The four-celled female gametophyte of Illicium (Illiciaceae; Austrobaileyales): implications for understanding the origin and early evolution of monocots, eumagnoliids,and eudicots

The recent consensus that Amborellaceae, Nymphaeales, and Austrobaileyales form the three earliest-diverging lineages of angiosperms has led comparative biologists to reconsider the origin and early developmental evolution of the angiosperm seven-celled/eight-nucleate (Polygonum-type) female gametophyte. Illicium mexicanum (Illiciaceae; Austrobaileyales) develops a four-celled/four-nucleate female gametophyte. The ontogenetic sequence of the Illicium female gametophyte is consistent with that of all other Austrobaileyales and also with all Nymphaeales and is likely a plesiomorphy of angiosperms. A character analysis based on more than 250 embryological studies indicates that a transition from an ancestrally four-celled/four-nucleate Illicium-like female gametophyte to a seven-celled/eight-nucleate female gametophyte occurred in the common ancestor of the sister group to Austrobaileyales (a clade that includes monocots, eumagnoliids, and eudicots). Comparative analysis of reconstructed ancestral female gametophyte ontogenies identifies specific early stages of ontogeny that were modified during this transition. These modifications generated two important angiosperm novelties-a set of three persistent antipodal cells and a binucleate central cell, which upon fertilization yields a triploid endosperm. Early angiosperms are anatomically quite diverse in these two features, although triploid endosperm, composed of one paternal genome and two maternal genomes, is a conserved feature of the overwhelming majority of angiosperms.