Abstract

Research on early-divergent angiosperms, including Amborella, the putative sister to all other extant angiosperms, is increasingly used as a yardstick to infer the nature of the hypothetical ancestral angiosperm. Some traits are relatively diverse (and hence relatively labile) in this phylogenetic grade, compared with the more derived eudicot clade, in which developmental patterns have become increasingly canalized. One of the many mysteries surrounding the origin of the angiosperms is the evolutionary origin of the Polygonum-type embryo sac (monosporic, eight-nucleate and seven-celled) that occurs in the majority of flowering plants. Observations on the megagametophyte of Amborella are conflicting, but a recent report of a supernumerary synergid in this genus1 raises the question of whether the Polygonum-type embryo sac is derived by duplication of a four-nucleate structure or by reduction from a multicellular structure. BioEssays 28: 1067–1071, 2006. © 2006 Wiley Periodicals, Inc.