A developmental and evolutionary analysis of embryology in Platanus (Platanaceae), a basal, eudicot

The Platanaceae are an early derived eudicot lineage and therefore occupy a key position for understanding reproductive character diversification associated with the early evolutionary radiation of flowering plants. We conducted an embryological study of Platanus racemosa in order to provide critical data on defining angiosperm reproductive characters for this important group. Female gametophyte development is monosporic. Embryogenesis occurs in a series of stages including zygote elongation and division, development of a linear proembryo, formation of the embryo proper, histogenesis, organogenesis, and growth. Endosperm development is a complex process that includes four distinct phases: free nuclear proliferation, cellularization of the chalazal zone, centripetal cellularization of the micropylar zone, and cellular differentiation and growth. Only the outer endosperm layer persists at seed maturity. Our findings differ significantly from previously published reports for Platanus, in which endosperm development was described as abinitio cellular. A comparison of endosperm development in Platanus with several closely and distantly related free nuclear taxa reveals considerable developmental variability, consistent with a hypothesis of multiple origins of free nuclear endosperm in angiosperms. Our analysis indicates that much remains to be learned about embryology in basal angiosperms. Additional developmental and comparative studies will likely reveal critical insights into the early evolution of flowering plants.