Ultrastructural study of the female gametophyte and the epistase in Cabombaceae and Nymphaeaceae

• Female gametophyte ultrastructure was studied in Nymphaeales for the first time.
• The epistase consists of cells compatible with a transfer cell function.
• Filiform apparatus is rudimentary in Nymphaeaceae and is absent in Cabombaceae.

Ultrastructural studies on the female gametophyte are restricted to species at relatively derived positions in the angiosperm phylogenetic tree. Therefore, this topic remains mostly unknown for the early-divergent lineages, in which a four-celled megagametophyte is common. Here, ultrastructure of the megagametophyte and micropylar nucellar epidermis was investigated in Cabomba caroliniana A. Gray (Cabombaceae), Nymphaea gardneriana Planch. and Victoria cruziana Orb. (Nymphaeaceae). The micropylar nucellar epidermis of the studied species differentiates into an epistase. These cells have metabolically active cytoplasm and thickened inner tangential walls. Epistase ultrastructure is compatible with a transfer cell specialization. This tissue may play an adaptive role in the secretion of chemotropic substances to direct the pollen tube growth toward the female gametophyte. The cytological characteristics of the female germ unit in members of Cabombaceae and Nymphaeaceae are generally similar to other angiosperms that develop a typical seven-celled, eight-nucleate female gametophyte; however, they differ in some specific points. In V. cruziana and N. gardneriana, the micropylar end of the synergids develops a rudimentary filiform apparatus with slight inward projections. By contrast, the synergids lack a filiform apparatus in C. caroliniana. Unlike most studied angiosperms, the filiform apparatus in the clade Cabombaceae-Nymphaeaceae is underdeveloped or absent, therefore character state transformations have occurred within basal angiosperms. The potential evolutionary shifts of this reproductive feature are highlighted.