Origin of facultative heterochromatin in the endosperm of Gagea lutea (Liliaceae)
Facultative heterochromatin occurs not only in certain animals in connection with sex determination but also in members of at least one plant genus, Gagea (Liliaceae s. str.), but here in the course of embryo sac development, fertilization, and endosperm formation. The present contribution intends to provide undebatable photographic and cytometric evidence, previously not available, for the events in the course of which three whole genomes in the pentaploid endosperm nuclei of Gagea lutea become heterochromatinized, In this plant, embryo sac formation usually follows the Fritillaria type, i.e., the embryo sac is tetrasporic, and a "1 + 3 position" of the spore nuclei is followed by a mitosis in which the three chalazal spindles fuse and two triploid nuclei are formed. A triploid chalazal polar nucleus is derived from one of these, which contributes to the pentaploid endosperm. These nuclei in the chalazal part of the embryo sac show stronger condensation compared with the micropylar ones. The pyenosis of the triploid polar nucleus is maintained and even enhanced during endosperm proliferation, while the micropylar polar nucleus and the sperm nucleus maintain their euchromatic condition. The origin of the heterochromatic masses in the endosperm nuclei from the three chalazal genomes of the central cell is unambiguously evident from the distribution of heterochromatic chromosomes in the first endosperm mitosis and the following interphase. DNA content measurements confirm a 3 : 2 relationship of heterochromatic and euchromatic chromosome sets, which is usually maintained up to the cellularized endosperm. Pycnotic nuclei in the chalazal part of megagametophytes are characteristic of several embryo sec types, but only for Gagea spp. it is documented that such nuclei can take part in fertilization and endosperm formation.