Karyotype characterization reveals active 45S rDNA sites located on chromosome termini in Smilax rufescens (Smilacaceae)

Karyotype structure and NOR activity in Brazilian Smilax Linnaeus, 1753 species (Smilacaceae)

The genus Smilax Linnaeus, 1753 (Smilacaceae) is a large genus of dioecious plants distributed in tropical, subtropical and temperate regions. Some Smilax species have medicinal importance and their identification is important for the control of raw material used in the manufacture of phytotherapeutical products. The karyotypes of seven Brazilian Smilax species were investigated. Mitotic metaphases of roots from young plants were analysed in Feulgen-stained preparations. The karyotypes were asymmetric and modal with 2n = 2x = 32 chromosomes gradually decreasing in size. In S. goyazana A De Candolle & C De Candolle, 1878, a polyploid species, 2n = 4x = 64. In all the species, the large and medium-sized chromosomes were subtelocentric and submetacentric and the small chromosomes were submetacentric or metacentric. Their karyotypes were quite similar, with differences in the arm ratio of some chromosomes. S. fluminensis Steudel, 1841 differed from the other species by having a large metacentric chromosome 1. These findings suggest that evolution occurred without drastic changes in the chromosomal structure in the species analyzed. Terminal secondary constrictions were visualized on the short arm of some chromosomes, but they were detected only in one homologue of each pair. Due to the terminal location and the degree of chromosome condensation, secondary constrictions were not visualized in some species. The nucleolus organizer regions (NORs) were mapped by silver-staining and fluorescent in situ hybridization (FISH) in S. rufescens Grisebach, 1842 and S. fluminensis. Silver-staining and FISH signals were colocalized on the short arms of six chromosomes in S. rufescens and four chromosomes in S. fluminensis. In FISH preparations, one of the largest chromosomes had the secondary constrictions highly decondensed in some cells. This finding and the heteromorphism observed in Feulgen-stained chromosomes suggest that differential rRNA gene expression between homologous rDNA loci can occur in some cells, resulting in different degrees of ribosomal chromatin decondensation. The presence of a heteromorphic chromosome pair in S. rufescens, S. polyantha Grisebach, 1842 and S. goyazana suggests a chromosomal sex determination in these dioecious species.

Chromosomal localization of 45S rDNA, sex-specific C values, and heterochromatin distribution in Coccinia grandis (L.) Voigt

Coccinia grandis is a widely distributed dioecious cucurbit in India, with heteromorphic sex chromosomes and X-Y sex determination mode. The present study aids in the cytogenetic characterization of four native populations of this plant employing distribution patterns of 45S rDNA on chromosomes and guanine-cytosine (GC)-rich heterochromatin in the genome coupled with flow cytometric determination of genome sizes. Existence of four nucleolar chromosomes could be confirmed by the presence of four telomeric 45S rDNA signals in both male and female plants. All four 45S rDNA sites are rich in heterochromatin evident from the co-localization of telomeric chromomycin A (CMA)(+ve) signals. The size of 45S rDNA signal was found to differ between the homologues of one nucleolar chromosome pair. The distribution of heterochromatin is found to differ among the male and female populations. The average GC-rich heterochromatin content of male and female populations is 23.27 and 29.86 %, respectively. Moreover, the male plants have a genome size of 0.92 pg/2C while the female plants have a size of 0.73 pg/2C, reflecting a huge genomic divergence between the genders. The great variation in genome size is owing to the presence of Y chromosome in the male populations, playing a multifaceted role in sexual divergence in C. grandis.