No abstract

Heterochromatin comprises a fraction of the genome usually with highly repeated DNA sequences and lacks of functional genes. This region can be revealed by using Giemsa C-banding, fluorochrome staining and cytomolecular tools. Some plant species are of particular interest through having a special type of heterochromatin denominated the cold-sensitive region (CSR). Independent of other chromosomal regions, when biological materials are subjected to low temperatures (about 0 °C), CSRs appear slightly stained and decondensed. In this study, we used Cestrum strigilatum (Solanaceae) to understand some aspects of CSR condensation associated with cytosine methylation levels, and to compare the behavior of different heterochromatin types of this species, when subjected to low temperatures.

Species of Cestrum (Linnaeus, 1753) have shown large diversity in the accumulation and distribution of repetitive DNA families, and B chromosomes have been described in seven species. Some types of repetitive DNA were identified in A and B chromosomes in species of this plant group, such as AT-rich SSR, 35S and 5S rDNA, C-Giemsa and C-CMA/DAPI bands and retrotransposons. To increase our understanding of the relationships of A and B chromosomes, the B of C. strigilatum Ruiz & Pavón, 1799 was microdissected, amplified and hybridized in situ against chromosomes of this species, and in six other species of this genus. FISH signals were observed in whole the B of C. strigilatum, including stretches of A chromosomes, as well as in some A chromosomes of all tested species. A strong FISH signal was seen adjacent to the 5S rDNA in the proximal region of pair 8 of all species and, due to this, we have searched for 5S rDNA fragments in the microdissected B chromosome. PCR and sequencing data evidenced 5S rDNA deletion along evolutionary pathways of the B of C. strigilatum. Although A and B chromosomes displayed redundancy in the repetitive DNA families in different species, the B of C. strigilatum seemed to differ from those Bs of other Cestrum species by the loss of rDNA fractions. A possible origin of Bs in Cestrum was discussed.

Plant genomes are variable in the accumulation and distribution of repetitive DNA families. Species of Cestrum show large diversity of repetitive DNA families, and B chromosomes have been described in seven species. Different DNA families have been identified in Cestrum, such as AT-rich SSR, 45S and 5S rDNA, C-Giemsa and C-CMA/DAPI bands and retrotransposons. To understand the relationships between B and A chromosomes of Cestrum, the B of C. strigilatum was microdissected, amplified, and fragments were used to produce a small library. Sequences showed the occurrence of stretches of SSR, minisats and LTR-RTs. The probe of B was hybridized in situ against chromosomes of eight Cestrum species. FISH signals were observed in the Bs of C. strigilatum and C. intermedium, besides stretches of A chromosomes of all species tested. Species showed hybridization signals in different positions, such as: i) signals adjacent to C-DAPI bands, ii) lightly dispersed signals throughout the chromosomes, and iii) an intense hybridization signals associated with 5S rDNA sites, in the proximal region of long arm of pair 8. Due to the strong FISH signal associated with 5S rDNA region of A chromosomes of all species, we search for 5S rDNA stretches in the microdissected B chromosome using PCR and Sanger sequencing. Data showed a possible degradation of 5S rDNA in the evolutionary pathways of the Bs. Although A and B chromosomes displayed redundancy in the repetitive DNA families in different species, the Bs of both C. strigilatum and C. intermedium seemed to differ from those of other species by the loss of rDNA fractions. These data indicate a common origin of Bs in Cestrum.