Systematics of Fagaceae: Phylogenetic Tests of Reproductive Trait Evolution

Abstract

The family Fagaceae includes nine currently recognized genera and ca. 1000 species, making it one of the largest and most economically important groups within the order Fagales. In addition to wide variation in cupule and fruit morphology, polymorphism in pollination syndrome (wind vs. generalistic insect) also contributes to the uniqueness of the family. Phylogenetic relationships were examined using 179 accessions spanning the taxonomic breadth of the family, emphasizing tropical, subtropical, and relictual taxa. Nuclear ribosomal DNA sequences encoding the 5.8S rRNA gene and two flanking internal transcribed spacers (ITS) were used to evaluate phylogenetic hypotheses based on previous morphological cladistic analysis and intuitive schemes. Parsimony analyses rooted with Fagus supported two clades within the family, Trigonobalanus sensu lato and a large clade comprising Quercus and the castaneoid genera ($$Castanea+Castanopsis$$ , Chrysolepis, Lithocarpus). Three DNA sequence data sets, 179‐taxon ITS, 60‐taxon ITS, and a 14‐taxon combined nuclear and chloroplast (matK), were used to test a priori hypotheses of reproductive character state evolution. We used Templeton’s (1983) test to assess alternative scenarios of single and multiple origins of derived and seemingly irreversible traits such as wind pollination, hypogeal cotyledons, and flower cupules. On the basis of previous exemplar‐based and current in‐depth analyses of Fagaceae, we suggest that wind pollination evolved at least three times and hypogeal cotyledons once. Although we could not reject the hypothesis that the acorn fruit type of Quercus is derived from a dichasium cupule, combined analysis provided some evidence for a relationship of Quercus to Lithocarpus and Chrysolepis, taxa with dichasially arranged pistillate flowers, where each flower is surrounded by cupular tissue. This indicates that a more broadly defined flower cupule, in which individual pistillate flowers seated within a separate cupule, may have a single origin.