A Systematic and Evolutionary Study of Oenothera (Onagraceae): Seed Coat Anatomy

Abstract

On the basis of a study of 32 of the 124 species representing the 14 sections of Oenothera, we find that the seed coats exhibit considerable anatomical diversity, particularly the exotesta, mesotesta, and endotesta. Features throughout the genus are: (1) endotegmen is one cell thick and persistent and (2) consists of longitudinally elongate, tannin-containing cells; (3) exotegmen consists of longitudinally elongate, thick-walled, pitted, lignified fibers; and (4) endotestal cells contain crystals. Unequivocal relationships are demonstrated for species with essentially identical seed features: (1) between section Kleinia and section Oenothera subsection Raimannia; and (2) among sections Hartmannia, Kneiffia, Xylopleurum, and Anogra. A cladistic analysis, using seed anatomy data, indicates that (1) O. havardii differs in several characteristics from members of section Gauropsis and should be excluded; and (2) seed anatomy of O. maysillesii differs in several features from the other species of subsection Emersonia; these same differences are shared with sections Ravenia, Contortae, Eremia, and Pachylophus, indicating that O. maysillesii is their sister group. The analysis indicates an early divergence of Oenothera into two major lineages: (1) species with thick endotesta (sections Contortae, Eremia, Kleinia, Oenothera, Pachylophus, and Ravenia); and (2) species with radially flattened endotestal cells (sections Anogra, Gauropsis, Hartmannia, Kneiffia, Lavauxia, Megapterium, Paradoxus, and Xylopleurum). Subsequent changes in seeds of both lineages include multiplication of the fibrous exotegmen, development of a crushed mesotesta or multiplication of the cell layers of the mesotesta, and specialization of the exotesta involving various radial cell enlargements. Diversification of the first lineage has involved changes in seed anatomy that correlate with ecological shifts, while to a much greater extent the evolution of the second lineage has not. Outer layers of the seed appear to be more open to adaptive modifications than inner ones.