Molecular phylogenetics and cupule evolution in Fagaceae as inferred from nuclear CRABS CLAW sequences
Fagaceae include nine genera and ca. 900 species, making it one of the largest and economically important groups within Fagales. Phylogenetic relationships remain unresolved despite a long history of systematic study. We used CRABS CLAW, a single-copy nuclear gene, to generate over 2,900 base pairs of new sequence data, and combined it with previously collected molecular data to examine morphological evolution in the Fagaceae. We clarified the relationships of Chrysolepis, Lithocarpus and Quercus, and in this improved phylogenetic context we inferred morphological transformations of the cupule. Phylogenetic analyses supported an origin for Quercus from a castaneoid ancestor and a novel sister group relationship for Asian Lithocarpus and Chrysolepis; however the monophyly of both Lithocarpus and subfamily Castaneoideae was rejected. Our data suggest that North American Lithocarpus densiflorus is distantly related to Asian Lithocarpus and more closely related to Quercus, supporting the hypothesis that the uniflorous cupules of L. densiflorus and Quercus are derived from dichasium cupules via loss of lateral flowers. The sister group relationship of Asian Lithocarpus and Chrysolepis is supported by the morphological synapomorphy that each fruit is surrounded by a cupule. A new hypothesis is proposed to explain the evolution of the unusual cupules of Asian Lithocarpus and Chrysolepis. In this model, every flower represents a reduced dichasium, and we interpret the cupules of these taxa to be compound in nature, formed by multiple single-flowered cupules.
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