Comparative molecular cytogenetics of major repetitive sequence families of three Dendrobium species (Orchidaceae) from Bangladesh.
OBJECTIVE
Dendrobium species show tremendous morphological diversity and have broad geographical distribution. As repetitive sequence analysis is a useful tool to investigate the evolution of chromosomes and genomes, the aim of the present study was the characterization of repetitive sequences from Dendrobium moschatum for comparative molecular and cytogenetic studies in the related species Dendrobium aphyllum, Dendrobium aggregatum and representatives from other orchid genera.


METHODS
In order to isolate highly repetitive sequences, a c(0)t-1 DNA plasmid library was established. Repeats were sequenced and used as probes for Southern hybridization. Sequence divergence was analysed using bioinformatic tools. Repetitive sequences were localized along orchid chromosomes by fluorescence in situ hybridization (FISH).


RESULTS
Characterization of the c(0)t-1 library resulted in the detection of repetitive sequences including the (GA)(n) dinucleotide DmoO11, numerous Arabidopsis-like telomeric repeats and the highly amplified dispersed repeat DmoF14. The DmoF14 repeat is conserved in six Dendrobium species but diversified in representative species of three other orchid genera. FISH analyses showed the genome-wide distribution of DmoF14 in D. moschatum, D. aphyllum and D. aggregatum. Hybridization with the telomeric repeats demonstrated Arabidopsis-like telomeres at the chromosome ends of Dendrobium species. However, FISH using the telomeric probe revealed two pairs of chromosomes with strong intercalary signals in D. aphyllum. FISH showed the terminal position of 5S and 18S-5.8S-25S rRNA genes and a characteristic number of rDNA sites in the three Dendrobium species.


CONCLUSIONS
The repeated sequences isolated from D. moschatum c(0)t-1 DNA constitute major DNA families of the D. moschatum, D. aphyllum and D. aggregatum genomes with DmoF14 representing an ancient component of orchid genomes. Large intercalary telomere-like arrays suggest chromosomal rearrangements in D. aphyllum while the number and localization of rRNA genes as well as the species-specific distribution pattern of an abundant microsatellite reflect the genomic diversity of the three Dendrobium species.
Begum
Rabeya
R
Department of Botany, University of Dhaka, Dhaka 1000, Bangladesh.
Alam
Sheikh Shamimul
SS
Menzel
Gerhard
G
Schmidt
Thomas
T
D001459:Bangladesh; D020732:Cytogenetic Analysis; D018744:DNA, Plant / Q000235:genetics*; D031667:Dendrobium / Q000235:genetics*; D018745:Genome, Plant / Q000235:genetics; D017404:In Situ Hybridization, Fluorescence; D016133:Polymerase Chain Reaction; D012091:Repetitive Sequences, Nucleic Acid
D018744:DNA, Plant