Delineation of methylation and histone modification: the epigenetic regulatory marks show slightly altered distribution with the elevation in ploidy level in the orchid Dendrobium nobile

Epigenetic regulatory DNA methylation and post-translational histone modifications are considered to be highly conserved. Recent findings however suggest that chromosomal distribution of individual modifications (acetylation, methylation, phosphorylation) can differ within and between the groups of eukaryotes, indicating the obvious possibility of evolutionary divergence in reading the ‘histone code’. Orchids are known for their complex genome organization and evolution. Current trends of formation of artificial and natural polyploid orchids generally provide superior genetic variability, size, substance and form of flowers to their diploid counterparts. Here, we report the comparative chromosomal distribution of 5mC and twelve selected modified histones (methylated, acetylated and phosphorylated) between diploid and autotetraploid Dendrobium nobile at critical cytogenetic landmarks (interphase, metaphase and anaphase) of mitotic cell division to understand the underlying consequences of polyploidization on the holistic chromatin environment in higher angiosperms like orchids. Comparative in situ immuno-detection of 5-mC sites revealed increment of DNA methylation in autopolyploid D. nobile in comparison to its diploid prototype. Notably, only H3K9me1 and H3K27me2 revealed its conserved chromosomal distribution between diploid and autotetraploid D. nobile by labeling the heterochromatin and euchromatin respectively. Whereas, other epigenetic methylated and acetylated histone marks showed more or less altered/facultative distribution pattern. Histone phosphorylation did not show any alteration in chromosomal distribution between diploid and autopolyploid D. nobile. It has been concluded that polyploidization events may influence/altered the chromosomal distribution of methylated and acetylated histone marks in higher plants. This study might serve as a baseline data to understand the affect of polyploidization on epigenetic alteration associated with DNA and histone modification that ultimately play a crucial role in speciation and genome evolution of higher angiosperms like orchids.