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Systematics |
Department of Botany, The Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, Illinois 60605-2496 USA; Missouri Botanical Garden, P. O. Box 299, St. Louis, Missouri 63166-0299 USA; and Laboratoire de Phanérogamie, Muséum National d'Histoire Naturelle, 16 rue Buffon, 75005 Paris, France
Received for publication May 31, 2001. Accepted for publication January 3, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Apiaceae • chloroplast DNA • ITS • ndhF, trnL-trnF noncoding region • Osmorhiza • phylogeny
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Osmorhiza was first recognized as a distinct genus by Rafinesque in 1818, although species of the genus were referred to other genera by previous researchers (e.g., Thunberg, 1784
; Michaux, 1803
; Persoon, 1805
; Sprengel, 1813
). Constance and Shan (1948)
conducted the first taxonomic study of the genus on a worldwide basis and proposed an infrageneric classification, which was adopted by Lowry and Jones (1984)
with a few modifications. They recognized two subgenera (Glycosma and Osmorhiza), the latter subdivided into three sections (Osmorhiza, Nudae, and Mexicanae, each with three species), which are characterized by the morphology of involucres, bractlets, styles, and fruits (Lowry and Jones, 1984
).
Osmorhiza shows several interesting biogeographic disjunctions: (1) members of section Osmorhiza demonstrate the classical eastern Asian–eastern North American disjunction; (2) two species (O. berteroi and O. depauperata) show an antitropical (often referred to as amphitropical) disjunction between temperate North and South America; and (3) O. berteroi and O. depauperata are also disjunct between eastern North America, the Great Lakes region, and western North America. The biogeographic diversification of Osmorhiza was recently evaluated using ITS sequences of the nuclear ribosomal DNA (Wen et al., 2002
), revealing the relative antiquity of the eastern Asian–eastern North American disjunction and the recent origin of both the antitropical as well as eastern and western North American disjunctions in Osmorhiza. The ITS phylogeny also suggests a relatively rapid diversification of the western North American taxa, even though they show a high level of morphological diversity.
The monophyly of Osmorhiza is well supported in a series of phylogenetic studies using ITS sequence data of Apiaceae tribe Scandiceae (Downie, Katz-Downie, and Spalik, 2000
). Wen et al. (2002)
clarified inter- and intraspecific relationships using a sample of 46 populations representing all ten species of Osmorhiza throughout its distribution based on ITS sequence data. This study suggested several relationships: (1) O. aristata from Asia occupies a basal position within the genus; (2) the nine New World species form a clade; (3) populations of O. berteroi from South America, western North America, eastern North America, and the Great Lakes region form a monophyletic group; (4) O. claytonii and O. longistylis from eastern North America form a clade; (5) O. brachypoda and O. purpurea are allied with a clade comprising O. depauperata and O. occidentalis; (6) several species (O. berteroi, O. brachypoda, O. depauperata, O. mexicana, O. mexicana subsp. bipatriata, O. occidentalis, and O. purpurea) form a largely western North American clade; and (7) O. glabrata from central Andes forms a trichotomy with the eastern North American clade and the western North American clade.
The objectives of the present study are to (1) assess the phylogeny for Osmorhiza using the chloroplast ndhF gene and the trnL-trnF noncoding region (trnL intron, and trnL [UAA] 3' exon-trnF [GAA] intergenic spacer); (2) compare the cpDNA phylogeny with the results of the previous ITS study; and (3) test the infrageneric classification of Lowry and Jones (1984)
within a molecular phylogenetic framework. We selected the ndhF gene and the trnL-trnF noncoding regions of chloroplast DNA because they display relatively fast nucleotide substitution rates and have been employed successfully in many other studies at the interspecific level (e.g., Gielly and Taberlet, 1994
, 1996
; Kita, Ueda, and Kadata, 1995
; Gielly et al., 1996
; Bakker et al., 2000
; Potter, Luby, and Harrison, 2000
). Also, a comparison of phylogenetic inferences based on chloroplast and nuclear markers may provide important insights into relationships and patterns of evolution within Osmorhiza. For example, at lower taxonomic levels (genus and below) such comparisons have revealed high levels of concordance in several studies (e.g., Baldwin, 1992
; Kim and Jansen, 1994
; Bayer, Soltis, and Soltis, 1996
; Bayer, Puttock, and Kelchner, 2000
; Choi and Wen, 2000
), but in a few cases, significant discordance has been found (e.g., Soltis and Kuzoff, 1995
; Soltis, Johnson, and Looney, 1996
). Such discordance may suggest hybridization or introgression (Soltis and Kuzoff, 1995
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), were selected as the outgroup.
Total DNA was extracted with the cetyltrimethylammonium bromide (CTAB) method of Doyle and Doyle (1987)
and purified over CsCl/ethidium bromide gradients. DNA amplifications were performed in 100-µL reactions containing approximately 50 ng genomic DNA, 20 nmol/L Tris buffer (pH 8.3) with 50 mmol/L KCl, 1.5 mmol/L MgCl2, and 0.1% Tween 20 (buffer designed by C. Bult), 0.15 mmol/L of each dNTP, 1 µmol/L of each primer, 5 units of Taq polymerase (Promega, Madison, Wisconsin, USA), and 5% DMSO (dimethyl sulfoxide). The primers to amplify ndhF and the trnL-trnF noncoding regions were those used in Olmstead and Sweere (1994)
and Taberlet et al. (1991)
, respectively. Double-stranded polymerase chain reaction (PCR) products were produced via 45 cycles of denaturation (94°C for 1 min), annealing (50°C for 2 min), and extension (72°C for 2 min). A 5-min final extension cycle at 72°C followed the 45th cycle to ensure the completion of novel strands. The PCR products were purified using Wizard PCR Preps DNA Purification System (Promega) prior to sequencing. The ndhF gene and the two noncoding regions were sequenced following the dideoxy chain termination method (Sanger, Nicklen, and Coulsen, 1977
) using the Sequenase Version 2.0 DNA Sequencing Kit (cat. no. US70770, Amersham, Cleveland, Ohio, USA) and alpha 33P-dATP as a radioactive tracer. Most mutations were base substitutions, thus allowing manual alignment.
Phylogenetic analysis was performed with PAUP* (version 4.02b, Swofford, 1999
) using maximum parsimony (Swofford et al., 1996
) and maximum likelihood (Felsenstein, 1981
) methods. Parsimony analyses were performed using a branch-and-bound search with MULPARS and furthest addition sequence options. The amount of support for monophyletic groups revealed in the maximally parsimonious tree(s) [MPT(s)] was examined with 500 bootstrap replicates (Felsenstein, 1985
) with the random addition and the heuristic search options using parsimony. The proportions of site differences were estimated using the Kimura two-parameter distance (Kimura, 1980
).
A partition homogeneity test (Farris et al., 1995
) was conducted with PAUP* (Swofford, 1999
) to determine the congruence of the chloroplast and nuclear data sets. The test was performed with 100 replicates, using an heuristic search option with simple addition, tree bisection-reconnection (TBR) branch-swapping, and gaps as missing data.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Several relationships are suggested by the parsimony analyses (Fig. 1): (1) the two populations of O. aristata from Asia form a basally branching monophyletic group; (2) the nine New World species form a monophyletic group (bootstrap value = 51%); (3) two subclades can be recognized among the New World taxa, O. claytonii + O. purpurea (bootstrap value = 58%) and O. depauperata + O. mexicana subsp. bipatriata (bootstrap value = 83%). Treating gaps as missing data, the Kimura two-parameter distance among species of Osmorhiza was estimated to be 0.00–0.464%. The highest divergence was between O. mexicana subsp. bipatriata from southwestern North America and O. claytonii from eastern North America; the lowest value was between O. purpurea from western North America and O. claytonii from eastern North America. Osmorhiza berteroi, sampled from five populations in South America and throughout North America, had a sequence divergence of only 0.00–0.056%.
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Several relationships are suggested by parsimony analyses of the trnL-trnF data (Fig. 2): (1) O. aristata from Asia is basal within Osmorhiza; (2) the nine New World species form a monophyletic group (bootstrap = 52%); (3) one accession of O. berteroi from Chile (Stuessy et al. 15556) and O. glabrata (bootstrap = 92%) form a subclade; (4) O. dapauperata and O. mexicana subsp. bipatriata form a subclade (bootstrap = 64%); and (5) O. occidentalis is relatively divergent from the other New World taxa, and multiple accessions of this species form a well-supported monophyletic group (bootstrap = 97%).
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Data incongruence
Assessment of congruence among trees using the partition-homogeneity test of PAUP* showed that the ITS and the cpDNA data sets were incongruent (P = 0.01), whereas the two cpDNA data sets were congruent (P = 0.58), with no conflicting branches detected between them (cf. Figs. 1 and 2). The topology from trnL-trnF noncoding regions had a higher resolution than that of the ndhF tree. Of the 2942 aligned positions in the combined cpDNA data set, 101 sites were variable and 30 sites were phylogenetically informative. Parsimony analysis of the combined chloroplast data set generated 14 393 MPTs with a total length of 116 steps, a CI of 0.879, an RI of 0.875, and an RC of 0.769. Phylogenies from the combined data (Fig. 3) and from separate analyses of the two noncoding regions (Fig. 2) had similar topologies, except that the combined analysis indicated the paraphyly of the two populations of O. longistylis. The maximum likelihood tree (MLT) has an identical topology to the MPT.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Although no morphometric analyses have been performed, these taxa clearly form a morphologically coherent group and have in the past been treated as a single species by some authors (e.g., Clarke, 1879
; Kuntze, 1891
; Boivin, 1968
). Wen et al. (2002)
suggested, however, that the morphological similarities exhibited by these species may involve shared plesiomorphic characters.
In contrast to the well-supported basal position of Osmorhiza aristata, the relationships among the New World species showed high discordance between cpDNA and nrITS phylogenies (cf. Figs. 3 and 4). For example, O. occidentalis is highly distinct from the other North American taxa in the cpDNA data sets and represents a major subclade. The ITS data (Fig. 4), on the other hand, suggest that O. occidentalis forms a clade with western North American O. depauperata. Osmorhiza occidentalis was first described in the monotypic genus Glycosma (Torrey and Gray, 1840
) and is distinguished from other Osmorhiza species by having unappendaged, glabrous fruit. Constance and Shan (1948)
treated Glycosma as a section within Osmorhiza, whereas Lowry and Jones (1984)
recognized it as a monotypic subgenus.
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Osmorhiza depauperata is disjunctly distributed in the Great Lakes region, northeastern North America and southern South America, whereas O. mexicana subsp. bipatriata is known from only a few localities in Coahuila and Nuevo León, Mexico, and in Texas. These two taxa form a clade in the chloroplast phylogeny, but the ITS data show that O. depauperata is sister to O. occidentalis.
The eastern North American taxa Osmorhiza claytonii and O. longistylis are placed in section Osmorhiza, along with the Asiatic O. aristata (Lowry and Jones, 1984
). In the ITS phylogeny, however, the section is paraphyletic. By contrast, the chloroplast DNA data sets suggest that O. claytonii and O. longistylis form a monophyletic group with O. purpurea, although this relationship is not strongly supported (bootstrap = 60%) (Fig. 3).
Osmorhiza mexicana subsp. mexicana, a member of section Mexicanae, is distributed in scattered populations from northern Mexico to North Argentina, thus occupying a much larger range than O. mexicana subsp. bipatriata. None of the analyses, however, suggest that these taxa form a monophyletic group. In the ITS phylogeny, the position of the two subspecies is unresolved, although both appear to be closely related to other western North American species. The chloroplast data sets also suggest that the subspecies of O. mexicana are not monophyletic, with the position of the typical subspecies resolved (Fig. 3) and O. mexicana subsp. bipatriata closely allied to O. depauperata (Figs. 1–3).
Comparisons of phylogenies based on ITS and chloroplast DNA data have been made in several previous studies (e.g., Franzke et al., 1998
; Choi and Wen, 2000
; McDade et al., 2000
; Potter, Luby, and Harrison, 2000
; Schwarzbach and Ricklefs, 2000
; Smith, 2000
). In the present analysis, phylogenies from the two data sets are largely incongruent. If separate data sets are incongruent as a result of evolutionary independence, then analysis of combined data sets may result in reduced or erroneous resolution with respect to the true organismal phylogeny (Hardig, Soltis, and Soltis, 2000
). We thus have not attempted a combined analysis of the data obtained on Osmorhiza.
Incongruence between nuclear and organellar phylogenetic trees is typically attributed to introgression of a cytoplasmic genome from one species into the nuclear background of another (e.g., Ferris et al., 1983
; Gyllensten and Wilson, 1987
; Harrison, Rand, and Wheeler, 1987
; Tegelstrom, 1987
; Soltis et al., 1991
; Rieseberg and Wendel, 1993
; Soltis and Kuzoff, 1995
; Soltis, Johnson, and Looney, 1996). No interspecific hybridization, however, has been reported for Osmorhiza.
Rare morphological intermediates have been observed among some species of Osmorhiza (Constance and Shan, 1948
; Lowry and Jones, 1984
), including between O. berteroi and O. occidentalis, O. glabrata and either O. berteroi or O. depauperata, and O. glabrata and O. mexicana subsp. mexicana. The high level of incongruence between nrDNA and cpDNA suggests that these morphological intermediates may be the result of hybridization and/or introgression. The morphologically variable taxa need to be examined along with their close relatives in areas where they co-occur. In this study, we examined a sample (Halse 5560) from a plant that co-occurs with O. berteroi and O. occidentalis and that shows some morphological intermediacy (especially the presence of some bristles on its fruit similar to those found in O. berteroi), but which is otherwise very similar to O. occidentalis. We examined the ITS and cpDNA profiles of three samples representing the presumed hybrid and typical O. berteroi (Halse 5559) and O. occidentalis (Halse 5561). Both cpDNA and nrITS data showed that the presumed hybrid had an ITS and cpDNA profile identical to that of O. occidentalis. There is thus no molecular evidence that hybridization has occurred in these populations. However, the data do not exclude the possibility that recent hybridization has occurred in which O. occidentalis is the maternal parent.
An independent study will be undertaken with at least one additional nuclear marker, such as waxy (Mason-Gamer and Kellogg, 1996) or adh (Gaut and Clegg, 1993
), to test the phylogenetic hypotheses and to examine the possible role of hybridization in the evolution of Osmorhiza.
Biogeography
The basally branching position of the Asiatic Osmorhiza aristata and the monophyly of the diverse New World clade (Figs. 1–4) support the hypothesis of a relatively ancient origin of the eastern Asian-eastern North American disjunction. The cladogenesis of Osmorhiza clearly appears to have been more rapid in the New World than in Asia. The relatively low DNA sequence divergence seen among the New World taxa, especially those in western North America, suggest rapid evolutionary radiation. Regarding the likely place of origin of the genus, the apparent close relationship of Osmorhiza with the Old World genera Myrrhis and Geocaryum (Downie, Katz-Downie, and Spalik, 2000
), coupled with the basal position of O. aristata, suggest that it took place in the Old World (Wen et al., 2002
). This interpretation differs from the one offered by Lowry and Jones (1984)
, who hypothesized a New World origin of Osmorhiza. More recent diversification among western North American Osmorhiza may have been facilitated by the availability of a broad array of habitats associated with the uplifting of the Rocky Mountains and the western cordillera in general, during the Tertiary (Barbour and Christensen, 1993
; Graham, 1993
).
Osmorhiza berteroi and O. depauperata show a similar pattern of antitropical disjunction between the temperate western North America and South America. Little sequence divergence was found among the populations of these species from the two hemispheres, a pattern similar to that observed in the nrITS sequences. Constance (1963)
suggested that these species might have migrated to South America in a step-wise manner along the western American cordillera during the Tertiary, with subsequent elimination of populations from the intervening tropical areas. However, the sequence divergence of cpDNA (0.00–0.056%) and nrITS (0.00–0.451%) between the western North American and South American populations of both species suggests a more recent origin. As indicated by Raven (1963)
, this pattern of disjunction corresponds closely to the migration routes of many bird species, which likely accounts for the distributions of O. berteroi and O. depauperata (Lowry and Jones, 1984
). Thus, the sequence divergence, the monophyly of both species, and their fruit morphology support the recent origin of the antitropical disjunction, perhaps by long-distance dispersal via birds from western North America to South America.
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FOOTNOTES |
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4 Author for reprint requests (wen{at}fieldmuseum.org)
.
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