Format: ORAL

Cecilia Zumajo-Cardona1, Dennis W. Stevenson2, Natalia Pabon-Mora3 Barbara A. Ambrose2

1 Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy. 2 New York Botanical Garden, Bronx, NY 10458, United States 3 Universidad de Antioquia, Medellín 050010, Colombia

The seed is a synapomorphy of all extant seed plants that develops from an ovule, i.e., an integumented megasporangium. As sporangia are conserved throughout vascular plants, the origin of the integument is a defining step in seed evolution. There are some key differences in the number of integuments present in seed plants as well as accessory structures that cover seeds in gymnosperms and some angiosperms. Due to extra structures covering the nucellus, the ovules of Gnetales (Ephedra, Gnetum, Welwitschia) are strikingly different from those of all other extant gymnosperms. In the genera Ephedra and Welwitschia two envelopes cover the nucellus in contrast to Gnetum that has three envelopes. In all three genera of Gnetales, it is generally agreed that the inner envelope is homologous to an integument that elongates and forms the micropyle in other gymnosperms. The homology of the additional envelopes remains equivocal, but it is not debated that these form a unique seed coat in the Gnetales. With an emphasis on the ovulate reproductive structures, here we present the morphoanatomy of the striking differences in the ovulate reproductive structures of representatives of the three gnetalean genera. With a search in our own generated transcriptomic data, we present the differences and similarities with what is already known for angiosperms. Developmental studies on the ovule of Gnetum gnemon suggests that canonical genes involved in integument development are not involved in the development of the additional envelopes, suggesting that at least from a molecular perspective there are major differences when compared to angiosperms. A holistic approach involving different perspectives may be required for a better understanding of the complex relationships and evolutionary history of the Gnetales.