera, lack the highly conserved structure of most telomeres, characterized by short repeats generated by telomerase, and have long repeats at their chromosome ends. Among Chironomus species with characterized telomeres Chironomus thummi is of particular interest because one of the telomeres, 3R, forms a giant puff in response to heat shock and other stress treatments. The puff contains nucleoprotein granules in which transcripts of the telomeric repeats are present. Most other nontelocentric telomeres irregularly form less pronounced heat shock puffs. One, the 4R telomere is, however, exceptional in being completely refractory to heat shock. We now pose the question whether the repeats in 3R and 4R have special sequence features. We find three different subfamilies of telomeric repeats in C. thummi, named TsA, TsB and TsC. They have an identical length (176 bp) and display base differences in defined regions, V1 and V2, connected by conserved segments. The TsA subfamily is localized exclusively at 3R, TsC only at 4R, whereas TsB repeats are shared by the remaining nontelocentric telomeres: 1R, 1L, 2R, 2L and 3L. Consequently both 3R and 4R have unique types of telomeric repeats. The 176 bp type repeats are absent from the telocentric, left end of chromosome 4. These results allowed us to differentiate in polytene chromosomes four types of telomeres characterized by tandemly repeated specific sequences.