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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 &amp; 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.

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