Abstract
The hickory genus (Carya) contains ca. 17 species distributed in subtropical and tropical regions of eastern Asia and subtropical to temperate regions of eastern North America. Previously, the phylogenetic relationships between eastern Asian and eastern North American species of Carya were not fully confirmed even with an extensive sampling, biogeographic and diversification patterns had thus never been investigated in a phylogenetic context. We sampled 17 species of Carya and 15 species representing all other genera of the Juglandaceae as outgroups, with eight nuclear and plastid loci to reconstruct the phylogeny of Carya. The phylogenetic positions of seven extinct genera of the Juglandaceae were inferred using morphological characters and the molecular phylogeny as a backbone constraint. Divergence times within Carya were estimated with relaxed Bayesian dating. Biogeographic analyses were performed in DIVA and LAGRANGE. Diversification rates were inferred by LASER and APE packages. Our results support two major clades within Carya, corresponding to the lineages of eastern Asia and eastern North America. The split between the two disjunct clades is estimated to be 21.58 (95% HPD 11.07-35.51) Ma. Genus-level DIVA and LAGRANGE analyses incorporating both extant and extinct genera of the Juglandaceae suggested that Carya originated in North America, and migrated to Eurasia during the early Tertiary via the North Atlantic land bridge. Fragmentation of the distribution caused by global cooling in the late Tertiary resulted in the current disjunction. The diversification rate of hickories in eastern North America appeared to be higher than that in eastern Asia, which is ascribed to greater ecological opportunities, key morphological innovations, and polyploidy.
Highlights
As one of the typical phytogeographic disjunctions in the Northern Hemisphere, the eastern Asian (EA)-eastern North American (ENA) floristic disjunction pattern has received considerable attention
Carya is strongly supported as monophyletic (MLBS 100%; posterior probability (PP) 1.00), and two major clades are well supported, which correspond to an EA group (MLBS 98%; PP 1.00) and an ENA group (MLBS 98%; PP 1.00), respectively
Within the EA group, C. cathayensis and C. kweichowensis are in a clade that is sister to a subclade of the remaining three species
Summary
As one of the typical phytogeographic disjunctions in the Northern Hemisphere, the eastern Asian (EA)-eastern North American (ENA) floristic disjunction pattern has received considerable attention (see 1-4 and references therein) This pattern is often explained by the boreotropical flora hypothesis [5]. A relatively continuous, homogenous mesophytic forest that spanned the Northern Hemisphere during the climatically warm mid-Tertiary became fragmented as global temperature cooled down in the late Tertiary and Quaternary [1,3,5,6,7,8,9] Both the Bering land bridge (BLB) [10,11] and the North Atlantic land bridge (NALB) [12,13] probably contributed to the floristic intercontinental exchanges to form the boreotropical flora.
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