Heterochromatin diversity and its co-localization with 5S and 45S rDNA sites in chromosomes of four Maxillaria species (Orchidaceae)

Marcelo Guerra,Leonardo P Felix,Juliano S Cabral

doi:10.1590/s1415-47572006000400015

Abstract

We investigated four orchids of the genus Maxillaria (M. discolor, M. acicularis, M. notylioglossa and M. desvauxiana) in regard to the position of heterochromatin blocks as revealed using chromomycin A3 (CMA) and 4'-6-diamidino-2-phenylindole (DAPI) fluorochrome staining and 5S and 45S rDNA sites using fluorescence in situ hybridization (FISH). The species showed differences in chromosome number and a diversified pattern of CMA+ and DAPI+ bands, including heteromorphism for CMA+ bands. The 5S and 45S rDNA sites also varied in number and most of them were co-localized with CMA+ bands. The relationship between 5S rDNA sites and CMA+ bands was more evident in M. notylioglossa, in which the brighter CMA+ bands were associated with large 5S rDNA sites. However, not all 5S and 45S rDNA sites were co-localized with CMA+ bands, probably due to technical constraints. We compare these results to banding data from other species and suggest that not all blocks of tandemly repetitive sequences, such as 5S rDNA sites, can be observed as heterochromatin blocks.

Highlights

We investigated four orchids of the genus Maxillaria (M. discolor, M. acicularis, M. notylioglossa and M. desvauxiana) in regard to the position of heterochromatin blocks as revealed using chromomycin A3 (CMA) and 4’-6-diamidino-2-phenylindole (DAPI) fluorochrome staining and 5S and 45S rDNA sites using fluorescence in situ hybridization (FISH)
The four species analyzed in our study showed differences in chromosome number and revealed a diversified pattern of CMA+ and DAPI+ bands, having 5S and 45S rDNA sites co-localized with CMA+ bands
The largest blocks of 5S rDNA always coincided with CMA+/DAPI– bands, while the sites of 45S rDNA always co-localized with CMA+ blocks of different fluorescence intensity (Figure 1)

Summary

Introduction

We investigated four orchids of the genus Maxillaria (M. discolor, M. acicularis, M. notylioglossa and M. desvauxiana) in regard to the position of heterochromatin blocks as revealed using chromomycin A3 (CMA) and 4’-6-diamidino-2-phenylindole (DAPI) fluorochrome staining and 5S and 45S rDNA sites using fluorescence in situ hybridization (FISH). The heterochromatin adjacent to the secondary constrictions is an exception, being constituted of tandemly repetitive 45S rRNA genes, which forms large rDNA blocks identified as positive regions for Cbanding and CMA staining (Fuchs et al, 1998; Guerra and Felix, 2000). The four species analyzed in our study showed differences in chromosome number and revealed a diversified pattern of CMA+ and DAPI+ bands, having 5S and 45S rDNA sites co-localized with CMA+ bands. The meaning of these results is discussed regarding the cytological characterization of heterochromatin

Methods

Results

Conclusion