Reconstruction of the evolutionary ontogeny of the endosperm by
phylogenetically anchored comparative embryology. A schematic
representation of the phylogenetic trees of (a) gymnosperms and (b)
angiosperms. (c) A series of example species illustrating the diversity
of embryo sac structures among extant plants and the subsequent
diversity of endosperm ploidy. The embryo sac depicted for Amborella
and Arabidopsis is of the Polygonum type. The variations in
endosperm ploidy for Piperaceae and Plumbaginaceae, the transient
presence of endosperm in Orchidaceae and its absence from
Podostemonaceae are all only predicted, on the basis of cytological
studies. The ploidy number of the endosperm has been precisely
measured in some basal angiosperms and is known in Arabidopsis.
For comparison to the angiosperm female gametophyte (embryo sac),
the female gametophyte of two gymnosperm species is shown with
copious cellularized haploid tissue and between one and five
archegonia, each harboring one egg cell. In the case of Ephedra, egg
cells are binucleate with a normal egg nucleus and a ventral canal
nucleus. In gymnosperms a situation of 'simple complex
polyembryony' occurs in which several egg cells can be fertilized (by
mitotically distinct sperm nuclei), and each zygote then generates
four embryo clones. The extent of this polyembryony varies between
species, and a simplified form is depicted here. Ultimately, only one
embryo will survive while the others degenerate (gray dashed lines in
the example of Abies). In the case of Ephedra, both nuclei of the egg
cell are fertilized by two sperm nuclei discharged by a single pollen
tube (double fertilization) and polyembryony also applies to both
fertilization products.
From Baroux et al. (2002). See also Friedman (2006).
phylogenetically anchored comparative embryology. A schematic
representation of the phylogenetic trees of (a) gymnosperms and (b)
angiosperms. (c) A series of example species illustrating the diversity
of embryo sac structures among extant plants and the subsequent
diversity of endosperm ploidy. The embryo sac depicted for Amborella
and Arabidopsis is of the Polygonum type. The variations in
endosperm ploidy for Piperaceae and Plumbaginaceae, the transient
presence of endosperm in Orchidaceae and its absence from
Podostemonaceae are all only predicted, on the basis of cytological
studies. The ploidy number of the endosperm has been precisely
measured in some basal angiosperms and is known in Arabidopsis.
For comparison to the angiosperm female gametophyte (embryo sac),
the female gametophyte of two gymnosperm species is shown with
copious cellularized haploid tissue and between one and five
archegonia, each harboring one egg cell. In the case of Ephedra, egg
cells are binucleate with a normal egg nucleus and a ventral canal
nucleus. In gymnosperms a situation of 'simple complex
polyembryony' occurs in which several egg cells can be fertilized (by
mitotically distinct sperm nuclei), and each zygote then generates
four embryo clones. The extent of this polyembryony varies between
species, and a simplified form is depicted here. Ultimately, only one
embryo will survive while the others degenerate (gray dashed lines in
the example of Abies). In the case of Ephedra, both nuclei of the egg
cell are fertilized by two sperm nuclei discharged by a single pollen
tube (double fertilization) and polyembryony also applies to both
fertilization products.
From Baroux et al. (2002). See also Friedman (2006).
Embryology
