DNA METHYLATION OF SEX-CHROMOSOMES IN A DIOECIOUS PLANT, MELANDRIUM-ALBUM

Melandrium album, a dioecious plant species, has two heteromorphic sex chromosomes with the XY constitution typical for male and the XX for female plants. This plant represents an experimental model system of s ex determination in which the Y chromosome plays a strongly dominant m ale role. We present data on the overall transcriptional activities of M. album sex chromosomes. DNA methylation patterns were analysed dire ctly at the level of chromosomes using in situ nick-translation of fix ed root mitotic chromosomes after nuclease digestion and in vivo label ling with S-adenosyl-L-[methyl-H-3] methionine as donor of methyl grou ps. Both techniques revealed that the two X chromosomes of female plan ts had different levels of DNA methylation. Cell treatment with a DNA hypomethylating drug, 5-azacytidine, significantly influenced the labe lling densities. These results imply that in female M. album plants, o ne of the two X chromosomes may be hypermethylated and inactive as des cribed for mammalian cells (Lyon hypothesis). A similar analysis made on male cells displayed a similar relative levels of methylation in au tosomes and sex chromosomes, thus indicating the transcriptional activ ity of both Y and X male chromosomes.