Abstract
There are differences in number and localization of nucleolus organizer regions (NORs) in genomes. In mammalian genomes, NORs are located on autosomes, which are often situated on short arms of acrocentric chromosomes and more rarely in telomeric, pericentromeric, or interstitial regions. In this work, we report the unique case of active NORs located on gonоsomes of a eutherian mammal, the Javan mouse-deer (Tragulus javanicus). We have investigated the position of NORs by FISH experiments with ribosomal DNA (rDNA) sequences (18S, 5.8S, and 28S) and show the presence of a single NOR site on the X and Y chromosomes. The NOR is localized interstitially on the p-arm of the X chromosome in close proximity with prominent C-positive heterochromatin blocks and in the pericentromeric area of mostly heterochromatic Y. The NOR sites are active on both the X and Y chromosomes in the studied individual and surrounded by GC enriched heterochromatin. We hypothesize that the surrounding heterochromatin might have played a role in the transfer of NORs from autosomes to sex chromosomes during the karyotype evolution of the Javan mouse-deer.
Highlights
Nucleolus organizer regions (NORs) are crucial for the formation of the nucleolus and contain multiple copies of the ribosomal clusters interdigitated with non-transcribed intergenic spacers [1]
The ribosomal RNA is further processed and packed into large (5.8S, 28S, and separate non-nucleolus 5S ribosomal DNA (rDNA)) and small (18S) subunits of the ribosome, that are responsible for RNA translation and critical for cellular functions [1]
We describe the sex chromosome localization of a NOR in a Javan mouse-deer karyotype (Tragulidae), provide its molecular cytogenetic characterization by localization of ribosomal genes, and investigate the activity of these NORs and surrounding regions
Summary
Nucleolus organizer regions (NORs) are crucial for the formation of the nucleolus and contain multiple copies of the ribosomal clusters interdigitated with non-transcribed intergenic spacers [1]. The ribosomal RNA is further processed and packed into large (5.8S, 28S, and separate non-nucleolus 5S rDNA) and small (18S) subunits of the ribosome, that are responsible for RNA translation and critical for cellular functions [1] These ribosomal DNA sequences exhibit a high degree of conservation across animals.
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