Ribosomal DNA locus evolution in Nemesia: transposition rather than structural rearrangement as the key mechanism?

Abstract

We investigated chromosome evolution in Nemesia using fluorescent in-situ hybridization (FISH) to identify the locations of 5S and 45S (18-26S) ribosomal genes. Although there was conservation between Nemesia species in chromosome number, size and centromere position, there was large variation in both number and position of ribosomal genes in different Nemesia species (21 different arrangements of 45S and 5S rRNA genes were observed in the 29 Nemesia taxa studied). Nemesia species contained between one and three pairs of 5S arrays and between two and four pairs of 45S arrays. These were either sub-terminally or interstitially located and 45S and 5S arrays were often located on the same chromosome pair. Comparison of the positions of rDNA arrays with meiotic chromosome behaviour in interspecific hybrids of Nemesia suggests that some of the changes in the positions of rDNA have not affected the surrounding chromosome regions, indicating that rDNA has changed position by transposition. Chromosome evolution is frequently thought to occur via structural rearrangements such as inversions and translocations. We suggest that, in Nemesia, transposition of rDNA genes may be equally if not more important in chromosome evolution.

Cassandraretrotransposons carry independently transcribed 5S RNA

“…In the plants (32,(43)(44)(45)(46)(47) and fungi (40), evidence has accumulated both for the lack of concerted evolution (48) and for variability and rapid rearrangements in 5S rRNA loci. An uncharacterized transpositional process even has been suggested to explain these phenomena (40,43,47).…”

The linked units of 5S rDNA and U1 snDNA of razor shells (Mollusca: Bivalvia: Pharidae)

“…However, how could this happen? Several surveys have suggested that rDNA (both 5S rDNA and the major ribosomal genes) frequently moves on from one location to another in the eukaryote genome (Rooney and Ward, 2005;Datson and Murray, 2006;Veltos et al, 2009;Nguyen et al, 2010), and several mechanisms have been proposed to explain this apparent mobility. Drouin and Moniz de Sá (1995) hypothesised that a 5S rDNA transposition could be produced at the DNA level mediated by extrachromosomal circular DNA or by an RNA intermediate.…”

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

“…As the number and position of the rDNA clusters are often species-specific, these chromosomal characters have been widely used in systematics and phylogenetic reconstructions. Studies on NOR variation in numerous plant, insect and vertebrate groups have invariably described changes in the number and chromosomal location of NORs even in closely related species, suggesting that rDNA clusters are highly mobile components of the genome (Gallagher et al, 1999;Datson and Murray, 2006;Cabrero and Camacho, 2008;Nguyen et al, 2010;da Silva et al, 2010;Cazaux et al, 2011). This important inter-species lability is generated either by chromosomal rearrangements or transposition events (Eickbush and Eickbush, 2007).…”

“…The vast majority of these investigations are based on the silverstaining method which presents two drawbacks: it reveals only the active fraction of the rRNA genes, that is, those that were transcribed during the previous interphase, and in some cases, it may lead to nonspecific staining of heterochromatic regions (Dobigny et al, 2003;Cabrero and Camacho, 2008;Carvalho et al, 2009). Access to the genes themselves by the fluorescence in situ hybridization (FISH) technique has provided a cytogenetic tool to explore the extent of NOR diversity between species irrespective of their expression status (Matsubara et al, 2004;Datson and Murray, 2006;Nguyen et al, 2008). However, many of these studies involve comparisons between species, and thus provide little information on the evolutionary processes acting on variation at the intraspecific level.…”