Localization of 45S rDNA and telomeric sites on holocentric chromosomes of Rhynchospora tenuis Link (Cyperaceae)

Abstract

Rhynchospora tenuis Link (Cyperaceae) is a weed widely distributed in Brazil that presents a small number of holocentric chromosomes (2n = 4) with some autopolyploid populations (2n = 8). The haploid number n = 2 is considered as a derivative of the base number x = 5. 45S rDNA probes and telomeric DNA were hybridized in both chromosome races of R. tenuis, looking for indications of chromosome fusions. The results showed that hybridization sites of the telomeric probe were restricted to end chromosome regions whereas rDNA sites were terminally located. The chromosome race with n = 4 exhibited a doubled number of sites, with similar size and location to the hybridized sequences, confirming its autopolyploid origin. Furthermore, the terminal location of the single 45S rDNA site in the 2n = 4 race suggested that disploid reduction in Rhynchospora, from n = 5 to n = 2, was followed by elimination or reorganization events, keeping the terminal distribution of these sites, as in an others species of the genus.

The chromosomes of New Zealand Schoenus species analysed by differential banding and fluorescence in situ hybridization

“…In Schoenus species variation in the number of 45S and 5S rDNA signals was observed but the positions of these sites were conserved. The results agree with the studies on Rhynchospora (Vanzela et al , 2003, Cuscuta approximata (Guerra and Garcia 2004) and Eleocharis (da Silva et al 2005) where the 45S rDNA signal was located terminally. Generally, nucleolar constrictions have been observed in the terminal regions of the holocentric chromosomes of some species in Cyperaceae (Hoshino et al 1993, Vanzela et al 1996, Luceno et al 1998.…”

“…It also appears that the number of signals is fewer in the diploid race than in the polyploidy one. Similar results have been found in the two races of R. tenuis (Vanzela et al 2003). Variation in the size and number of 45S rDNA signals was detected in eight species of Rhynchospora.…”

“…Interestingly, 4-8 signals appeared in the species with 2n=10 and 30 signals in R. rosemariana with 2n=50 and the polarization of the rDNA signals in the interphase has been related to the building of a The telomere probe hybridized to terminal positions of all the chromosomes of S. apogon confirming the presence of Arabidopsis-type telomere sequence in holocentric Schoenus. Similar results were seen in Luzula (Fuchs et al 1995) and Rhynchospora (Vanzela et al 2003) suggesting a uniformity of telomere sequences in the two holocentric genera. However, ectopic telomeric sites have been observed in Eleocharis (da Silva et al 2005) and a variety of other plants but it is unclear whether these are related to past translocation events.…”

Holocentromeres inRhynchosporaare associated with genome-wide centromere-specific repeat arrays interspersed among euchromatin

“…To test the assumption that all holocentromeres are devoid of centromere-specific repeats, we analyzed R. pubera (12,(20)(21)(22). We report here on the first (to our knowledge) conserved centromere-specific repeats of a holocentric species and propose a model of chromosome organization for species with centromeric repeats distributed throughout the entire genome.…”

Chromosome numbers in plant cytotaxonomy: concepts and implications

“…Theoretically, fusion of holokinetic chromosomes may occur by breaks and reunion at telomeric repeats or at other chromosomal sites. Indeed, Da Silva et al (2005) observed a site of telomeric DNA at the median region of the largest chromosome pair of E. subarticulata (n = 3), whereas in R. tenuis (n = 2) no interstitial site of telomeric DNA was found (Vanzela et al, 2003).…”