Heterochromatin and rDNA sites distribution in the holocentric chromosomes of Cuscuta approximata Bab. (Convolvulaceae)

Abstract

Cuscuta is a widely distributed genus of holoparasitic plants. Holocentric chromosomes have been reported only in species of one of its subgenera (Cuscuta subg. Cuscuta). In this work, a representative of this subgenus, Cuscuta approximata, was investigated looking for its mitotic and meiotic chromosome behaviour and the heterochromatin distribution. The mitotic chromosomes showed neither primary constriction nor Rabl orientation whereas the meiotic ones exhibited the typical quadripartite structure characteristic of holocentrics, supporting the assumption of holocentric chromosomes as a synapomorphy of Cuscuta subg. Cuscuta. Chromosomes and interphase nuclei displayed many heterochromatic blocks that stained deeply with hematoxylin, 4',6-diamidino-2-phenylindole (DAPI), or after C banding. The banded karyotype showed terminal or subterminal bands in all chromosomes and central bands in some of them. The single pair of 45S rDNA sites was observed at the end of the largest chromosome pair, close to a DAPI band and a 5S rDNA site. Two other 5S rDNA site pairs were found, both closely associated with DAPI bands. The noteworthy giant nuclei of glandular cells of petals and ovary wall exhibited large chromocentres typical of polytenic nuclei. The chromosomal location of heterochromatin and rDNA sites and the structure of the endoreplicated nuclei of C. approximata seemed to be similar to those known in monocentric nuclei, suggesting that centromeric organization has little or no effect on chromatin organization.

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.…”

“…reported similar fluorescent banding pattern in species of Rhynchospora. The banding pattern variations have been suggested to be related to the structural rearrangements as seen in monocentrics and found that the kinetochores organization has little or no effect on chromatin organization Garcia 2004). Therefore, no single typical pattern of distribution of GC/AT-rich regions can be concluded from the results of the present or previous investigation on holocentric chromosomes and may suggest that the holocentricity of chromosomes does not limit or regulate the disposition of these regions.…”

Estudo citogenético e evolutivo em espécies brasileiras de Eleocharis (Cyperaceae)

“…CMA 3 + /DAPI − , CMA 3 + /DAPI + , and CMA 3 − /DAPI + , which varied in size and position; however, only the karyotype of R. globosa was differentiated owing to accumulation of AT-rich blocks. In the second case, Guerra and García (2004) described a large amount of heterochromatin (more AT-than GC-rich bands) in Cuscuta approximata (Cuscutaceae). In fact, the accumulation and distribution of heterochromatin in holocentric chromosomes seems to be very similar to that known for monocentric chromosomes (Guerra 2000).…”

“…The pTa71 probe localised hybridisation sites always at the chromosome ends, as previously reported by in Rhynchospora, by Furuta and Hoshino (1999) in E. mamillata var. cyclocarpa and by Guerra and García (2004) in C. approximata (Cuscutaceae). These latter authors hybridised chromosomes with 18S rDNA probes and found 12 terminal hybridisation sites, with two small chromosomes with rDNA sites at both extremities.…”

“…24 Hoshi (1995) and Furuta and Kondo (1999) also reported multiple and terminal rDNA 25 sites in Drosera. However, in Cuscuta approximata one pair of 45S rDNA and three of 1 5S rDNA sites were reported (Guerra and García 2004). In spite of general tendency 2 towards a terminal location of 45S rDNA, the 5S rDNA was reported preferentially in 3 interstitial regions in plants and animals, and in Eleocharis the interstitial occurrence of 4 5S rDNA was associated to chromosome rearrangement (Da Silva et al 2008b).…”