Facultative heterochromatin occurs not only in certain animals in connectio
n with sex determination but also in members of at least one plant genus, G
agea (Liliaceae s. str.), but here in the course of embryo sac development,
fertilization, and endosperm formation. The present contribution intends t
o 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 f
rom one of these, which contributes to the pentaploid endosperm. These nucl
ei in the chalazal part of the embryo sac show stronger condensation compar
ed with the micropylar ones. The pyenosis of the triploid polar nucleus is
maintained and even enhanced during endosperm proliferation, while the micr
opylar polar nucleus and the sperm nucleus maintain their euchromatic condi
tion. The origin of the heterochromatic masses in the endosperm nuclei from
the three chalazal genomes of the central cell is unambiguously evident fr
om the distribution of heterochromatic chromosomes in the first endosperm m
itosis and the following interphase. DNA content measurements confirm a 3 :
2 relationship of heterochromatic and euchromatic chromosome sets, which i
s usually maintained up to the cellularized endosperm. Pycnotic nuclei in t
he chalazal part of megagametophytes are characteristic of several embryo s
ec types, but only for Gagea spp. it is documented that such nuclei can tak
e part in fertilization and endosperm formation.