Female gametophyte development in Kadsura: Implications for Schisandraceae, Austrobaileyales, and the early evolution of flowering plants

• Published in: International journal of plant sciences Vol. 164; no. 5; pp. 293 - 305
• Main Authors: Friedman, W.E, Gallup, W.N, Williams, J.H
• Format: Journal Article
• Language: English
• Published: Chicago The University of Chicago Press 01.09.2003; University of Chicago, acting through its Press
• Subjects: Amborella; Austrobaileyaceae; Austrobaileyales; Cells; evolution; Flowers & plants; gametocytes; Illiciaceae; Kadsura; Kadsura japonica; megasporogenesis; Modularity; plant development; plant morphology; Schisandraceae; Trimeniaceae; developmental evolution; modularity; cell cycle; angiosperm; gametes; evolution; embryology
• ISSN: 1058-5893; 1537-5315
• DOI: 10.1086/376877

Recent phylogenetic analyses of angiosperms have identified a set of "basal" angiosperm lineages (Amborella, Nymphaeales, and a clade that includes Illiciaceae, Schisandraceae, Trimeniaceae, and Austrobaileyaceae) that are central to the study of the origin and early diversification of flowering plants. Prior to this phylogenetic revelation, much of the work on the embryology of ancient angiosperm lineages focused on core magnoliids (e.g., Magnoliales, Winterales). It is now apparent that little is known about the basic embryological features of the most ancient extant lineages of flowering plants, particularly with respect to the nature and development of the female gametophyte and the ploidy and genetics of the endosperm. Here, we report that Kadsura japonica (Schisandraceae) develops a four-celled female gametophyte with an egg cell, two synergids, and a uninucleate central cell. The pattern of free-nuclear divisions in the female gametophyte of Kadsura precisely matches what has recently been reported for four-celled gametophytes in the Nymphaeales. Following the first mitosis, migration of one of the two nuclei to the chalazal pole of the female gametophyte, as in Polygonum-type female gametophytes, does not occur. Rather, both nuclei remain close together in the micropylar domain where they undergo one additional mitotic division to yield four free nuclei before cellularization. Microspectrofluorometric analysis of relative DNA content of the central cell nucleus in Kadsura shows that this nucleus is haploid and contains the 1C quantity of DNA prior to fertilization. Thus, the endosperm of Kadsura should be diploid and biparental, as it is in Nuphar and other Nymphaeales. It now appears that four-celled female gametophytes, with consequent production of diploid endosperms, are common among the most ancient lineages of angiosperms, with the sole exception, to date, of Amborella. Finally, based on an analysis of the modular nature of the angiosperm female gametophyte, we provide evidence that four-celled female gametophytes that yield diploid biparental endosperms are likely to be plesiomorphic for flowering plants.