Seed Coat Anatomy and Relationships of Ludwigia Sects. Microcarpium, Dantia, and Miquelia (Onagraceae), and Notes on Fossil Seeds of Ludwigia from Europe

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Abstract
Seed coat anatomy was investigated in all species of three closely related sections of Ludwigia, i.e., sects. Microcarpium (14 species), Dantia (five species), and Miquelia (one species), and in two fossil seeds of Ludwigia from Europe. Eight species of sect. Microcarpium have a thin endotesta, as do other Ludwigia, whereas the other six species of the section have a thick endotesta, as do sects. Dantia and Miquelia. The thin endotesta, also found in other Onagraceae, is clearly a plesiomorphic characteristic, and the thick endotesta is an apomorphic one. The present study supports the hypothesis that sects. Dantia and Miquelia were independently derived from a common ancestor from within sect. Microcarpium, specifically from those species with thick endotesta. On the basis of different character state combinations of capsule wall structure, capsule dehiscence, and seed coat structure, we recognize five species groups within sect. Microcarpium: group 1, which comprises species with plesiomorphies alone--L. alata, L. lanceolata, L. linearis, and L. suffruticosa: group 2, which comprises only L. microcarpa with a plesiomorphic capsule wall structure and capsule dehiscence and an apomorphic seed coat structure: group 3, which comprises two species with a plesiomorphic capsule wall structure and apomorphic capsule dehiscence and seed coat structure--L. curtissii and L. simpsonii; group 4, which comprises species with apomorphic capsule wall structure and dehiscence, and a plesiomorphic seed coat structure--L. pilosa, L. polycarpa, L. ravenii, and L. sphaerocarpa: and group 5, which comprises species with apomorphies alone--L. glandulosa, L. linifolia and L. stricta. Groups 2, 3 and 4 may have been derived from hybridization between ancestral species of other groups. This hypothesis of the hybrid origin of groups 2, 3 and 4, as well as the species groupings themselves, is supported by biosystematic studies. Of two fossil seeds from Europe, L. collinsoniae is assigned to the complex consisting of sects. Microcarpium, Dantia and Miquelia since it shares their apomorphic seed coat structure, but the sectional assignment of the other fossil seed, L. corneri is unclear. Our study indicates that no direct relationships exist between the two fossil species or between L. collinsoniae and L. palustris of sect. Dantia, the only modern species in Europe.