Linkage of 35S and 5S rRNA genes in Artemisia (family Asteraceae): first evidence from angiosperms

Abstract

Typically in plants, the 5S and 35S ribosomal DNA (rDNA) encoding two major ribosomal RNA species occur at separate loci. However, in some algae, bryophytes and ferns, they are at the same locus (linked arranged). Southern blot hybridisation, polymerase chain reactions (PCR), fluorescent in situ hybridisation, cloning and sequencing were used to reveal 5S and 35S rDNA genomic organisation in Artemisia. We observed thousands of rDNA units at two-three loci containing 5S rDNA in an inverted orientation within the inter-genic spacer (IGS) of 35S rDNA. The sequenced clones of 26-18S IGS from Artemisia absinthium appeared to contain a conserved 5S gene insertion proximal to the 26S gene terminus (5S rDNA-1) and a second less conserved 5S insertion (5S rDNA-2) further downstream. Whilst the 5S rDNA-1 showed all the structural features of a functional gene, the 5S-rDNA-2 had a deletion in the internal promoter and probably represents a pseudogene. The linked arrangement probably evolved before the divergence of Artemisia from the rest of Asteraceae (>10 Myrs). This arrangement may have involved retrotransposons and once formed spread via mechanisms of concerted evolution. Heterogeneity in unit structure may reflect ongoing homogenisation of variant unit types without fixation for any particular variant.

Karyological Features of <i>Achyrocline</i> (Asteraceae, Gnaphalieae): Stable Karyotypes, Low DNA Content Variation and Linkage of rRNA Genes

“…In relation to Southern blot analyses, García et al [2009] indicated that a ladder of fragments is expected in species with tandem arrays of 5S rDNA since in most angiosperms these genes contain a single conserved Bam HI site [Röser et al, 2001]. A similar outcome was observed in Citrus medica (used here as an outgroup).…”

“…The most common situation found in higher plants is that both the 5S and 18S-5.8S-25S rDNA sequences are organized separately in tandem arrays distributed in one or more loci per genome [Srivastava and Schlessinger, 1991]. However, exceptions to this rule have been reported for several unicellular eukaryotes [Drouin and Moniz de Sá, 1995], bryophytes [Sone et al, 1999] and more recently in Artemisia (Asteraceae) [García et al, 2009]. Our FISH analyses in three species of Achyrocline have shown that there is only one 5S and one 45S rDNA site in these species and that both co-localize in the pericentromeric region of chromosome pair 10.…”

“…Our results show that 5S and 45S rDNA are not organized in tandem arrays in these Achyrocline species. García et al [2009] reported that the 5S rDNA sequence is inserted in the intergenic spacer (IGS) of the 45S rDNA unit in Artemisia species. Curiously, this genus belongs to the tribe Anthemidae, subfamily Asteroideae, the same subfamily to which Achyrocline belongs.…”

Early evolutionary colocalization of the nuclear ribosomal 5S and 45S gene families in seed plants: evidence from the living fossil gymnosperm Ginkgo biloba

“…The 45S-5S unit arrangements were not considered to be present in seed plants until recently, when García et al (2009García et al ( , 2010 detected this type of linkage in a wide sample of taxa belonging to the phylogenetically advanced Asteraceae family. Our finding that the 45S-5S loci are linked in Ginkgo suggests a colocalization of the two nuclear ribosomal gene families in the early evolutionary history of seed plants, indicating that a reassessment of the organization and molecular evolution of rDNA genes of basal plants is still necessary, as the results of Wicke et al (2011) should be viewed with great caution.…”

“…Recently, however, it has been unequivocally shown for the first time, using molecular cytogenetics, Southern blot hybridization, PCR-based techniques and DNA sequencing, that 5S genes can be embedded within the 45S multigene family in angiosperms (Asteraceae; García et al, 2009;García et al, 2010). The detailed survey within Asteraceae has revealed that in about 25% of the species in this family, the two ribosomal gene families have been combined into a single, linked 45S-5S unit (García et al, 2010).…”