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Evolution of reproductive structures in grasses (Poaceae) inferred by sister-group comparison with their putative closest living relatives, Ecdeiocoleaceae¹

²Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK; ³Seed Conservation Department, Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK; ⁴Department of Biology, University of Missouri– St. Louis, One University Boulevard, St Louis, Missouri 63121 USA; ⁵Royal Botanic Gardens, Sydney, NSW 2000, Australia

ABSTRACT

Despite much recent activity in the phylogeny and developmental genetics of grasses, the enigmatic homologies of their reproductive structures remain largely unresolved, partly because their highly derived morphology has resulted in a unique associated terminology. Outstanding questions include whether grass lodicules and stamens are derived from a single perianth or stamen whorl, respectively, whether the grass caryopsis is homologous with a nut, and how the scutellum evolved. We investigated the reproductive structures of the putative sister group of grasses, the southwestern Australian family Ecdeiocoleaceae, which includes two genera, Ecdeiocolea and Georgeantha. The zygomorphic arrangement of the four (rather than six) stamens in male flowers of Ecdeiocolea indicates that they may represent three outer stamens plus the adaxial inner stamen. Within Ecdeiocoleaceae, characters such as the highly unusual nucellus structure of Ecdeiocolea are autapomorphic. Sister-group comparisons indicate that some characteristic grass features, notably the scutellum, do not occur in their putative closest relatives and that more data are needed on early-diverging grass genera to resolve these issues. The grass caryopsis could represent one end of a transformation series embodied by the reduced gynoecial structure and indehiscent fruit of other Poales such as Flagellaria, Joinvillea, and Ecdeiocolea.

Key Words: caryopsis • Ecdeiocolea • flower • Georgeantha • grasses • monocots • Poaceae • scutellum

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