Delimitations of species are crucial for correct and precise identification of taxa. Unfortunately “species” is more a subjective than an objective concept in taxonomic practice due to difficulties in revealing patterns of infra- or inter-specific variations. Molecular phylogenetic studies at the population level solve this problem and lay a sound foundation for DNA barcoding. In this paper we exemplify the necessity of adopting a phylogenetic concept of species in DNA barcoding for tree peonies (Paeonia sect. Moutan). We used 40 samples representing all known populations of rare and endangered species and several populations of widely distributed tree peonies. All currently recognized species and major variants have been included in this study. Four chloroplast gene fragments, i.e. ndhF, rps16-trnQ, trnL-F and trnS-G (a total of 5040 characters, 96 variable and 69 parsimony-informative characters) and one variable and single-copy nuclear GPAT gene fragment (2093–2197 bp, 279 variable and 148 parsimony-informative characters) were used to construct phylogenetic relationships among the taxa. The evolutionary lineages revealed by the nuclear gene and the chloroplast genes are inconsistent with the current circumscriptions of P. decomposita, P. jishanensis, P. qiui, and P. rockii based on morphology. The inconsistencies come from (1) significant chloroplast gene divergence but little nuclear GPAT gene divergence among population systems of P. decomposita + P. rockii, and (2) well-diverged nuclear GPAT gene but little chloroplast gene divergence between P. jishanensis and P. qiui. The incongruence of the phylogenies based on the chloroplast genes and the nuclear GPAT gene is probably due to the chloroplast capture event in evolutionary history, as no reproductive barriers exist to prevent inter-specific hybridization. We also evaluated the suitability of these genes for use as DNA barcodes for tree peonies. The variability of chloroplast genes among well-defined species or population systems of a species complex is 4.82 times the figure within the groups, and the GPAT gene is twice as variable between the groups as within the groups. The number of completely divergent sites is sufficient to mark the two subsections, the two species in subsection Delavayanae, and the well-divergent species in subsection Vaginatae. But the genes currently used either from the chloroplast genome or from the nuclear genome alone cannot correctly assign samples of P. decomposita, P. jishanensis, P. qiui, or P. rockii to the species as currently defined. We conclude that (1) DNA barcoding should be based on prior phylogenetic studies to understand the evolutionary lineages and how well the taxonomic species correspond to the lineages; (2) it is unlikely to find a single short fragment as a barcode for every plant and such a fragment could result in misidentification when a chloroplast capture event happened in the evolutionary history of plants like tree peonies; and (3) we suggest striving for a universal marker at the familial level and locally universal barcodes within a family instead of looking for a universal barcode for all plants.

DNA barcoding: species delimitation in tree peonies

The majority of tetraploid peonies are allopolyploids derived from crosses between phylogenetically distinct diploid lineages Tetraploid Paeonia obovata was previously considered to be an autopolyploid because it is morphologically indistinguishable from the diploid of the same species The presence of the Adh2 gene in tetraploid P obovata but the inability to amplify the Adh2 gene from Chinese diploids of P obovata, however, suggests that the tetraploid was not an autotetraploid derivative of the geographically adjacent diploid populations in China The Adh gene phylogenies rather suggest that the tetraploid originated from crosses between two geographical races of diploid P obovata distributed in China and Japan The intermediate status of tetraploid P obovata between auto- and allopolyploidy highlights the need for population genetic analyses of polyploid origins along the continuous range of genomic divergence Here we present a model that describes the probabilities of polyploid formation and establishment as a function of genomic divergence between diploid progenitors The probability of polyploid formation (P f ) is obtained from the multiplication of the probability of production of unreduced gametes (Pg) and the probability of 'hybridization' (P h ) P f stays relatively stable when the genomic divergence is low, and then decreases progressively rapidly with the increase of genomic divergence between diploid progenitors The probability of polyploid establishment (P e ), which depends on the rate of appearance of stable beneficial gene combinations and the rate of fertility restoration, is positively correlated with the genomic divergence of diploid parents Multiplication of P f and P e gives an overall probability of polyploid origins (P o ) that varies continuously along the genomic divergence between diploid progenitors

Origins of polyploids: an example from peonies ( Paeonia ) and a model for angiosperms

Tree peony, being crowned the title "King of Flowers" in China, is of great medicinal, ornamental, and economic values. In the present study, the phylogeny of the wild tree peony species (section Moutan, Paeonia, Paeoniaceae), represented by twelve accessions collected from all eight species in the section, was investigated based on the DNA sequence in five DNA fragments from both nuclear (Adh1A, Adh2 and GPAT) and chloroplast (trnS-trnG and rps16-trnQ) genomes, as well as morphological characters. Both maximum parsimony (MP) and Bayesian inference of phylogeny (BI) trees were reconstructed based on the combined data of the DNA sequences and morphological data, respectively. The MP and BI trees have the similar topology, and the sect. Moutan clearly branched into two clades. One clade consists of two species, P. delavayi and P. ludlowii, corresponding to the subsect. Delavayanae, and another clade is composed of other six species. Within the second clade, the six species can be divided into three subclades consisting of P. rockii and P. decomposita, P. jishanensis and P. qiui, P. suffruticosa and P. ostii, respectively. Among the three subclades, P. jishanensis/P. qiui is most closely related to P. suffruticosa/P. ostii. These results provide up to date the clearest picture of the phylogeny of wild tree peony species in the sect. Moutan.

Phylogenetic analysis of Paeonia sect. Moutan (Paeoniaceae) based on multiple DNA fragments and morphological data

In this paper we argue for the specific status of Paeonia suffruticosa Andrews, for the rationality of P. suffruticosa ssp. yinpingmudan D. Y. Hong, K. Y. Pan & Z. W. Xie and its wildness, and for the treatment of P. suffruticosa ssp. atava (Bruhl) S. G. Haw & Lauener as a subspecies of P. rockii, P. rockii ssp. atava (Bruhl) D. Y. Hong & K. Y. Pan. P. jishanensis T. Hong & W. Z. Zhao is justified as a legitimate name, while P. spontanea (Rehder) T. Hong & W. Z. Zhao is recognized as a superfluous name. Halda's four combinations and two new hybrid names in sect. Moutan DC. were treated as synonymy. As a result, one new combination is made and five new synonyms are proposed here in this paper. Key words Paeonia, Paeonia suffruticosa, Paeonia suffruticosa ssp. yinpingmudan, Paeonia rockii ssp. atava, new combination, new synonymy.

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Notes on taxonomy of Paeonia sect. Moutan DC. (Paeoniaceae)

Tree peonies belong to Paeonia L. section Moutan DC., Paeoniaceae. All wild tree peonies are endemic to China, which is also the first domestication place of cultivated tree peonies. A new taxonomic system of Sect. Moutan was proposed by Hong and Pan (Hong D.Y. and Pan K.Y. 1999a. Acta Phytotax. Sin. 37: 351–368; 1999b). In their system eight species, of which three each has two subspecies are recognized. Wild tree peonies are distributed in Henan, Gansu, Shaanxi, Shanxi, Anhui, Hubei, Sichuan, Yunnan and Xizang (Tibet) of China. The most densely populated area is Henan province, where five wild species (subspecies) are found and supposed to be related to the origin of cultivated tree peonies. Since 1990 considerable work on taxonomy of tree peonies has been done by Chinese botanists, which is summarized in this paper. In addition, the nomenclature history of the tree peony species listed in Hong and Pan's (Hong D.Y. and Pan K.Y. 1999a. Acta Phytotax. Sin. 37: 351–368; 1999b) system is reviewed and the information on their geographical distribution and ecological habitats is presented.

Taxonomy, Geographic Distribution and Ecological Habitats of Tree Peonies

Paeonia decomposita was described in 1939 by Handel-Mazzetti on the basis of the specimen H. Smith 4641 from Chosodjo, N.W. Sichuan, China. Ster (1946: 142) considered it conspecific with P. suffruticosa Andr., stating: "I have seen a photograph of the type specimen (H. Smith, No. 4641) of P. decomposita ..., and if this photograph is compared with Rock's specimen of P. suffruticosa in the Kew Herbarium they will be found to be alike". Rock's specimen of P. suffruticosa was since described as a new subspecies, P. suffruticosa subsp. rockii S. W. Haw & Lauener (1990), which has been raised to specific level, P. rockii (S. G. Haw & Lauener) T. Hong & J. J. Li (in T. Hong & al., 1992). In his revision of Chinese peonies, Fang (1958: 302, 314-315) stated, under P. suffruticosa, that P. decomposita was an imperfectly known species, which he treated as a doubtful synonym of P. suffruticosa in the English abstract. In the same paper he described a new woody species, P. szechuanica Fang, also from N.W. Sichuan. Pan (1979: 41) treated P. decomposita as a straightforward synonym of P. suffruticosa and recognized P. szechuanica as a correct name. H.-Y. Hong (in Fu 1992: 536-537) included P. szechuanica as an endangered species. Osti (1994) also used the name P. szechuanica, ignoring P. decomposita. Therefore, P. decomposita has not again been recognized as a correct name since its publication.

Pan Kai-yu

Papers

The identity of Paeonia decomposita Hand.-Mazz.