Flower and Fruit Development of Three Species of Hydrophylleae Sheds New Light on Flower Evolution in Hydrophyllaceae

Abstract

Premise of research. Hydrophyllaceae are characterized by simple floral organization contrasting with a peculiar floral architecture resulting from complex compartments formed by stamen–corolla tube modifications. Additionally, the internal ovary architecture of Hydrophyllaceae shows significant variation, while the developmental trajectory of the gynoecium is relatively conserved. Despite insights from recent studies of the flower and fruit evolution of the family, there are only a few ontogenetic studies of Hydrophyllaceae, and a complete understanding of the underlying processes has not yet been achieved.Methodology. Here, we use scanning electron microscopy and micro–computed tomography to investigate the flower and fruit ontogeny of two genera of Hydrophylleae, Emmenanthe and Pholistoma, with a particular focus on the gynoecium and modifications of the stamen–corolla tube.Pivotal results. Our results complement two previously published data sets, broadening our understanding of Hydrophylleae evolution. Hydrophylleae comprise only a few monotypic or small genera, but their floral evolution appears to be remarkably complex, in terms of both gynoecial structure and perianth modifications. We find 10 stamen–corolla tube modifications, although these may be rudimentary in species previously considered as lacking them altogether (e.g., Emmenanthe). The relative conservation of perianth architecture contrasts with the highly variable internal ovary architecture of Hydrophylleae. There is considerable divergence in ovule/seed number, as well as in the details of (parietal) placentation and septation, and we propose a hypothetical evolutionary series for the internal ovary architectural diversity of Hydrophyllaceae.Conclusions. We propose that—starting from a fairly conserved floral organization—minor heterochronous shifts in both perianth and ovary development can explain most of the morphological diversity found in the flowers and fruits of Hydrophylleae.