Retroelements are a diversified fraction of eukaryotic genomes, with the Ty1/copia and Ty3/gypsy groups being very common in a large number of plant genomes. We isolated an internal segment of the Ty3/gypsy retroelement of Cestrum strigilatum (Solanaceae) using PCR amplification with degenerate primers for a conserved region of reverse transcriptase. The isolated segment (pCs12) was sequenced and showed similarity with Ty3/gypsy retroelements of monocotyledons and dicotyledons. This segment was used as probe in chromosomes of C. strigilatum and Cestrum intermedium. Diffuse hybridization signals were observed along the chromosomes and more accentuated terminal signals in some chromosome pairs, always associated with nucleolus organizer regions (NORs). The physical relationship between the hybridization sites of pCs12 and pTa71 ribosomal probes was assessed after sequential fluorescence in situ hybridization (FISH). Hybridization signals were also detected in the B chromosomes of these species, indicating an entail among the chromosomes of A complement and B-chromosomes.

We studied the karyotypes of four Brazilian Cestrum species (C. amictum, C. intermedium, C. sendtnerianum and C. strigilatum) using conventional Feulgen staining, C-Giemsa and C-CMA3/DAPI banding, induction of cold-sensitive regions (CSRs) and fluorescent in situ hybridization (FISH) with rDNA probes. We found that the karyotypes of all four species was 2n = 2x = 16, with, except for the eighth acrocentric pair, a predominance of meta- and submetacentric chromosomes and various heterochromatin classes. Heterochromatic types previously unreported in Cestrum as neutral C-CMA3(0)/DAPI0 bands, CMA3+ bands not associated with NORs, and C-Giemsa/CSR/DAPI- bands were found. The heterochromatic blocks varied in size, number, position and composition. The 45S rDNA probe preferentially located in the terminal and subterminal regions of some chromosomes, while 5S rDNA appeared close to the centromere of the long arm of pair 8. These results suggest that karyotype differentiation can occur mainly due to changes in repetitive DNA, with little modification in the general composition of the conventionally stained karyotype.