Endosperm development and evolution in basal flowering plants.

摘要

A phylogenetically-based, comparative, embryological study of 13 basal angiosperm taxa was undertaken. Specific goals were to (1) provide a full developmental analysis of each species, (2) compare patterns to determine phylogenetic character distribution, (3) reconstruct ancestral characters for angiosperms, and (4) explore scenarios of ontogenetic evolution that occurred during the early radiation of flowering plants.; Embryology was fully investigated in Platanus racemosa (Platanaceae), a basal eudicot. Female gametophyte development is monosporic. Embryogenesis involves zygote elongation and division, development of a linear proembryo, formation of the embryo proper, histogenesis, organogenesis, and growth. Endosperm development is complex and includes four distinct phases: free nuclear proliferation, cellularization of the chalazal zone, centripetal cellularization of the micropylar zone, and cellular differentiation and growth. Comparison of endosperm in Platanus with other free nuclear taxa reveals considerable developmental variability, consistent with a hypothesis of multiple origins of free nuclear endosperm.; Endosperm development was compared in 12 additional basal angiosperm taxa. Endosperm ontogeny was analyzed as a series of stages (characters) during which differential modes of development occur among taxa (character states). Differential developmental fate (of chalazal and micropylar endosperm domains) is common to all taxa studied, and may characterize endosperm of all angiosperms. Bipolar endosperm development in basal angiosperms bears a marked similarity to angiosperm embryogenesis, which may have implications for understanding the evolutionary origin of endosperm. Variability in endosperm developmental characters indicates that many ontogenetic transformations occurred during early radiation of the flowering plants.; I compared endosperm development in Amborella, Nuphar (Nymphaeales), and Illicium (Illiciales), representing the three lineages recently identified as the earliest diverging, extant angiosperms. Amborella and Illicium share a bipolar, cellular pattern of development, characterized by an oblique first division, that phylogenetic mapping resolves as ancestral for flowering plants. A series of character state transformations occurred in the Nymphaeales which led first to a modified cellular pattern (present in Nuphar) with a transverse first division. The transverse cellular pattern was transformed to a helobial pattern that is present in the Cabombaceae (Nymphaeales). This is the first explicit hypothesis presented for endosperm ontogenetic evolution, and the origin of the helobial pattern.