Himalayan Origin and Evolution of Myricaria (Tamaricaeae) in the Neogene

Abstract

Background

Myricaria consists of about twelve-thirteen species and occurs in Eurasian North Temperate zone, most species in the Qinghai-Tibet Plateau (QTP) and adjacent areas.

Methodology/Principal Findings

Twelve species of Myricaria plus two other genera Tamarix and Reaumuria in Tamaricaceae, were sampled, and four markers, ITS, rps16, psbB-psbH, and trnL-trnF were sequenced. The relaxed Bayesian molecular clock BEAST method was used to perform phylogenetic analysis and molecular dating, and Diva, S-Diva, and maximum likelihood Lagrange were used to estimate the ancestral area. The results indicated that Myricaria could be divided into four phylogenetic clades, which correspond to four sections within the genus, of them two are newly described in this paper. The crown age of Myricaria was dated to early Miocene ca. 20 Ma, at the probable early uplifting time of the Himalayas. The Himalayas were also shown as the center of origin for Myricaria from the optimization of ancestral distribution. Migration and dispersal of Myricaria were indicated to have taken place along the Asian Mountains, including the Himalayas, Kunlun, Altun, Hendukosh, Tianshan, Altai, and Caucasus etc., westward to Europe, eastward to Central China, and northward to the Mongolian Plateau.

Conclusions/Significance

Myricaria spatiotemporal evolution presented here, especially the Himalayan origin at early Miocene ca. 20 Ma, and then migrated westward and eastward along the Asian mountains, offers a significant evolutionary case for QTP and Central Asian biogeography.

Introduction

The Tamaricaceae contains about eighty species [1] and four genera: Tamarix, Myricaria, Reumuria, and Hololachna [2]. This family, and Frankeniaceae, are defined as the salt-gland anatomical lineage [3]. Myricaria consists of about twelve - thirteen species [4]–[7] and occurs in Northern Temperate zone of Eurasia, mainly along the Asian mountains. There are eight species in Himalayas, many are endemic, thus forming a center of diversity for Myricaria (see Figure 1).

Figure 1. Distribution of Myricaria species (a,b), the information obtained from floras and herbaria, mainly in China.

Geographical division of QTP eastern and western portions is shown in a red broken line (c). The Himalayan origin and dispersal routes along the Asian mountains are illustrated in arrows (a).

Desvaux (1825) established the genus Myricaria and Niedenzu [8] presented the first classification. Zhang & Zhang [4] studied Myricaria in China and recognized ten species; they presumed the Himalayas to be the center of origin based mainly on the distribution of species. Gorschkova [7] described six species belonging to two sections in the flora of the former USSR.

Another issue relevant to Myricaria systematics is the species Myricaria elegans. Ovezinrlikov & Kinzikaeva [9] erected the genus Myrtama based on this species but it caused some controversy. Zhang et al. [10] used ITS sequence data to study the relationships within Tamaricaceae and regarded Myrtama as an intermediate genus between Myricaria and Tamarix [11]. After sampling four species from Myricaria and sequencing ITS, rbcL, and tRNAs Ser (GCU) and Gly (UCC), Gaskin et al. [1] found Myrtama and Hololachna to be distinct within Tamaricaceae, as did Zhang et al. [10]. However, based on additional sequence data, Hua et al. [12] and Wang et al. [13] confirmed that Myrtama should be included in Myricaria. Sampling ten species of Myricaria and sequencing cpDNA psbA–trnH and the rpL16 intron, Liu et al. [14]–[15] investigated the species-level phylogeographical patterns of Myricaria in western China as well as the origin of M. laxiflora, a unique subtropical species of conservation concern from the Three Gorges of the Yangtze River in Sichuan and Hubei provinces. The Himalayas were proposed as the center of origin of Myricaria by Liu et al. [14] with the estimated age of origin 1.46–2.30 Ma.

Closely associated with the distribution pattern of Myricaria and related taxa, the QTP and Himalayan uplift during the Neogene are hypothesized to be a major influence on organism evolution in Asia (e.g. [16]–[17]). Following collision of the Indian and Eurasian continents at ca. 50 Ma, the altitude and range of the QTP near the Oligocene-Miocene boundary became sufficient to trigger a reorganization of the Asian climate, as evidenced by the beginning of loess deposition in the Chinese Loess Plateau and the Junggar Basin [18]–[20]. Some evidence confirms that the central areas of the QTP were raised to present altitudes by that time [16], [21]–[22] and uplift of the Himalayas may have also begun at that time [22]–[23]. Uplift of peripheral portions of the plateau has continued at various intervals [24]–[28]. A major uplift of QTP is often suggested to have occurred at 8 Ma, which also coupled with global cooling, even though Molnar [29] considered this uplift evidence to be inconclusive. Uplift of the QTP and global climate cooling and aridification [27] have been suggested causes for the evolution of many organisms [30]–[36]. As these studies have shown, rapid diversification of lineages in the QTP resulted in the migration of some species into other temperate regions, such as Central Asia, the Arctic, the Mediterranean (Caucasus-Alps) and southern Asia. Of these, connections between the QTP and adjacent arid, more northern areas can often be discerned, for example in recent studies on Hippophae rhamnoides (Elaeagnaceae) [37], Caragana (Fabaceae) [38], and Astragalus (Fabaceae) [39]. Linkage of the QTP and Africa and/or the Mediterranean is illustrated by Begonia [40] and Uvaria (Annonaceae) [41]. An example linking the QTP and Southeast Asia is Paini (Anura: Dicroglossidae) [42].

The origin of Myricaria has been associated with the QTP and Himalayas but justification has been weak. Zhang & Zhang [4] presumed that Myricaria originated from the Himalayas, only based on species distribution of the genus, whereas same opinion conducted by Liu et al. [14] from a phylogeography. Here we attempt to examine the origin and evolution of this genus and link it to the Himalayan uplift to explain the causes of its evolutionary patterns. In addition, the classification and distribution of Myricaria are examined using molecular phylogeny and biogeography.

Materials and Methods

Taxa sampled

Twelve species (seventeen samples) of Myricaria plus seven species from the outgroups Tamarix and Reummuria served as sources of DNA material (Table 1). The herbaria utilized in China were as follows: HNWP (Northwest Institute of Plateau Biology, Chinese Academy of Sciences (CAS), Xining, Qinghai); SHI (Shihezhi University, Shihezhi, Xinjiang); and XJBI (Xinjiang Institute of Ecology and Geography, CAS, Urumqi, Xinjiang), as well as the LE (Komarov Botanical Institute, Russian Academy of Sciences, St. Petersburg, Russia).

Table 1. List of Sampled Taxa, Vouchers and Genebank Accession Numbers.

Species	Voucher	Source	ITS	trnL-trnF	rps16	psbB-psbH
Myricaria Desv.
M. alopecuroides Schrenk.	P. Yan 3650 (SHI)	Tashikurgan, Xinjiang, China, alt. 3650m	KJ729654	KJ729806	KJ729756	KJ729705
M. alopecuroides Schrenk. 1	Tibet-Xinjiang Exp. Team 1034 (HNWP)	Sukepiya, Yecheng, Xinjiang, China, alt. 2800m	KJ808603	KJ808634	KJ808619	KJ808649
M. bracteata Royle	Y.H. Wu 36461 (HNWP)	Nuomuhong, Dulan, Qinghai, China, alt. 2840m	KJ729655	KJ729807	KJ729757	KJ729706
M. elegans Royle 1	P. Yan 3999 (SHI)	Bandir, Tashekurgan, Xinjiang, China, alt. 3000m	KJ808604	KJ808635	KJ808620	KJ808650
M. elegans Royle 2	P. Yan 7178 (SHI)	Mazhaxi, Yecheng, Xinjiang, China, alt. 3600m	KJ808605	KJ808636	KJ808621	KJ808651
M. elegans Royle 3	P. Yan 7378 (SHI)	Ritu, Tibet, China, alt. 4600m	KJ808606	KJ808637	KJ808622	KJ808652
M. germenica (L.) Desv.	I.O. Baitulin, Aralbaiev s.n. (LE)	Zajsanskaya depression, E. Kazakistan	KJ808607	KJ808638	----	----
M. laxiflora (Franch.) P.Y. Zhang et Y.J. Zhang 1	Wuhan Bot Gard	Wuhan Bot Gard, Hubei, China	KJ808608	KJ808639	KJ808623	KJ808653
M. laxiflora (Franch.) P.Y. Zhang et Y.J. Zhang 2	Wuhan Bot Gard	Wuhan Bot Gard, Hubei, China	KJ808609	KJ808640	KJ808624	KJ808654
M. paniculata P.Y. Zhang et Y.J. Zhang	B.Z. Guo; W.Y. Wang 21930 (HNWP)	Linzhi, Tibet, China, alt. 2000m	KJ808610	----	----	KJ808655
M. platyphylla Maxim.	Z.Y. Yang; L.M. Ke 5711 (XJBI)	Houxia, Urumqi, Xinjiang, China	KJ808611	KJ808641	KJ808625	KJ808656
M. prostrata Hook.f. et Thomson ex Benth. et Hook.f.	P. Yan 7242 (SHI)	Hechakou, Hetian, Xinjiang, China, alt. 5000m	KJ808612	KJ808642	KJ808626	KJ808657
M. pulcherrima Batalin	L.M. Ke 121 (XJBI)	Ermuchang, Shaya, Xinjiang, China alt. 4350m	KJ808613	KJ808643	KJ808627	KJ808658
M. rosea W.W. Sm.	R.F. Huang G89-485 (HNWP)	Milinpaiqu, Tibet, China, alt. 4530m	KJ808614	----	KJ808628	KJ808659
M. squamosa Desv.	P. Yan 4002 (SHI)	Bandir, Tashekurgan, Xinjiang, China, alt. 3002m	KJ729658	KJ729810	KJ729760	KJ729709
M. squamosa Desv. 1	Y.H. Wu 3077 (HNWP)	Beishan, Huzhu, Qinghai, China, alt. 2700m	KJ808615	KJ808644	KJ808629	KJ808660
M. wardii C.Marquand Sun YX	R.H. Ree, S.K. Wu 30159 (LE)	Linzhi-Bomi, Tibet, China, alt. 3550m	KJ808616	KJ808645	KJ808630	KJ808661
Reaumuria Linn.
R. kaschgarica Rupr. 1	Tibet-Xinjiang Exp. Team 5166 (HNWP)	Tashekurgan, Xinjiang, China	KJ808617	KJ808646	KJ808631	KJ808662
R. kaschgarica Rupr. 2	Y.M. Duan 84-A-012 (XJBI)	Ruoqiang, Xinjiang, China, alt. 3080m	----	KJ808647	KJ808632	KJ808663
R. soongarica (Pall.) Maxim.	Tibet-Xinjiang Exp. Team 5098(SHI)	Tashekurgan, Xinjiang, China, alt. 2300m	KJ808618	KJ808648	KJ808633	KJ808664
Tamarix L.
T. karakalensis Freyn	K.B. Blinkovsky 12 VIII 1953 (LE)	C. Kopetdag, Ashkhabad, Turcominia	KJ729659	KJ729811	KJ729761	KJ729710
T. laxa Willd.	O.N. Demina 18 V 2001 (LE)	Orlovsky, Bostov, Russia	KJ729660	----	KJ729762	KJ729711
T. meyeri Boiss.	M.R. Tanybaeva 12 V 2007 (LE)	Turkestan Ridge, Kirgiztan	KJ729661	KJ729812	KJ729763	KJ729712
T. ramosissima Ledeb.	N.A. Brykova s.n. 10 VII 1998 (LE)	Orlovsky, Bostov, Russia	KJ729662	KJ729813	KJ729764	KJ729713

Herbaria: HNWP (Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, Qinghai, China); SHI (Shihezi University, Shihezi, Xinjiang, China), XJBI (Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang, China); LE (Komarov Botanical Institute, Russian Academy of Sceinces, Russia, St. Petersburg, Russia).

DNA sequencing

Total genomic DNA was extracted using the CTAB method [43]. The polymerase chain reaction (PCR) was used for amplification of double stranded DNA. The 25 µl reaction system contained 0.25 µl of Ex Taq, 2.5 µl of 10× Ex Taq buffer (Mg²⁺ concentration of 25 mM), 2.0 µl of dNTP mix (2.5 mM concentration for each dNTP), 1 µl of the forward and reverse primers at 5 umol/µl, and 0.5 µl of template DNA. The following primers were used: for ITS: ITS1-f (5′-AGA AGT CGT AAC AAG GTT TCC GTA GC-3′) and ITS4-r (5′-TCC TCC GCT TAT TGA TAT GC-3′), for trnL-trnF: trnL-f (5′-CGA AAT CGG TAG ACG CTA CG-3′) and trnF-r (5′-ATT TGA ACT GGT GAC ACG AG -3′), for the intron of rps16: rps16-f (5′-GTG GTA GAA AGC AAC GTG CGA CTT-3′), and for rps16-r (5′-TCG GGA TCG AAC ATC AAT TGC AAC-3′) [44]; and the intergenic spacer psbB-psbH: psbB-psbH-r (5′-TTCAACAGTTTGTGTAGCCA-3′) and psbB-psbH-f (5′-AGATGTTTTTGCTGGTATTGA-3′) [45].

The protocol for amplification consisted of an initial hotstart at 95°C for 2 min, followed by 30 cycles of denaturation at 94°C for 30 s, annealing at 52°C for 30 s, extension at 72°C for 90 s, and a final extension at 72°C for 10 min. PCR products were purified using the PEG precipitation procedure [46]. These were sequenced using an ABI Prism 3770 Genetic Analyzer (Shanghai Sanggon Biological Engineering Technology & Service, Shanghai, China). Sequences were aligned using CLUSTAL X software [47], and then adjusted by hand. All gaps were treated as missing characters. Finally, a combined dataset consisting of four sequences of ITS and three cpDNA trnL-trnF, rps16, and psbB-psbH, was prepared for phylogenetic analysis.

Phylogenetic analysis and divergence time estimate

The sequence dataset from twelve species (seventeen samples) of Myricaria plus seven species of Tamarix and Reummuria yielded 3202 aligned nucleotide characters from four genes: ITS, trnL-trnF, rps16, and psbB-psbH. The incongruence length difference (ILD) test of the four gene datasets was carried out using PAUP* [48], to assess potential conflicts between the different DNA fragments. This test was implemented with 100 partition-homogeneity test replicates, using a heuristic search option with simple addition of taxa, TBR branch swapping and MaxTrees set to 1000. 0.222 of incongruence length difference (ILD) tests [48] showed that the four gene datasets were not incongruent.

Estimation of phylogenetic relationships and divergence time was conducted using a Bayesian method implemented in BEAST 1.5.4, employing a relaxed clock model [49]–[50]. We used the uncorrelated lognormal molecular clock model with a Yule process for the speciation model, GTR+I+G for the substitution model (estimated for the dataset), and a normal distribution with SD of 1 as priors on the calibration nodes to accommodate for calibration uncertainty. Minimum ages for the two normal priors were constrained to the root of all taxa and a node respectively, the family Tamaricaceae 70 Ma, and genus Tamarix 25 Ma, with a detailed description as follows. A Markov chain Monte Carlo analysis was run for 50 million generations and sampled every 1,000 generations, and two independent runs were performed to confirm the convergence of the analysis. The stationarity of each run was examined using the effective sampling size of each parameter (>200). The last 40 million generations were used to construct the maximum clade credibility tree and the associated 95% highest posterior density distributions around the estimated node ages.

Optimization of ancestral distributions

Tamaricaceae root constrained

Tamaricaceae is included in the order Caryophyllales [2], [51] and has no reliable macrofossil record. According to molecular dating [52]–[54], the divergence time of the order has been estimated as ca. 100 Ma. Tiffney [55] considered that the extant woody families originated during the Cretaceous to early Eocene, while herbaceous families appeared during the late Oligocene to Miocene. For instance, the woody families Ulmaceae and Fabaceae appeared at about 70 Ma [54], [56] and 70–60 (-50) Ma [54], [56], [57] respectively. Families related to the Tamaricaceae, such as Polygonaceae and Amaranthaceae/Chenopodiaceae, have an approximate age of ca. 65 Ma [58]. The two subfamilies of the Caryophyllaceae have an approximate age of 40–55 Ma according to the age of the inflorescence fossil Caryophylloflora paleogenica, and the family has an possible age of ca. 73 (60–80) Ma [59]–[60]. Even though the ancestor of the related families Tamaricaceae and Frankeniaceae has been dated to 43–30 Ma [56], Tamaricaceae itself has had variable dating results. Bell et al. dated it to (72-) 60–58 (-44) [54], while Wikstrom et al. placed it at 52–37 Ma [56], and Schuster et al. at (125-) 118.7–110 (-90.7) [61]. In the light of these estimates, a balanced age for Tamaricaeae could be suggested as about 70 Ma; this estimate was chosen as the family root for molecular dating.

The earliest reliable fossil record of Tamarix is Miocene, from the Yunnan province of China [62]. Most of the available fossils are from the Miocene, therefore, the genus might be hypothesized to have had an origin at least in early Miocene. However, considering its wide distribution in Europe, Africa, Asia, and North America, and our limited samples mainly from China, we conservatively assigned an age of late Oligocene-early Miocene, at ca. 25 Ma for Tamarix.

Areas

In accordance with the distribution of Myricaria species along the Asian mountains (Figure 1), we divided the distribution into six areas, namely, A: the eastern Himalayas, including the eastern QTP, the Hengduan mountains, and northern and central China; B: the western Himalayas, including the western QTP and the Pamir-Alai, Kunlun-Altun, and Hendukosh mountains; C: the Tianshan mountains and Junggar-Turan deserts; D: Altai-Siberia; E: the Mongolian Plateau; and F: Asia Minor-Caucasus-Europe. These six areas are distinct in biodiversity, vegetation, and floristics [16]–[18], [30], [32], [36]–[38].

Optimization of ancestral distributions

To infer biogeographical events, three methods were used: a parsimony-based procedure Diva [63], S-Diva [64] and a maximum likelihood-based DEC model (Lagrange; [65]–[66]). These three approaches are simultaneously considered so that to assess the relevant biogeographical processes, such as vicariance, dispersal, and extinction.

Diva

Dispersal–vicariance analysis optimizes distributions for each node of the tree by minimizing the number of assumed dispersals and extinctions, and favors vicariance events [63], [67]. The Diva program reconstructs widespread ancestral distributions, restricting them to single areas. Because allopatric speciation by vicariance is the null model in Diva, vicariance and range division would always be the preferred explanation if the ancestors were widespread. To avoid inferring a widespread ancestor at the root because of the presence of widespread extant taxa, a limit of two areas was set (maxareas  =  2) in Diva [63]. The phylogenetic typology of the BEAST tree (Figure 2) was input for Diva analysis.

Figure 2. Chronogram of Myricaria and outgroups Tamarix and Reaumuria in Tamariaceae, with maximum clade credibility performed by BEAST.

Dating values are plotted at the right of the nodes, and posterior probability support of more than 95% is labeled as “*” at the nodes. Two vertical lines are labeled at 20 Ma and 8 Ma, corresponding respectively to two stages and two high-altitude ranges (blue ranges in A, B) of the QTP uplift, Himalayan motion, and rapid and major-range uplift. Four sections within Myricaria are shown, along with flowers and degree of union of the filaments. In section Alpinae, flower and filament status refers to M. rosea, and in section Renantherae, the flower and filaments, above right, refer to M. bracteata. The filaments below left refer to M. germanica and those below right refer to M. squamosa.

S-Diva

(or Bayes-Diva) [64] is a program which complements Diva and implements the methods of Nylander et al. [68] and Harris et al. [21], determining statistical support for ancestral range reconstructions using multiple trees from Bayesian analysis. This has the advantage that uncertainties in phylogenetic inference can be taken into account. One hundred Bayesian MCMC trees with the last stable typologies from BEAST, and a BEAST tree typology (Figure 2) were input into the S-Diva program.

Lagrange

A valuable new biogeographical method is parametric likelihood analysis, with a dispersal–extinction–cladogenesis model [67], as implemented in Lagrange v. 2.0.1 [65]. This method calculates the likelihood of biogeographical routes and areas occupied by the most recent common ancestor (MRCA) for a given phylogenetic tree topology (BEAST tree, Figure 2) and the present distribution of taxa. Therefore, dispersal and vicariance of lineages, represented by connection areas, can be estimated by the probabilities. This is thus a form of MRCA area reconstruction differing from the parsimony approach of Diva.

Results

Phylogenetic analysis and divergence time estimate

The phylogenetic tree obtained from Bayesian inference in BEAST showed that Myricaria is monophyletic and Myrtama should be included in Myricaria rather than treated as a distinct genus (Figure 2). Within Myricaria, four clades were recognized, two corresponding to the existing sections Parallelantherae Ndz. and Renantherae Ndz, the other two represent new groups to be named as sections Alpinae and Laxiflorae (see Appendix S1). Flowers and filaments of the plants are illustrated in Figure 2, to show the characteristics of the four sections. In the present phylogenetic tree, the clades of the genus and the sections have strong support, confirming the validity of taxa at the ranks of genus and section. Section Renantherae comprises the most species in the genus, and has two subclades. The two widely distributed species of this section, M. alopecuroides and M. squamosa, are located in each subclade. The crown age of Myricaria was ca. 20 Ma, and 8.83∼6.35 Ma for four sections.

Optimization of ancestral distributions

The results of the three approaches Diva, S-Diva, and Lagrange (Figure 3) showed a consistent and strongly supported pattern, particularly at the ancestral nodes for Myricaria (AB), and among the four sections, Parallelantherae (B), Alpinae (A), Laxiflorae (A), and Renantherae with AB from Diva and S-Diva, whereas only with ABCDEF/B from Lagrange. On the whole, AB, A, and B, namely the Himalayas and the QTP, should be considered as ancestral areas in Myricaria. The events occurring in areas C, D, E, and F, were considered to be dispersals, several of which can be distinguished. M. prostrata occurs in the Himalayas, and its western Himalayan distribution was indicated to be a dispersal event from the eastern Himalayas. M. bracteata in sand lands of the Mongolian Plateau was shown to be a migrant from the eastern Himalayas, Hengduan Mountains, and Northern China. Whereas the distribution of M. germanica in Asia Minor-Caucasus-Europe was came from the western Himalayas.

Figure 3. Biogeographical ancestral optimization performed with Diva, S-Diva and Lagrange.

Pie charts at the internal nodes represent the calculated probabilities (relative frequencies) of alternative ancestral areas (reconstructions), and were produced by S-Diva; letters are labeled at the right and above nodes, and probabilities from S-Diva are at the right and below nodes; letters above and below at branches are from Lagrange, indicating the highest probability migration routes and inheritance of area by upper and lower descendant branches, respectively. Located at the front of M. prostrata, M. bracteata, and M. germanica, vertical lines on the branches indicate dispersals involving these three species. Area letters as stated in the text: A: eastern Himalayas, including the eastern QTP, Hengduan mountains, northern China, and central China of East Asia; B: western Himalayas, including the western QTP, and Pamir-Alai, Kunlun-Altun, and Hendukosh mountains; C: Tianshan-Tungger-Turan; D: Altai-Siberia; E: Mongolian Plateau; and F: Asia Minor-Caucasus-Europe.

Discussion

Phylogenetic division of sections within Myricaria

Niedenzu [8] divided Myricaria taxa into two sections: Parallelantherae Ndz. and Renantherae Ndz. Gorschakova [7] accepted this classification system in the Flora of the USSR. Zhang & Zhang [4], however, considered that the establishment of infrageneric ranks was not appropriate due to its complicated and variable morphological characters. Therefore, in the Flora of China [5]–[6] there is no division of infrageneric sections.

However, our phylogenetic tree (Figure 2) yielded a clear phylogenetic division of four sections, including two that are new. Of them the Himalayan and QTP section Alpinae, containing M. prostrata, M. rosea, and M. wardii, is characterized by the prostrate and recumbent, and with an adaptation to high altitudes of 3000–5300 m. Section Laxiflorae, comprises of only species M. laxiflora, endemic to the subtropical area of the Sichuan and Hubei provinces in eastern China. Detailed descriptions of two new sections are given in Appendix S1.

Myricaria elegans Royle was originally described from the Kunawar region of the western Himalayas [69]. Based on this species and its characters of 10 stamens, flat leaves, and no obvious style, the genus Myrtama was established [9]. Qaiser & Ali [70] named it Tamaricaria, whereas Baum [71] moved Myricaria elegans to Tamarix as T. ladachensis. As mentioned, Zhang et al. [10] and Gaskin et al. [1] accepted Myrtama at generic rank. Zhang et al. [10] considered that Myricaria elegans was an intermediate and hybrid genus between Myricaria and Tamarix, related more to Tamarix. However, Hua et al. [12] and Wang et al. [13], based on sequence data, found that it would be appropriately placed in Myricaria. Our results (Figure 1) also show that inclusion of Myricaria elegans in Myricaria is suitable, since the whole of Myricaria, including Myricaria elegans, has strong support (100%) (Figure 2). This is in accordance with evidence from morphological classification [4] and molecular phylogeny [12]–[14]. The former conclusion supporting retention of Myrtama [10] was only based on ITS sequence data, which is probably not sufficient evidence [72]. While in the phylogeny of Gaskin et al. [1] (their Figure 2), only four species were sampled, and four species as clade had a strong bootstrap support (99%) for the inclusion in Myricaria [1].

Himalayan origin, ancestral inheritance, and multi-diversification in the Himalayas

Our estimated crown age of ca. 20 Ma for Myricaria (Figure 2) falls into the probable early range of the Himalayan uplift in early Miocene [22], consequently allows us to speculate that uplift of this mountain range caused the origin of Myricaria. The biogeographical analytical result of Diva, S-Diva and Lagrange, showing the combined Himalayan area AB as the ancestral area for Myricaria (see Figure 3), which also supports a Himalayan origin. The present molecular dating results are in contrast to previous phylogeographical opinion of a main divergence event at the implausible age of 1.46–2.30 Ma in the Plio-Pleistocene [14].

The southern and northern slopes of the Himalayas differ dramatically in temperature and precipitation. Myricaria species occurring on the northern slope are generally xeric, same as those of the main plateau [73]. The western portions of the Himalayas and adjacent QTP are more arid than the eastern parts [27], [73], see Figure 1c. Probably these differences are the cause of the persistent diversification between the eastern (A) and western Himalayas (B) for the Myricaria lineages (Figures 1 and 3). The eastern A contains the alpine species M. rosea, and M. wardii, and the western B includes M. elegans and M. prostrata. These four species are endemic to the Himalayas, and occupy two of the four Myricaria sections. In particular, the western Himalayas (B) is an important geographical node and dispersal center for Myricaria, with movement toward the Pamir-Alai, Hendukosh, Tianshan, and Kunlun-Altun mountains, etc. Noticeable, the Himalayas as a union was divided into A and B two times (see Figure 3), once at the time of generic origin and diversification, and the other at the diversification node of the sections Renantherae and Laxiflorae. Overall, the Himalayan areas AB, A, and B as the ancestors occurred at least seven times at the nodes in Figure 3. In detail, the Himalayan union AB as an ancestral area appeared at two nodes, with estimated ages of 20.25 Ma (genus crown age) and 10.07 Ma, the eastern Himalayas A at three nodes with ages of 19.84 Ma, 6.83 Ma (section Alpinae crown age), and 6.35 Ma (section Laxiflorae crown age), and the western Himalayas B occupied two, with ages of 8.83 Ma (section Parallelantherae crown age) and 5.81 Ma. All of these stressed the diversification and geographical heritage of Himalayan ancestry.

Many plant groups of the Northern Temperate Zone are hypothesized to have originated from the QTP [16], [30]–[36], especially from the Hengduan Mountains; few have originated from the Himalayas. Myricaria presents a case of Himalayan montane origin.

In some cases, the Himalayas are regarded as a corridor of plant species migration linking East Asia and Central Asia and/or the Mediterranean [16], [35], [74]–[77]. The Himalayas have acted as a migration corridor for Sino-Japanese elements westward and Mediterranean elements eastward, such as Hippophae rhamnoides (Elaeagnaceae) [37], Pogonophace (Astragalus, Fabaceae) [72], and Phyllolobium [77] (Fabaceae). They also served as a migration route westwards to Central Asia from East Asia for Caragana (Fabaceae) [78], Begonia migration was through the Himalayas eastwards from Africa [40]. Our Myricaria analysis shows endemism, origin, and remarkable multiple diversifications within the Himalayas, but with weak or absent migration here. The Himalayas therefore seems to serve as the center of origin for Myricaria rather than as a corridor. This is rather like as these QTP endemic plant groups, such as Nannoglottis (Asteraceae) [79], Ligularia complex (Asteraceae) [80], Saussurea (Asteraceae) [81], Dolomiaea, Diplazoptilon and Xanthopappus (Asteraceae) [82], and Aconitum (Ranunculaceae) [83], all of them are hypothesized to have originated from native QTP during Miocene-Pliocene.

Diversification of Myricaria sections

Our estimated crown ages for the four sections of Myricaria were about 8.83∼6.35 Ma (Figure 2), this is a remarkable molecular clock response to the rapid and major-range uplift of QTP at ca. 8 Ma. and the diversifications of four sections all occurred in the Himalayas (Figure 3): sections Alpinae and Laxiflorae in the eastern Himalayas (A), section Parallelanthera in western Himalayas (B), and section Renantherae in Himalayan union (AB). We have drawn the distinct morphological characters of four sections in Figure 2. This could be regarded as the response of four sections diversifications to the uplift in Himalayas.

The time of Myricaria origin and diversification related to Himalayan uplift in early Miocene, and the subsequent sectional diversifications at about 8.83∼6.35 Ma, correspond respectively to the two labels of the QTP uplift, Himalayan motion, and rapid and major-range uplift [21]–[29]. These diversification times of the genus and sections are very similar to those of the genus Caragana [38], which also has a 16 Ma time of generic origin and diversification ages of sections at about 8 Ma. However, here we provide the Himalayan places of origin and diversification for Myricaria, while those of Caragana remain unknown. The evolution of Myricaria is also temporally similar to that of the Asian spiny frogs Paini [42]. Uplift of the Himalayas, is hypothesized to be most likely due to a cut off of the genetic exchange at ca. 19 Ma, resulting in the splitting of the subgenera Nanorana and Paa of Paini began in the Miocene near 10 Ma [42].

Migration along Asian mountains

For plant migration and dispersal, mountains generally act as a route or corridor [84], such as the Himalayan corridor mentioned above. Whereas the Himalayas for Myricaria are regarded as the center of origin, other distributions outside of the Himalayas and QTP can be understood as dispersals or migrations eastward, westward and northward along Asian mountains (see Figure 1). In fact, the results of vicariance and dispersal from biogeographical analysis (Figure 3) show that except for divergence of phylogenetically basal clades located in the eastern and western Himalayas (A and B), most remaining events, occurring in areas such as the Tianshan-Jungger-Turan (C), Altai-Sibiria (D), Mongolian Plateau (E), and Asia Minor-Caucasus-Europe (F) resulted from dispersal events. As evidenced from Myricaria (see Figure 1), dispersal and migration were possible from the Himalayas to Asia Minor-Caucasus-Europe as shown by M. germanica, and to the sand lands of the Mongolian Plateau by M. bracteata. These distributions along the Asian mountains are very similar to those of Hippophae rhamnoides (Elaeagnaceae) [37], a species with another interesting distribution in North Temperate Eurasia. Hippophae rhamnoides includes nine subspecies, and has been shown to have originated from the QTP, or more exactly, the eastern QTP-Hengduan Mountains, and then to have radiated and dispersed in different directions. Here, Myricaria originated in the Himalayas union, not in the eastern Himalayas only as H. rhamnoides. However, the northwestern Himalayas for H. rhamnoides and Myricaria played an important node role in connecting with Central Asia and Europe, and both dispersal route and direction is very similar.

Supporting Information

Appendix S1

Two new sections within Myricaria.

(DOC)

Funding Statement

This study was funded by China National Key Basic Research Program (No. 2012FY111500, 2014CB954201), CAS Important Direction for Knowledge Innovation Project (No. KZCX2-EW-305), and the Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.