Phylogenetic and biogeographic complexity of Magnoliaceae in the Northern Hemisphere inferred from three nuclear data sets
Introduction
The intercontinental disjunctions of plants in the Northern Hemisphere are considered to be the most complex biogeographic pattern observed at the global scale (Wen, 1999, Wen, 2001, Milne and Abbott, 2002, Donoghue and Smith, 2004, Milne, 2006). Such disjunctions are generally thought to be remnants of a more continuously distributed, mixed mesophytic forest during the Tertiary, known as the “boreotropical flora” (Wolfe, 1975). It subsequently became fragmented due to geologic and climatic changes. Some of the ancient relict mesic forests that once covered much of the temperate regions of the Northern Hemisphere can be found today in the southeast region of USA as well as in eastern to central China and central to southern Japan.
Although the “boreotropical flora” hypothesis has been well accepted, questions still remain (Donoghue and Smith, 2004). These concern their overall phylogenetic relationships, morphological correlations (convergence/stasis) of temperate pairs in different areas, and morphological divergence between temperate and tropical relatives (Wen, 1999, Donoghue and Smith, 2004, Ickert-Bond et al., 2007). The disjunct taxa with relatives in the tropics are biogeographically much more complicated (Wen, 1999). To better understand the complexity requires the evaluation of phylogenetic relationships and divergence times in a broader phylogenetic framework, including closely related elements spanning from the temperate regions to the subtropical and tropical zones.
One potentially important study group showing such a disjunct pattern is the family Magnoliaceae, which is distributed from the north temperate to the tropical regions. Based on the fossil record (Tao and Zhang, 1992, Frumin and Friis, 1996, Frumin and Friis, 1999), this family has been considered to be one of the earliest extant lineages of flowering plants (93.5–110 mya) and has played a crucial role toward our understanding of the origin and diversification of angiosperms. Both morphological and molecular evidence have supported this scenario (Takhtajan, 1969, Cronquist, 1981, Mathews and Donoghue, 1999, Parkinson et al., 1999, Qiu et al., 1999, Soltis et al., 1999, Graham and Olmstead, 2000).
Magnoliaceae is comprised of ca. 220–240 species characterized by an androecium of numerous spirally arranged stamens, a gynoecium with many simple carpels spirally arranged on an elongated axis, and separate tepals (Law, 1984, Law, 1996, Law, 2004, Liu et al., 1995, Figlar and Nooteboom, 2004). Roughly two-thirds of the species are currently distributed in temperate and tropical regions of eastern to southeastern Asia (Fig. 1). The other third occur in the New World from the temperate eastern North America through tropical South America as far as Brazil and Bolivia (Dandy, 1927, Dandy, 1971, Dandy, 1978b, Law, 1984, Nooteboom, 1993, Nooteboom, 1998, Thorne, 1993, Frodin and Govaerts, 1996). The distribution of Magnoliaceae in the north temperate as well as tropical regions in Asia and the Americas (Fig. 1), makes it an excellent model for understanding the evolution of intercontinental temperate disjunctions and their interactions with the tropical members.
Magnoliaceae are usually divided into two subfamilies: Magnolioideae and Liriodendroideae (Law, 1984, Nooteboom, 1985). Except for the very distinct Liriodendroideae (including only Liriodendron with two species), taxonomists have long debated over the classification of Magnolioideae due to a paucity of phylogenetically useful characters (Dandy, 1927, Dandy, 1978a, Dandy, 1978b, Law, 1984, Law, 1996, Qiu et al., 1995a, Qiu et al., 1995b, Jobes, 1998, Nooteboom, 1985, Nooteboom, 1998, Chen and Nooteboom, 1993, Azuma et al., 1999, Azuma et al., 2001, Kim et al., 2001, Sun and Zhou, 2004, Xu and Rudall, 2006). Most recently, a new classification for Magnolioideae was proposed by Figlar and Nooteboom (2004) based on the chloroplast phylogenetic results and morphological re-examinations (also see Figlar, 2006). This classification recognized only one genus Magnolia in the Magnolioideae subfamily.
Molecular phylogenetic studies on Magnoliaceae have been conducted using chloroplast data (e.g., Qiu et al., 1995a, Qiu et al., 1995b, Azuma et al., 1999, Azuma et al., 2001, Azuma et al., 2004, Shi et al., 2000, Kim et al., 2001, Wang et al., 2006). One of the first important findings from molecular evidence was the polyphyly of Magnolia section Rhytidospermum, with the North American Magnolia tripetala closely related to the Asian counterparts, rather than to M. macrophylla, or M. fraseri from the same continent (Qiu et al., 1995a, Qiu et al., 1995b). The studies also indicated that Manglietia and Michelia were nested within Magnolia. Kim et al. (2001) used ndhF sequences of 99 taxa representing all of the traditional Magnoliaceae lineages and produced a most parsimonious tree containing 11 clades. However, many clades were weakly supported and the relationships among these clades remained unclear. Other molecular studies produced similar results (e.g., Azuma et al., 2000, Azuma et al., 2001, Azuma et al., 2004, Ueda et al., 2000).
The limited divergence of chloroplast sequences and the relative morphological homogeneity in Magnoliaceae have made it difficult to resolve the phylogenetic relationships within the family (Qiu et al., 1995a, Azuma et al., 2001, Kim et al., 2001, Li and Conran, 2003). Nuclear data have not been employed for phylogenetic studies in Magnoliaceae so far. Since nuclear markers have been generally shown to provide stronger phylogenetic signals than many plastid sequences (Wolfe et al., 1987, Small et al., 2004), we herein use three nuclear genes to resolve the phylogenetic relationships of Magnoliaceae. Phytochrome evolution in land plants has been shown to result from a series of gene duplications (e.g., PHYA, PHYB, PHYC, and PHYE) that have led to independent and functionally distinct lines (Mathews et al., 1995, Mathews and Sharrock, 1996, Manabe and Nakazawa, 1997, Mathews et al., 2003). Nucleotide variation at phytochrome loci has been useful in various phylogenetic studies of basal angiosperms and of several other angiosperm families (Kolukisaoglu et al., 1995, Mathews and Donoghue, 1999). The low-copy nuclear LEAFY (LFY) gene has been used for phylogenetic analyses of plants with its introns showing a relatively high nucleotide substitution rate (Hoot and Taylor, 2001, Archambault and Bruneau, 2004). The Arabidopsis GA INSENSITIVE (GAI) locus and related genes of the DELLA subfamily encode growth regulators and have been implicated in quantitative variation for developmental traits (Peng et al., 1999, Silverstone et al., 1998, Thornsberry et al., 2001, Boss and Thomas, 2002). The GAI-like gene sequence (GAI1) derived from a grapevine dwarf mutant was examined in a phylogenetic context within the Vitaceae and has been shown to be a potentially important marker (Wen et al., 2007).
Biogeographic studies have been conducted on some disjunct lineages in Magnoliaceae (Parks et al., 1983, Parks et al., 1994, Parks and Wendel, 1990, Qiu and Parks, 1994, Qiu et al., 1995a, Qiu et al., 1995b, Azuma et al., 2001). Liriodendron was estimated to have diverged in the mid-Miocene based on allozyme and restriction fragment length polymorphism (RFLP) analyses of cpDNA and paleobotanical evidence (Parks and Wendel, 1990). Subsequently, the divergence time of the genus was specifically estimated to be 27.9 ± 4.4 mya based on plastid trnK, psbA_trnH, and atpB_rbcL sequences using strict molecular clocks calibrated with fossil evidence (Azuma et al., 2001). Other lineages in the family suggested additional disjunctions. Magnolia sect. Rhytidospermum, the North American M. tripetala was shown to be sister to the Asian counterparts and their divergence was estimated to be during the late Miocene to early Pliocene using the allozyme and RFLP data of cpDNA (Qiu et al., 1995a, Qiu et al., 1995b), and 20.9 ± 3.3 to 27.9 ± 4.4 mya using the cpDNA sequences by Azuma et al. (2001). Divergence times in Magnoliaceae previously were all dated based on strict molecular clocks and discrepancies were found between different data sets. To update our biogeographic understanding of the family, we performed age estimation using “relaxed clock” analyses and multiple fossil calibrations (Renner, 2005).
This study employed sequences of three nuclear markers to infer the phylogenetic relationships in Magnoliaceae, with comparison of previous chloroplast results. Divergence times for intercontinental disjunct clades were estimated using the nuclear sequences under a relaxed molecular clock. Geologic data were placed in the phylogenetic contexts to gain insights into the biogeographic origin and interactions of temperate and tropical elements disjunct between eastern Asia and eastern North America. The phylogenetic framework and the temporal scale that we present provide a foundation to examine the complexity and relationships of biogeographic diversification of angiosperms between temperate and tropical zones in the Northern Hemisphere.
Section snippets
Plant sampling and sequencing
A total of 104 samples representing 86 species and subspecies, and varieties were sequenced in this study (Appendix 1). This sampling scheme covers all sections and nearly all subsections (excepting subsections Dugandiodendron, Splendentes, and Maingola) of the most recent classification system of Magnoliaceae by Figlar and Nooteboom (2004). We followed Frodin and Govaerts (1996) for generic circumscription and scientific names, and Figlar and Nooteboom (2004) for the classification system.
Phylogenetic analyses based on nuclear data sets
The PHYA matrix comprised 1070 aligned positions without indels. The putative LFY sequence was 887 bp in aligned length including both a coding region (1–330) with a 3-bp indel and an intron ranging from 331 to 887. The alignment of the GAI1 sequences generated a data matrix of 1300 positions with a 9-bp indel. The strict consensus trees for each nuclear sequence significantly supported the separation of the two subfamilies (Liriodendroideae and Magnolioideae), but the phylogenetic relationships
Phylogenetic relationships within Magnoliaceae
All nuclear data clearly supported the separation of the two subfamilies, the speciose Magnolioideae and the monogeneric Liriodendroideae, as had been shown previously based on chloroplast data (Chase et al., 1993, Qiu et al., 1993, Azuma et al., 2001, Kim et al., 2001). Each of the three nuclear genes, PHYA, LFY, and GAI1 separately exhibited a relatively low level of sequence divergence, and thus the phylogenetic relationships within the subfamily Magnolioideae were not well resolved (their
Acknowledgments
This study was supported by grants from the National Basic Research Program of China (973 Program, 2007CB411601), the Natural Science Foundation of China (NSFC 30625004 and 40771073 to H. Sun), and the John D. and Catherine T. MacArthur Foundation (to J. Wen), and by the Laboratory of Analytical Biology of the Smithsonian Institution’s National Museum of Natural History. We appreciate the valuable comments from R. Figlar of the Magnolia Society International, South Carolina, USA.
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