Comparative fruit structure in Hydatellaceae (Nymphaeales) reveals specialized pericarp dehiscence in some early-divergent angiosperms with ascidiate carpels

Abstract

We provide the first detailed description of specialized anatomical mechanisms of fruit dehiscence in two sections of Trithuria (Hydatellaceae), a basal angiosperm lineage with a markedly ascidiate carpel. The dehiscent fruits of Hydatellaceae are unique among angiosperms. Earlier studies suggested the follicle as a primitive type of dehiscent angiosperm fruit, developing from a conduplicate carpel. In contrast with follicles, the single-seeded apocarpous (or unicarpellate) dehiscent fruits of Hydatellaceae dehisce by three equally spaced valves separating from three fruit ribs. There is no evidence of pseudomonomery. In mature fruits, all cell layers except the outer epidermis of the ovary are obliterated, so that only one pericarp layer (the exocarp) remains, except in the regions around the vascular bundles and at the fruit apex and base. Morphological cladistic analyses reveal data incongruence between characters describing fruit morphology and other morphological features of Hydatellaceae, including those linked with dioecy/monoecy. As a result, strict consensus trees inferred from maximum parsimony analyses based on each of the two sub-sets of morphological data are more resolved than ones inferred from the analysis of the entire morphological dataset. Fruit characters correspond closely with the recently published molecular phylogeny of the sole genus, Trithuria. The occurrence of an apparently rudimentary mechanical tissue at the fruit apex of most Hydatellaceae with indehiscent fruits suggests that the stem group of Hydatellaceae was characterized by dehiscent fruits with lignified endocarp fibres. The occurrence of Strelitzia-type surface waxes characteristic of Trithuria sect. Trithuria provides further evidence of the homoplasy of this epicuticular wax type at the scale of seed plants.