An Updated Infrageneric Classification of the North American Oaks (Quercus Subgenus Quercus): Review of the Contribution of Phylogenomic Data to Biogeography and Species Diversity

Paul S Manos,Andrew L Hipp

doi:10.3390/f12060786

Abstract

The oak flora of North America north of Mexico is both phylogenetically diverse and species-rich, including 92 species placed in five sections of subgenus Quercus, the oak clade centered on the Americas. Despite phylogenetic and taxonomic progress on the genus over the past 45 years, classification of species at the subsectional level remains unchanged since the early treatments by WL Trelease, AA Camus, and CH Muller. In recent work, we used a RAD-seq based phylogeny including 250 species sampled from throughout the Americas and Eurasia to reconstruct the timing and biogeography of the North American oak radiation. This work demonstrates that the North American oak flora comprises mostly regional species radiations with limited phylogenetic affinities to Mexican clades, and two sister group connections to Eurasia. Using this framework, we describe the regional patterns of oak diversity within North America and formally classify 62 species into nine major North American subsections within sections Lobatae (the red oaks) and Quercus (the white oaks), the two largest sections of subgenus Quercus. We also distill emerging evolutionary and biogeographic patterns based on the impact of phylogenomic data on the systematics of multiple species complexes and instances of hybridization.

Highlights

We describe the regional patterns of oak diversity within North America and formally classify 62 species into nine major North American subsections within sections Lobatae and Quercus, the two largest sections of subgenus Quercus
We present a subsectional classification for the American oak clade based largely on our phylogenomic work of the past five years [15,19,23,24,25]
We have provided a review and synthesis of the systematics of the oaks within the Flora of the North American region, as informed by recent analyses using phylogenomic analyses of nuclear sequence data

Summary

Introduction

The oaks (Quercus, Fagaceae) comprise more than 435 species distributed across temperate and tropical regions of the northern hemisphere [1,2,3]. They are among the best-known and most ecologically significant forest trees in North America Oaks are united by a suite of distinctive floral traits—in particular, pendant male catkins and tricarpellate female flowers with linear styles and expanded stigmas—and a signature fruit trait: the circular single fruit or acorn seated within a cup-shaped extra-floral accessory structure, a specialized involucre called a cupule. While other genera of the family feature acorns and valveless rounded cupules, the combination of floral character states shared by oaks is unique within the family [11,12]

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Conclusion