Abstract
Chromosomal localization of ribosomal RNA coding genes has been studied by using FISH (fluorescence in situ hybridization) in 21 species from the genus Chironomus Meigen, 1803. Analysis of the data has shown intra- and interspecific variation in number and location of 5.8S rDNA hybridization sites in 17 species from the subgenus Chironomus and 4 species from the subgenus Camptochironomus Kieffer, 1914. In the majority of studied species the location of rDNA sites coincided with the sites where active NORs (nucleolus organizer regions) were found. The number of hybridization sites in karyotypes of studied chironomids varied from 1 to 6. More than half of the species possessed only one NOR (12 out of 21). Two rDNA hybridization sites were found in karyotypes of five species, three – in two species, and five and six sites – in one species each. NORs were found in all chromosomal arms of species from the subgenus Chironomus with one of them always located on arm G. On the other hand, no hybridization sites were found on arm G in four studied species from the subgenus Camptochironomus. Two species from the subgenus Chironomus – Chironomus balatonicus Devai, Wuelker & Scholl, 1983 and Chironomus “annularius” sensu Strenzke, 1959 – showed intraspecific variability in the number of hybridization signals. Possible mechanisms of origin of variability in number and location of rRNA genes in the karyotypes of species from the genus Chironomus are discussed.
Highlights
The ribosomal RNA genes in eukaryotic genomes are multiply repeated and form the family of ribosomal genes
Karyotypes of most Chironomus species studied in this work have four polytene chromosomes, which corresponds to the haploid chromosome set n = 4 (Figs 1–4)
DNA-probe used for FISH analysis consists of two main components: gene coding 5,8S rRNA and internal transcribed spacer (ITS1)
Summary
The ribosomal RNA genes in eukaryotic genomes are multiply repeated and form the family of ribosomal genes. They are arranged in clusters comprising hundreds of tandemly repeated units, each consisting of three genes – 18S, 5.8S, and 28S rRNA – separated by transcribed and untranscribed intergenic spacers (Long and David 1980). The clusters of ribosomal RNA genes in chromosomes are located to the nucleolus organizer regions (NORs). Studies involving numerous animal and plant groups have demonstrated that the number of NORs and their location on chromosomes may differ in distant species, and in closely related ones. Research into NOR variation in karyotypes has clarified the patterns in chromosome evolution of many insect groups (Cabrero and Camacho 2008, Cabral-de-Mello et al 2010, 2011, Grzywacz et al 2011, Oliveira et al 2011, Neto et al 2013)
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