Chromosomal dynamics of nucleolar organizer regions (NORs) in the house mouse: micro-evolutionary insights

Abstract

Variation in the number and chromosomal location of nucleolar organizer regions (NORs) was studied in the house mouse, Mus musculus (2n¼40). From an origin in Western Asia, this species colonized the Middle East, Europe and Asia. This expansion was accompanied by diversification into five subspecies. NOR diversity was revealed by fluorescence in situ hybridization using 18S and 28S probes on specimens spanning Asia to Western Europe. The results showed that the house mouse genome possessed a large number of NOR-bearing autosomes and a surprisingly high rate of polymorphism for the presence/absence of rRNA genes on all these chromosomes. All NOR sites were adjacent to the centromere except for two that were telomeric. Subspecific differentiation established from the NOR frequency data was concordant with the overall pattern of radiation proposed from molecular studies, but highlighted several discrepancies that need to be further addressed. NOR diversity in M. musculus consisted of a large number of polymorphic NORs that were common to at least two subspecies, and a smaller number of NORs that were unique to one subspecies. The most parsimonious scenario argues in favor of a subspecific differentiation by lineage sorting of ancestral NOR polymorphisms; only the unique NORs would have appeared by interchromosomal transposition, except for the two telomeric ones that may have originated by hybridization with another species. Such a scenario provides an alternative view from the one prevailing in most systematic and phylogenetic analyses that NORs have a high transposition rate due to concerted evolution of rRNA genes.

Differentiation of Sex Chromosomes and Karyotype Characterisation in the Dragonsnake <b><i>Xenodermus javanicus</i></b> (Squamata: Xenodermatidae)

“…The variation in NORs size and/or location has been frequently reported in various vertebrates [e.g. Gornung et al, 2011;Britton-Davidian et al, 2012;Gornung, 2013]. Therefore, we suggest that the polymorphism of the XJA pair can be attributed to the variation in the numbers of copies of the rDNA gene.…”

Building up the nucleus: nuclear organization in the establishment of totipotency and pluripotency during mammalian development

“…As the NORs of most Mus musculus subspecies are located on six autosomes and as are all centromerically located (Britton-Davidian et al 2012;Gibbons et al 2015), it was initially suggested that rDNA might be a player for the peripheral NPB localization of pericentromeres. However, 3D mapping of rDNA by DNA-FISH in the zygote revealed that chromosomes without NORs also associate with NPBs, and not all rDNA-bearing chromosomes have their centromeres on the NPB surface, but some sit close to the nuclear periphery (Aguirre-Lavin et al 2012).…”

Construction of synthetic nucleoli in human cells reveals how a major functional nuclear domain is formed and propagated through cell division

“…To date, the DNA requirement and molecular mechanism allowing the propagation through mitosis of human nucleoli remain to be elucidated. The positioning of rDNA arrays within heterochromatic regions is an evolutionarily conserved feature (Henderson et al 1972;Britton-Davidian et al 2012), hinting at a role for chromosome context in nucleolar biology. Furthermore, even though acrocentric chromosome p-arms are still missing from current human genome assemblies, our recent work has established that sequences surrounding rDNA arrays are conserved among all human acrocentric chromosomes (Floutsakou et al 2013).…”