B chromosomes of Puschkinia libanotica are characterized by a reduced level of euchromatic histone H3 methylation marks

Epigenetic marks in the chromosome complement of the liliaceous plant Puschkinia libanotica were studied using immunofluorescence to detect histone modifications. In particular, a comparison was made between the euchromatic and heterochromatic components of supernumerary B chromosomes (Bs) relative to the A chromosomes of the basic diploid set, in order to provide further insights into the enigmatic properties of these 'genetically silent' Bs. No differences were found between A and B chromosomes for the heterochromatic dimethylation marks of histone H3 at lysine positions 9 and 27. However, distinctions between A and B chromosomes were revealed for the euchromatic di- and trimethylation marks of histone H3 at lysine 4. The results indicate that the less-transcriptionally active Bs are not marked by an enriched level of heterochromatic histone marks, but rather by a low level of euchromatin-associated histone modifications.