Spacer size heterogeneity in ribosomal DNA ofChironomus thummiis due to a 120 bp repeat homologous to a predominantly centromeric repeated sequence

Abstract

The rDNA of Ch. tepperi is homogeneous in structure with a repeat size of 8.4 kb. This size seems to be typical for the basic repeat unit in Chironomus species. Ch. th. piger rDNA cistrons are slightly increased in length (9.0 kb). In the non-transcribed spacer (NTS) an appr. 0.18 kb segment is additionally present in about 50 % of the repeats. Ch. th. thummi DNA contains largely heterogeneous rDNA repeats, mainly between 10 and 16 kb. The heterogeneity is due to varying numbers of 120 bp elements present in the NTS. The different spacer size classes are not randomly distributed. The short repetitive 120 bp elements (Cla I elements) hybridize in situ with the nucleolus and with centromere regions. The Cla I elements are regularly present in the thummi NTS, but are absent in the piger NTS. Only very few piger rDNA cistrons may contain Cla I elements.

Occurrence of 9 homologous repeat units in the external spacer region of a nuclear maize rRNA gene unit

“…With the exception of a 30 bp deletion in the first repeat, two minor insertions in the third repeat and a 31 bp insertion in the fourth repeat, the repeats exhibit the same length and show 80-95% homology to one another. Similar observations have been made for several other organisms (7,(9)(10)(11)13). This suggests, that the repeat units are submitted to restrictions of mutational change and hints at a functional importance of these structures.…”

“…The transcription units themselves are separated from each other by nontranscribed external spacer regions, the size of which varies from approximately 1 to 30 kb depending on the organism (5-8). The external spacer regions very often contain repeats of 60-700 nucleotides in length (7)(8)(9)(10)(11)(12)(13). The number, arrangement and DNA sequence of the repeats have been found to be highly variable between closely related species and even between dif-ferent organisms of the same species (8,10,(12)(13)(14).…”

“…The external spacer regions very often contain repeats of 60-700 nucleotides in length (7)(8)(9)(10)(11)(12)(13). The number, arrangement and DNA sequence of the repeats have been found to be highly variable between closely related species and even between dif-ferent organisms of the same species (8,10,(12)(13)(14). The location and sequence of the initiation site of transcription of the precursor RNA is different from species to species, although some homology around the transcription start has been observed in the case of rat, mouse and man (15).…”

Characteristics of the nuclear (18S, 5.8S, 28S and 5S) and mitochondrial (12S and 16S) rRNA genes ofApis mellifera(Insecta: Hymenoptera): structure, organization, and retrotransposable elements

“…Relative to coding regions of the rRNA genes, complete IGS sequences from arthropods have only been characterized for a few taxa, and many of these are dipterans: D. melanogaster (Simeone et al ., 1985; Tautz et al ., 1987; Hayward & Glover, 1988; Tautz et al ., 1988), D. orena , D. virilis , D. hydei (Tautz et al ., 1987), Aedes albopictus (Baldridge & Fallon, 1992), A. aegypti (Wu & Fallon, 1998), Anopheles sinensis (Whang et al ., 2002), the tstetse fly, Glossina sp. (Glossinidae) (Cross & Dover, 1987a,b), the black fly, Simulium sanctipauli (Simuliidae) (Morales‐Hojas et al ., 2002) and several species of Chironomus midges (Chironomidae) (Degelmann et al ., 1979; Israelewski & Schmidt, 1982; Israelewski, 1983). Other published arthropod complete IGS sequences are from the German cockroach Blattella germanica (Blattaria: Blattellidae) (Mukha et al ., 2002), the bulldog ant, Myrmecia croslandi (Hymenoptera: Formicidae) (Ohnishi & Yamamoto, 2004) and the crustaceans Daphnia pulex (Cladocera) (Crease, 1993) and Tigriopus californicus (Copepoda) (Burton et al ., 2005).…”

Ribosomal RNA Genes of the Anopheles gambiae Species Complex

“…However, Chironomus thummi thummi spacer architecture is similar to the other Diptera, with varying numbers of a 120-bp subrepeat in the region immediately proximal to the ETS (62). Unlike the other Diptera, in which the subrepeats are unique to the rDNA, this 120-bp repeat exhibits more than a 90% sequence homology with a satellite DNA family present in approximately 70,000 copies per genome in centromeric heterochromatin (48,62).…”