Tree top climbing : towards an evolutionary tree of Hydrangea section Cornidia

As traditionally recognized, the tribe Hydrangeeae (Hydrangea s.l.) of the Cornales family Hydrangeaceae includes the warm temperate and tropical genera Hydrangea s.s. plus Broussaisia, Cardiandra, Decumaria, Deinanthe, Dichroa, Pileostegia, Platycrater and Schizophragma. A number of species of this tribe are popular woody ornamentals for their conspicuous inflorescences with attractive marginal flowers. The most recent revision of Hydrangea s.s. by McClintock (1957) subdivides this genus in the sections Hydrangea and Cornidia, each further divided in subsections. The H. sect. Cornidia comprises a yet undefined number of evergreen root-climbing species of nearly exclusive American distribution, with the exception of one species from Taiwan and the Philippines. The present study focuses on the tribe Hydrangeeae (Hydrangea s.l.) and H. sect. Cornidia. Knowledge of the phylogenetic relationships within the tribe has progressively improved in a series of molecular and morphological studies consistently recovering Hydrangea s.s. as paraphyletic with respect to the other genera in the tribe. A recent study by our research group included a number of Hydrangeeae representatives, recognizing two highly supported main clades within the tribe. However, due to heterogeneous distributions of rates of evolution and limited plastid variability, relationships among the different “genera” remained poorly understood. Despite the long record of phylogenetic studies in the tribe Hydrangeeae, a limited number of species from H. sect. Cornidia could be included in previous phylogenetic studies preventing a robust test of the monophyly of this section and its subsections, as well as a better knowledge of the evolutionary history of this group. The present PhD research project was divided in four subprojects. The aims of the first two were to gain insight in the phylogenetic relationships within Hydrangea s.l. and H. sect. Cornidia, respectively. In order to achieve these, we evaluated the performance of a number of coding and non-coding regions from the chloroplast and nuclear genomes for resolving and supporting phylogenetic relationships in these groups. The third subproject connects our fundamental phylogenetic research of Hydrangea s.l. to an applied study on hortensia breeding. Although hortensias are already more than two centuries important for horticultural trade, crosses between distantly related species have been problematic in this group hindering exploitation of economic value. The aim of this subproject was to inform the selection of potentially successful hortensia hybrid crosses, based on our recently improved phylogenetic knowledge in the group and genetic distances calculated from our extensive chloroplast nucleotide database. The fourth subproject focused on the characterization of the root climbing growth form in H. sect. Cornidia, using a multidisciplinary approach integrating plant biomechanics, architecture and anatomy. Our results represent a concrete progress in the reconstruction of a solid phylogenetic framework for Hydrangea s.l. and H. sect. Cornidia facilitating the application of the evidently needed taxonomical changes in these groups. Additionally, our results highlight the importance of “phylogenetic thinking” and its value in plant breeding. Furthermore, we present a contribution to the knowledge of the biology of the previously barely studied H. sect. Cornidia.