STRUCTURE AND DNA METHYLATION PATTERN OF PARTIALLY HETEROCHROMATINISED ENDOSPERM NUCLEI IN GAGEA-LUTEA (LILIACEAE)

Pentaploid endosperm nuclei in certain Gagea species exhibit large mas ses of sticky and dense chromatin, not observed in somatic nuclei, The se heterochromatin masses most probably stem from the triploid chalasa l polar nucleus of the embryo sac, thus representing an example of fac ultative heterochromatinisation in plants, In the present: investigati on, we studied the nuclei in Gagea lutea (L.) Ker-Gawl. endosperm tiss ue. The position of the heterochromatin in interphase nuclei was obser ved by confocal laser scanning microscopy (CLSM) and the DNA methylati on status of the euchromatin and heterochromatin was analysed by immun olabelling with an antibody raised against 5-methylcytosine (anti-5-mC ). In young endosperms. heterochromatin was relatively dispersed. occu pying some peripheral and inner parts of the nuclei. In a later endosp erm development, the nuclei became smaller and more pycnotic, and the heterochromatin masses were placed predominantly near the nuclear peri phery. The distribution of anti-5-mC labelling on the heterochromatic regions was unequal: some parts appeared hypermethylated while other p arts were, like the euchromatin, not labelled. During mitosis, the lab elling intensity of all the chromosomes was approximately the same, th us indicating that there are no cytologically detectable methylation d ifferences among the individual sets of chromosomes. However, differen ces in the anti-5-mC signal intensity along individual chromosomes wer e observed, resulting in banding patterns with highly positive bands a pparently representing constitutive heterochromatic regions. From thes e results it is obvious that facultative heterochromatinisation. in co ntrast to constitutive heterochromatinisation, need not be strictly ac companied by a prominent DNA hypermethylation.