The recent unprecedented concurrence of several independent phylogenetic analyses of basal angiosperm taxa has led to the identification of the three earliest diverging lineages of extant flowering plants: Amborella trichopoda, Nymphaeales, and Illiciales. With a more confident identification of the earliest angiosperm lineages, we are now in a better position to resolve character polarity and identify character state transitions during the earliest radiation of extant flowering plants. Because of its phylogenetic position as sister to all other angiosperms, Amborella is critical to assessing angiosperm character polarity and evolution. However, little is known about the basic biology of Amborella, which is endemic to New Caledonia, not widely cultivated, and thus insufficiently studied. In particular, embryological data are completely lacking. Endosperm is the sexually-derived embryo-nourishing structure that is unique to the life cycle of angiosperms. We provide the first report of endosperm development in Amborella. The endosperm exhibits a bipolar, cellular developmental pattern that is quite similar to other basal angiosperm taxa we have investigated, most notably Illicium, another representative of the three earliest-diverging lineages of flowering plants. In contrast, endosperm in Amborella is quite unlike endosperm in Cabomba and other Nymphaeales. The implications of these findings for understanding the origin and evolution of this distinctive aspect of the reproductive biology of flowering plants are discussed.

Key words: Amborella, basal angiosperms, character evolution, development, endosperm, reproductive biology