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Department of Biology, Indiana University, Bloomington, Indiana 47405-6801
Received for publication February 23, 1998. Accepted for publication August 11, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Asteraceae • Astereae • biogeography • DNA sequence • internal transcribed spacer (ITS) region • molecular phylogenetics • nuclear ribosomal DNA (nrDNA) • taxonomic radiation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). It is the largest tribe in North America, including over 70 genera and 
1100 species (compiled from Kartesz, 1994
, excluding Hawaiian and Mexican taxa). In North America, the species are predominantly annuals and herbaceous perennials, and are important components of most terrestrial habitats on the continent. Representatives of the tribe in North America include some of the largest and most geographically widespread genera in the family, including Aster L., Erigeron L., and Solidago L. The phylogenetic relationships among many North American genera have been studied by Lane et al. (1996)
using chloroplast restriction site data. However, this study includes only three genera of predominantly non-North American taxa (Baccharis L., Bellis L., Felicia Cass.), making it difficult to critically evaluate the relationship of North American Astereae to the remainder of the tribe, or to assess the origin of North American taxa. Because long-distance dispersal is so well documented within the family (e.g., Baldwin et al., 1991
; Liston and Kadereit, 1995
), a study of the origins of North American taxa must entertain the possibility of multiple historical, transcontinental origins, and should include a robust sampling of species from other continents.
Few higher level phylogenetic hypotheses for the Astereae have been proposed, due to morphological uniformity and/or paucity of morphological characters that are consistent within genera (Nesom, 1994b
). Consequently, some genera in the tribe, most notably Aster, Erigeron, and Haplopappus DC., became taxonomic dumping grounds for large numbers of morphologically similar but distantly related taxa. Tribal-level treatments by Bentham (1873)
and Hoffman (1890)
are artificial, emphasizing generic groups with capitula "homochromous" vs. "heterochromous," and Grau's (1977)
synopsis of the tribe resorts to strictly geographical delimitations. However, recently, Haplopappus s.l. has been segregated into natural genera (Lane and Hartman, 1996
, and references therein), and similar studies in Erigeron (e.g., Nesom, 1989
, 1994a
; Noyes and Rieseberg, 1996
) and Aster (e.g., Nesom, 1994c
; Xiang and Semple, 1996
) are progressing toward the circumscription of evolutionarily meaningful taxa.
Zhang and Bremer (1993)
and Nesom (1994b)
each examined tribal-level evolutionary patterns using morphology, but reached highly disparate conclusions. Zhang and Bremer (1993)
divided the tribe into 23 informal groups and scored one representative genus of each group for 26 morphological characters. Based on the results of cladistic analysis, they recognized three subtribes: Grangeinae, Asterinae, and Solidagininae. The Grangeinae, according to Zhang and Bremer, occupies a basal position in the tribe and is sister to the Asterinae and Solidagininae. The Grangeinae (sensu Zhang and Bremer) includes Grangea Adanson and eight other genera of Africa and tropical Asia; it is a group that had been considered distinct from other Astereae by Bentham (1873)
, Hoffman (1890)
, and more recently, Fayed (1979)
. The frequent occurrence of pinnatifid-pinnatisect leaves in the group provides the single character distinguishing the Grangeinae (sensu Zhang and Bremer) from other Astereae, and suggests a morphological link to Tribe Anthemideae. The Solidagininae (sensu Zhang and Bremer) is predominantly North American, but also includes Engleria O. Hoffm. and Pteronia L. from Africa. This subtribe includes mostly homochromous genera and thus includes those North American genera with yellow rays such as Solidago, Chrysopsis (Nutt.) Ell., and Grindelia Willd., among others. The Asterinae (sensu Zhang and Bremer) is a large and diverse assemblage that includes mostly heterochromous genera from North America, Europe, South America, Africa, Asia, and Australia. The North American Asterinae are intermixed with taxa from other continents, suggesting that geography is uninformative as to evolutionary relationship. One possible explanation for the low levels of geographic patterning in the Astereae, based on this morphological analysis, is that intercontinental dispersal occurred frequently in the history of the tribe.
Nesom (1994b)
provided a comprehensive summary of the Astereae that includes (1) a detailed rationale (based primarily on morphological but to some extent also upon molecular grounds) for the formal division of the tribe into 14 subtribes, (2) proposed groupings of taxa within subtribes, and (3) hypotheses both for intertribal relationships and for the origin of North American Astereae. Nesom's hypotheses of relationship are subjective (phyletic), based on informed intuition rather than cladistics, and his groupings differ considerably from the 23 informal groups of Zhang and Bremer (1993)
. The 14 proposed Astereae subtribes and their putative relationships are provided in Fig. 1. According to Nesom, nine subtribes are Southern Hemisphere in affinity, seven of which constitute an assemblage that he coined the Grangeoid Complex. The remaining five subtribes are principally Northern Hemisphere and form a monophyletic group. The core of North American Astereae, in Nesom's view, comprises four of these five Northern Hemisphere subtribes (Chrysopsidinae, Machaerantherinae, Solidagininae, and Symphyotrichinae), and includes both homochromous and heterochromous genera. Hereafter, these will be referred to as the "North American core." The fifth Northern Hemisphere subtribe, the Asterinae (including Aster s.s.) is hypothesized to represent an independent Asian radiation. The base of the Nesom (1994b)
tree is an unresolved polytomy involving the Grangeoid Complex, the Northern Hemisphere assemblage, and the subtribes Hinterhuberinae and Baccharidinae (Fig. 1). Grangea and 14 other genera are grouped by Nesom (1994b)
into the Grangeinae, which occupies a relatively derived position within the Grangeoid Complex (Fig. 1). This is in contrast to the hypothesis of Zhang and Bremer (1993)
that afforded the Grangeinae basal status within the Astereae.
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hypothesis, the North American Astereae, in addition to the four North American core subtribes (detailed above), also includes one major group (subtribe Conyzinae), 11 genera that are classified with Southern Hemisphere subtribes, and one genus classified with Asian Asterinae. Nesom proposes that these taxa, based on morphology, are of Southern Hemisphere or Asian origin and now occur in North America via dispersal. The Conyzinae includes Erigeron (400+ spp.), Conyza L. (60+ spp.), and four small genera endemic to South America: Neja D. Don, six spp.; Hysterionica Willd., seven spp.; Leptostelma D. Don, three spp.; and Apopyros Nesom, two spp. (Nesom, 1994b
). Erigeron is most prevalent in North America (240+ spp.) but species are also endemic to Eurasia and South America. Conyza is also widespread with taxa described from North America, South America, and Africa. It is believed that the Conyzinae form a natural group with the exception of hypotheses (based on morphology) proposing that African Conyza (40 spp.) may be more closely related to other African genera than to other Conyzinae (Nesom, 1990b
, 1994b
). With support from floral morphology and the occurrence of a diversity of Conyzinae in South America, Nesom groups the Conyzinae with the six other Southern Hemisphere subtribes constituting the Grangeoid Complex (Fig. 1). According to Nesom (1994b)
, the Conyzinae most likely arose in the Southern Hemisphere and later dispersed to North America, where they experienced a secondary radiation.
The 12 other genera of North American Astereae that also fall outside of the North American core are all relatively small in size and endemic to North America. Eleven of these genera are classified within three Southern Hemisphere subtribes with principal distributions on different continents (Brachycominae [Australia]: Aphanostephus DC., Astranthium Nutt., Dichaetophora A. Gray, Geissolepis B. L. Robinson, Townsendia W. Hook.; Feliciinae [Africa]: "Monoptilon group" [Chaetopappa DC., Monoptilon Torrey & A. Gray], "Pentachaeta group" [Pentachaeta Nutt., Rigiopappus A. Gray, Tracyina S. F. Blake]; Hinterhuberinae [South America]: Ericameria Nutt.). Boltonia L'Her. is postulated to be closely related to Asian taxa and classified as Asterinae. In summary, to explain the occurrence of the Conyzinae and these 12 endemic genera in North America requires a minimum of five independent dispersal events, four from the Southern Hemisphere and one from Asia. Nesom's (1994b)
treatment of Astereae is fundamentally different from that of Zhang and Bremer (1993)
in that it suggests a strong role for geography in the delimitation of major groups (rather than heterochromy vs. homochromy), and rejects the Grangeinae as basal within the tribe. However, both models suggest an important role for intercontinental dispersal in explaining diversification patterns in North American Astereae.
An additional controversy, central to understanding the origin of North American Astereae, concerns the delineation and circumscription of Aster. Aster s.l. (400 spp.) is morphologically heterogeneous and geographically widespread, with centers of diversity in North America and Eurasia. The genus has a complex taxonomic history, as botanists over the last two centuries have attempted to carve out natural groups (reviewed in Nesom, 1994c
; Xiang and Semple, 1996
). Because many segregates have been proposed without regard to broader patterns of diversity in the genus, these classifications have generally not been accepted by the botanical community. Two recent works help to bring a broader perspective to Aster. Nesom (1994c)
conducted a comprehensive phyletic morphological assessment of Aster s.l.; Xiang and Semple (1996)
examined 48 species of Aster s.l. in a chloroplast restriction site analysis that emphasized North American diversity but also included the type species, European Aster amellus L. Both studies provide compelling justification for the recognition of several North American segregate genera such as Doellingeria Nees, Eucephalus Nutt., Ionactis E. Greene, Oclemena E. Greene, Oreostemma E. Greene, and, to varying degrees, Sericocarpus Nees and Symphyotrichum Nees. The two studies differ markedly, however, in that Nesom (1994c)
hypothesized that Aster s.s. (including the type species, A. amellus) is restricted to a group of 180 species occurring for the most part in Eurasia. He further proposed that the closest relatives of Aster s.s. are 12 small Eurasian genera including, among others, Crinitaria Cass., Psychrogeton Boiss., Kalimeris Cass., and Callistephus C. A. Mey. These genera constitute subtribe Asterinae (sensu Nesom, 1994b
). Most North American Aster segregates, on the other hand, are classified as subtribe Symphyotrichinae (Nesom, 1994b
) and are members of the North American core group. In contrast, Xiang and Semple (1996)
proposed that Aster s.s. comprise not only Eurasian taxa but a large group of North American species that had been recognized by Nesom (1994c)
as the segregate genus Eurybia (Cass.) S. F. Gray (Symphyotrichinae). The Xiang and Semple (1996)
proposal is supported by cladograms showing Eurasian Aster amellus among North American Eurybia (sensu Nesom). According to their hypothesis, Aster s.s. arose in North America and subsequently dispersed throughout Eurasia. In summary, where Nesom (1994c)
proposed a fundamental difference between North American and Eurasian Aster, Xiang and Semple (1996)
postulated an intimate relationship.
This research examines Astereae evolution from a molecular phylogenetic perspective and tests hypotheses concerning the origin and biogeographical relationships of the tribe in North America by analyzing sequence variation in the internal transcribed spacer (ITS) region and the 5.8S cistron of nuclear ribosomal DNA for a sample of 62 taxa that includes: seven outgroup taxa from Tribes Anthemideae, Calenduleae, Gnaphalieae, and Inuleae; 28 North American Astereae; and 27 Astereae sampled from South America, Europe, Asia, Africa, and Australia. ITS sequence analysis is a proven powerful tool for testing phylogenetic hypotheses within or among closely related genera (e.g., Bain and Jansen, 1995
; Baldwin et al., 1995
; Kim, Crawford, and Jansen, 1996
). However, recent efforts reveal that ITS variation can also be used to interpret evolutionary history at higher levels, for instance within tribes of Asteraceae (Francisco-Ortega et al., 1997
) and Poaceae (Hsiao et al., 1995a
, b
), at the family level (e.g., Apiaceae; Downie and Katz-Downie, 1996
), and even among flowering plant families (Hershkovitz and Zimmer, 1996
). Here, we use ITS sequence data to (1) construct a phylogeny of the Astereae, (2) test existing classifications of Nesom (1994b)
and Zhang and Bremer (1993)
, (3) assess the geographic origin of North American taxa, and (4) test conflicting hypotheses of Nesom (1994c)
and Xiang and Semple (1996)
concerning the relationship of Aster s.s. to North American segregate genera. In addition, this study is potentially informative regarding the importance of dispersal in the evolution and diversification of the Astereae and for evaluating molecular and character trait evolution in the tribe. Because numerous North American Astereae are included in this study, comparisons are also possible with the more comprehensive study of Lane et al. (1996)
.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, and that for Artemisia canariensis (Besser) Less. from Francisco-Ortega et al. (1997)
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) and molecular studies (e.g., Jansen et al., 1990
). Fifty-five Astereae taxa are included, with sampling guided by Nesom's (1994b)
tribal treatment, and including representatives of all 14 subtribes. Forty of the taxa represent the nine proposed Southern Hemisphere subtribes (Table 1). This sample includes representatives of each of the 11 genera native to North America, but presumed by Nesom (1994b)
to be Southern Hemisphere in origin and taxonomic affinity. The Conyzinae, likewise hypothesized by Nesom (1994b)
to be Southern Hemisphere in origin, is represented by nine accessions and includes taxa from North America, South America, Europe, and Africa. The sample for the five Northern Hemisphere subtribes (15 taxa) includes 11 species, representing the four subtribes that constitute the North American core, plus four Asterinae (three Eurasian, one North American). Seven of the samples, Aster s.s., Crinitaria, Doellingeria, Kalimeris, Oreostemma, Sericocarpus, and Symphyotrichum, represent Aster s.l. from North America, Europe, and Asia.
The Astereae sample also represents 19 of the 23 generic groups recognized by Zhang and Bremer (1993)
; only four of their generic groups ("Apodocephala," "Gutierrezia," "Haplopappus," and "Petradoria") were not included in this analysis. Lane et al. (1996)
reported that most genera of the "Gutierrezia" and "Petradoria" groups of Zhang and Bremer (1993)
form a well-supported clade that includes (among others) two genera included in this study, Euthamia Nutt. and Solidago. The "Haplopappus" group of Zhang and Bremer (1993)
comprises five genera. In Lane et al. (1996)
, four of these form a clade that includes Machaeranthera Nees (sampled here). This comparison with Lane et al. (1996)
suggests that the taxonomic diversity present in the "Gutierrezia," "Haplopappus," and "Petradoria" groups of Zhang and Bremer (1993)
is moderately well represented, although indirectly, in this study. Adequate material of the "Apodocephala group" (Apodocephala Baker and Vernoniopsis Humbert, of Madagascar) was not available for study. It should be noted that Apodocephala is considered by Nesom (1994b)
to be a member of the Heliantheae rather than Astereae. Vernoniopsis is classified by Nesom (1994b)
as subtribe Baccharidinae, represented by three accessions in this study.
DNA isolation, PCR amplification, sequencing
Total DNA was isolated either from 0.10 g fresh or 0.02 g dry leaf tissue. For dried source materials (herbarium specimens), best results were generally obtained if collections were made after 1980 and were dried without the use of ethanol. For most samples, the CTAB (hexadecyltrimethylammonium bromide) extraction procedure of Doyle and Doyle (1987)
was used, except that all volumes were reduced to allow DNA isolations to be performed in 1.5-mL microcentrifuge tubes. Also, sodium metabisulfite (1% w/v) was added to the DNA isolation buffer. Total DNAs isolated with this method were cleaned with the Elu-Quik DNA purification kit (Schleicher and Schuell, Keene, New Hampshire). DNA for a few samples was isolated using the DNeasy Plant Mini Kit (Qiagen, Chatsworth, California) using 0.02 g dry leaf tissue. This latter method precluded the need for further DNA purification; isolated DNAs were used directly in PCR amplifications.
ITS amplifications were performed in 50-µL reactions using 10–20 ng of genomic DNA, 30 mmol/L tricine pH 8.4, 2 mmol/L MgCl2, 50 mmol/L KCl, 100 µmol/L each dNTP, 2.0 units Taq polymerase, and 0.15 µmol/L primer (equimolar). For most samples, ITS1, 5.8S, and ITS2 were amplified as a single molecule using primers "ITS-4" (White et al., 1990
) and a modification of "ITS-5" (White et al., 1990
) based on sequence reported for Glycine (Eckenrode, Arnold, and Meagher, 1985
; 5’-GGAAGGAGAAGTCGTAACAAGG-3’). A few genomic DNAs were partially degraded requiring two separate amplifications for the ITS1 and ITS2 regions. For these samples, ITS1 was amplified with "ITS-5" and a modification of "ITS-2" (White et al., 1990
) based on sequence obtained from Glycine (D. Nickrent, personal communication, Southern Illinois University at Carbondale; 5'-GCTACGTTCTTCATCGATGC-3';). The 5.8S-ITS2 region was amplified using a modification of "ITS-3" (White et al., 1990
) primer based on Glycine (D. Nickrent, personal communication, Southern Illinois University at Carbondale; 5';-GCATCGATGAAGAACGTAGC-3') and "ITS-4.' Cycling conditions for amplification consisted of 30 cycles of 95°C for 1 min, 55°C for 1 min, 72°C for 1 min, followed by a final extension of 7 min at 72°C. Amplification products were purified using the Elu-Quik purification kit.
Sequencing of ITS1 was accomplished using primers "ITS-1" (White et al., 1990
) and the modified version of "ITS-2" given above. ITS2 was sequenced using a modified "ITS-3" (above) and primer "ITS-4." Most purified double-stranded templates were sequenced using Sequenase 2.0 (United States Biochemical, Cleveland, Ohio) and 35S-dATP on denaturing 5% acrylamide gels. The sequences were read in both forward and reverse directions. Gels were transferred to 3M Whatman filter paper, vacuum dried, and exposed to Kodak X-Omat AR film for 18–36 h. Sequences were read manually and entered into the sequence manipulation program SeqApp (version 1.9; copyrighted 1992, Don Gilbert, Indiana University). For a few samples, the double-stranded PCR products were cycle-sequenced (in both directions) according to protocols accompanying the Applied Biosystems Prism 377 automated sequencer, using reagents provided with the Thermosequenase Dye-Terminator Cycle Sequencing Kit (Amersham Life Sciences, Inc., Cleveland, Ohio). Data obtained using this method were proofread and edited with the sequence analysis program Sequencher, version 3.0 (Gene Codes Corporation; Ann Arbor, Michigan) and then compiled into SeqApp.
ITS boundaries and sequence divergence
The boundaries of the ITS1 and ITS2 sequences were made by comparison to published nrDNA sequences of other genera of Asteraceae (Baldwin, 1992
; Kim and Jansen, 1994
; Bain and Jansen, 1995
; Bayer, Soltis, and Soltis, 1996
), and those published for Daucus and Vicia (Yokota et al., 1989
), and for Oryza (Takaiwa, Oono, and Sugiura, 1985
). In these latter two studies, boundaries between spacers and ribosomal regions were determined by S1 nuclease mapping, and via RNA-DNA sequence comparisons, respectively. Because nucleotide variation was detected in sequenced regions of 5.8S, one base position at the 5’ end and 62 bases at the 3’ end of 5.8S were also included in this analysis. Sequence alignments for all analyses were conducted, in part, using the program Clustal W (Thompson, Higgins, and Gibson, 1994
). Pairwise and multiple alignment gap and extension penalties were set at 15 and 5, respectively, with the alignment transition weight of 0.5 and the "Delay Divergent" option set at 40%. The resulting matrix was examined visually and adjusted where deemed necessary. Also, highly variable regions that appeared to be misaligned were removed from all analyses. PAUP test version 4.0.0d61 ("PAUP*"; by permission of D. Swofford, Smithsonian Institution, personal communication) was used to calculate nucleotide sequence divergence based on Kimura's two-parameter model (Kimura, 1980
), G + C content of the sequence analyzed, and transition/transversion ratios for the most parsimonious trees.
Sequence analyses
Two different phylogenetic analyses were conducted. An initial analysis included the seven non-Astereae outgroup taxa plus a 16-taxa sample of Astereae. The second analysis used only and all Astereae sequences. The first analysis was done to identify basal taxa within the Astereae that could be used as outgroups in the expanded analysis. Selection of representative Astereae taxa for use in the reduced analysis was based, in part, on preliminary phylogenetic analyses that indicated that the Astereae comprise three major groups of taxa: (1) Felicia, Amellus L., Commidendron Berch. ex DC., and select Hinterhuberinae; (2) mostly Southern Hemisphere genera; and (3) mostly North American genera. Four, six, and six representative sequences were selected from these three groups, respectively, and combined with outgroup taxa for analysis. The need to conduct a separate, reduced data set analysis was indicated by preliminary phylogenetic analyses of all-inclusive data sets that resulted in poor resolution within the Astereae, presumably related to the relatively higher proportion of bases excluded from data sets including both Astereae and outgroup taxa (see discussion below).
Fitch maximum parsimony was performed on the combined ITS1, ITS2, and partial 5.8S data set using PAUP* with ACCTRAN, MULPARS, and TBR options. Multiple islands of equally parsimonious trees were sought by performing 100 random entries in all heuristic searches. Gaps in the aligned sequences were coded either as missing data or were treated as informative by coding each single base gap as a "fifth base" using the GAPS = NEWSTATE option in PAUP*.
Both unweighted and character-state weighted parsimony were conducted with both gap treatments for the reduced and Astereae-rooted data sets. Transversions were weighted over transitions by a 1.45:1 ratio in the reduced data set and by a 1.54:1 ratio in the Astereae-rooted analysis by means of the USERTYPE STEPMATRIX of PAUP*. These ratios were obtained from analyses of unweighted trees by PAUP*. Support for monophyletic groups was evaluated using 100 bootstrap replicates (Felsenstein, 1985
). These analyses were conducted using PAUP* with the ACCTRAN, MULPARS, and TBR options and a full heuristic search with ten random addition sequences of taxa per replicate.
The g1 statistic (Hillis and Huelsenbeck, 1992
) was calculated by computing the tree-length distribution of 100 000 random parsimony trees using the RANDOM TREES command of PAUP*. Consistency Index (CI) (Kluge and Farris, 1969
) and Retention Index (RI) (Farris, 1989
) were also calculated.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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For the reduced data set, 52 bp, constituting four fragments, could not be aligned unequivocally and were excluded from the analysis- ITS1, positions 113–130; ITS2, positions 354–374, 415–420, and 523–529. Alignment of the remaining 524 characters required the insertion of 68 gaps, 55 of which (81%) are 1 bp in length. Only 28 (41%) of the gaps are phylogenetically informative and comprise 25 1-bp, two 2-bp, and one 5-bp gap insertions. Within any single sequence, the longest contiguous string of gaps did not exceed 5 bp, except for a 21-bp gap in Achillea near the 5’ end of ITS2.
In the Astereae-rooted analysis, 35 bases were excluded that constitute three fragments- ITS1, positions 127–137; ITS2, positions 360–372 and 551–561. Alignment of the remaining 539 characters required the insertion of 65 gaps, most of which (45; 69%) are 1 bp in length. Twenty-eight of the gaps (43%) are phylogenetically informative, and include 21 1-bp, five 2-bp, and two 3-bp gap insertions. All contiguous gap insertions in the Astereae-rooted data set were less than 6 bp except for Brachyscome, which required insertion of a large 54-bp gap and a nearby 9-bp gap near the center of ITS1.
Sequence divergence/information content
For analyses of the reduced and Astereae-rooted data sets conducted with gaps treated as missing data, there were 212 (40%) and 229 (42%) parsimony-informative sites, respectively. Including gaps as informative characters increased the number of informative sites to 224 (43%) and 244 (45%) for the two data sets. Sequences of outgroup taxa are highly dissimilar to each other, yielding an average pairwise distance value based on a Kimura two-parameter model of 27.9%. Similarly, the average pairwise distance between the 16 Astereae used in the reduced analysis and the seven outgroup taxa is 28.3%. The 55 taxa included within the Astereae-rooted analysis are considerably more similar, yielding an average distance value of 13.9%. Lowest distances were obtained for comparisons between Rigiopappus and Tracyina (1.4%), and between Chrysothamnus Nutt. and Solidago (2.4%). Highest distances within the Astereae correspond to comparisons between Chiliotrichum Cass. and Aphanostephus (25.6%) and between Chiliotrichum and Hysterionica (25.4%).
The g1 statistic was used to evaluate the degree of skewness in 100 000 randomly generated trees in both data sets. For the reduced data set, g1 =-0.792 and -0.847 for trees generated from analyses with gaps treated as missing and as "fifth base," respectively. Values of -0.393 and -0.451 were obtained for the corresponding Astereae analyses. These results indicate that both data sets are skewed significantly from random (P < 0.01 for g1 = -0.09 for 250 variable sites and >25 taxa). These values indicate that there is a high level of phylogenetic signal in the ITS sequences (Hillis and Huelsenbeck, 1992
).
Reduced data set analysis
Four trees resulting from weighted and unweighted parsimony analysis of the reduced data set, with gaps treated as either missing data or as a "fifth base," are shown in Fig. 2. The two "gaps missing" treatments each yielded single most parsimonious trees (Fig. 2A, B). When gaps were coded as a "fifth base," unweighted analysis produced four equally most parsimonious trees while the weighted analysis yielded two shortest solutions. Resulting strict consensus trees are shown in Fig. 2C, D. Treating gaps as a "fifth base" appears to result in trees with higher information content, as these trees display 13 and 14 nodes, respectively, with >50% bootstrap support, while only 11 and 9 similarly supported nodes occurred in shortest trees with gaps treated as "missing." The topologies of the four trees are nearly identical. The monophyly of the Astereae is strongly supported in each, with bootstrap values ranging from 98 to 100% (indicated by arrow 1, Fig. 2). Results are also consistent in providing strong support for the monophyly of the taxa selected to represent the Anthemideae (Achillea, Artemisia L., Ursinia Gaertn.) and the Gnaphalieae (Anaphalis and Pseudognaphalium).
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Astereae-rooted analysis
Relevant data from the four rooted analyses of 55 Astereae taxa using unweighted and weighted maximum parsimony, with gaps treated as missing or as a "fifth base," are presented in Table 2. Figure 3 depicts the strict consensus tree resulting from weighted parsimony analysis with gaps coded as a "fifth base" and displays bootstrap support for all nodes; subtribe membership is according to Nesom (1994b)
and Zhang and Bremer (1993)
, and geographic origin of each taxon is indicated. All four strict consensus trees are highly resolved; three exhibit 31 nodes with >50% bootstrap support. The strict consensus tree resulting from an unweighted analysis with gaps treated as missing exhibits 30 nodes with 
50% support (Table 2). Figure 4 depicts, in phylogram form, one of the 18 equally parsimonious trees resulting from the weighted analysis with gaps coded as "missing." The topology of this tree is moderately skewed, suggesting evolutionary rate heterogeneity for ITS among taxa.
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) to be derived from relatives on other continents. There are two non-North American accessions that occur within this otherwise strictly North American clade, Erigeron uniflorus L. (Europe) and Hysterionica (South America). Both occur nested within a highly supported clade that includes other Conyzinae taxa. Bootstrap support for the node beneath the North American clade is moderate (60–74%; Node 2, Fig. 3, Table 2). Doellingeria umbellata (Miller) Nees appears as the sister to the remaining accessions in the clade. Bootstrap support of the monophyly of the remaining 29 North American taxa ranges from 98 to 99% (Node 3, Fig. 3, Table 2). All cladograms show a poorly resolved assemblage of 18 taxa, the Southern Hemisphere grade, between the basal group and the North American clade. Six of these taxa are Eurasian in origin, three are Australian, four are African, three are South American, and two, Lagenifera panamensis S. F. Blake and Laennecia sophiifolia (Kunth) Nesom, are native to North America (Fig. 3). According to Nesom (1994b)
, all but three of these taxa (Aster, Crinitaria, and Kalimeris [Asterinae]) have Southern Hemisphere affinities. Podocoma notobellidiastrum (Griseb.) Nesom appears as the closest relative to the North American clade in all four analyses; however, the bootstrap support for this relationship is weak (<50%).
In contrast to the Southern Hemisphere grade, the North American clade exhibits considerable phylogenetic structure. None of the four consensus depictions of this clade contain polytomies, and over 20 nodes in each are supported by 50% bootstrap support. The relationships among Conyzinae taxa are complex. Six species of Conyzinae are found within the North American clade: Conyza canadensis (L.) Cronq., Hysterionica, and four Erigeron species. These taxa form a well-supported clade (100% bootstrap support in all analyses) that also includes Aphanostephus (Brachycominae) (Fig. 3). These taxa, plus Chrysopsis, Heterotheca, and Geissolepis, form a ten-taxa clade that receives 68 and 71% bootstrap support in analyses with gaps coded as a "fifth base." There is less support for this association with analyses that code gaps as "missing" data. Three species of Conyzinae occur elsewhere in cladograms: Erigeron byei Sundberg & Nesom associates with Boltonia in the North American clade, and Conyza gouanii (L.) Willd. and C. pyrrhopappa A. Rich. (Africa) appear as members of the Southern Hemisphere grade.
Four of seven representatives of Aster s.l. occur within the North American clade. Doellingeria occurs as the sister to all other North American taxa. Oreostemma and Symphyotrichum occur in a well-supported clade (95–100% bootstrap support) that also includes Grindelia and Machaeranthera. Sericocarpus appears as the sister to Solidagininae genera Solidago and Chrysothamnus. The remaining three species of Aster s.l. occur in the Southern Hemisphere grade: Aster amellus, the type species, occurs with segregate genus Kalimeris (100% bootstrap support in all cladograms). Crinitaria occurs within a separate clade.
The results also show that the 11 representatives of the four subtribes constituting the North American core (sensu Nesom; Chrysopsis, Chrysothamnus, Doellingeria, Euthamia, Grindelia, Heterotheca, Machaeranthera, Oreostemma, Sericocarpus, Solidago, and Symphyotrichum) do not form a monophyletic group. Instead, they are depicted in all cladograms (for example Fig. 3) as comprising five independent clades interspersed among other North American genera. Lower nodes in the North American clade generally receive low bootstrap support (<50%), with the taxa forming five or six consistently well supported small- to medium-sized clades.
Based on average genetic distances, members of the North American clade are more distant from basal group taxa (0.191) than they are from members of the Southern Hemisphere grade (0.145), as would be anticipated from their relative positions in Fig. 3. The average pairwise distance among North American clade members is 0.127. This is significantly greater (P < 0.01; Games-Howell method [Sokal and Rohlf, 1995
]) than the average distance among Southern Hemisphere taxa (0.088), but similar to the average for basal group members (0.138).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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and Nesom (1994b)
show little support for the former and only moderate congruence with the latter. The evolutionary relationships supported by ITS do not uphold the Zhang and Bremer (1993)
hypothesis that the Astereae comprise a basal Grangeinae, homochromous Solidagininae, and heterochromous Asterinae. Grangea (Grangeinae) does not occur in the basal group of the ITS phylogeny but instead occurs within the Southern Hemisphere grade. Additionally, there is no ITS evidence to support a broadly inclusive homochromous Solidagininae, because yellow-rayed Astereae do not form a monophyletic group. Although most of the yellow-rayed taxa do form a part of the North American clade, Pteronia (South Africa) is a member of the basal group and therefore presumably only very distantly related. Furthermore, the nine Solidagininae genera (sensu Zhang and Bremer) occur in five different clades, intermixed with Asterinae. This suggests that while ray color may be phylogenetically informative among close relatives, it is a poor indicator of cladistic relationships at higher levels. Several genera of predominantly heterochromous Astereae, such as Erigeron and Felicia, also include homochromous species, suggesting that ray color may be fairly labile in the tribe. Knowledge of the genetics of ray color in Astereae could be invaluable for interpreting the evolutionary patterns of this trait. Other more restricted studies, of Aster (Xiang and Semple, 1996
), and of North American Astereae (Lane et al., 1996
), similarly find little support for the Zhang and Bremer (1993)
classification.
The ITS phylogenies (e.g., Fig. 3) roughly resemble Nesom's (1994b)
Astereae hypothesis (Fig. 1). Although there are significant differences, Nesom's Grangeoid Complex and Northern Hemisphere assemblage very broadly parallel the ITS Southern Hemisphere grade and North American clade, respectively. Principal differences relate to the positions of Asterinae, Conyzinae, and Feliciinae. The ITS phylogenies indicate that the Asterinae are allied with Southern Hemisphere taxa rather than with other Northern Hemisphere species; this is particularly noteworthy because it has important implications for interpreting relationships within Aster s.l. (see below). The repositioning of the Asterinae in ITS phylogenies, relative to Nesom's (1994b)
hypothesis, is anticipated by achene morphology. According to Nesom (1994b)
, the Grangeoid Complex subtribes are characterized by two-nerved, flattened, glandular achenes, while Northern Hemisphere taxa typically produce eglandular, multinerved, more-or-less terete achenes. However, the Asterinae, as circumscribed by Nesom (1994b)
, are unique among Northern Hemisphere taxa, in that most produce two-nerved, flattened, glandular achenes. The ITS phylogenies suggest that Asterinae achene morphology indicates Southern Hemisphere affinities. The ITS phylogenies position the Conyzinae within the North American clade, rather than with Southern Hemisphere taxa. Achene morphology is equivocal, in this instance, as the achenes of Conyzinae, although eglandular like those of most other North American taxa, are flattened and therefore similar to Southern Hemisphere species. Because ITS evidence strongly suggests that the Conyzinae are derived members of the North American clade, flattened achenes appear to represent parallelism or convergence rather than homology with Southern Hemisphere species. The Feliciinae also occur in a basal position in the ITS phylogeny; they are included within a broader discussion of basal taxa below.
Five of the seven basal Astereae taxa in the ITS phylogeny are Hinterhuberinae or Baccharidinae, subtribes hypothesized by Nesom (1994b)
to be isolated within the Astereae (Fig. 1). The ITS data do not support the monophyly of these subtribes, however, because each also includes members (Baccharidinae: Psiadia, Baccharis; Hinterhuberinae: Diplostephium Kunth, Ericameria) that occur in either the Southern Hemisphere grade or the North American clade. The basal position of the Feliciinae is unexpected in the ITS phylogenies since Nesom (1994b)
had hypothesized a derived position within the Grangeoid Complex (Fig. 1). The ITS data support Nesom's (1994b)
contention that Grangea (Grangeinae), rather than basal, is relatively derived, and also Nesom's expanded concept of Grangeinae, which includes taxa related to the African genus Nidorella. The clade containing Nidorella and Grangea has high bootstrap support and is well nested within the Southern Hemisphere grade.
The Astereae basal group is morphologically diverse. Commidendron (four spp.; St. Helena), Pteronia (80 spp., Africa), Olearia (130 spp., Australia), and Chiliotrichum (seven spp., South America) are shrubs to small trees, while Felicia (85 spp., Africa) and Amellus (12 spp.; Africa) both comprise annuals to small shrubs, and Oritrophium (15 spp.; South America) species are scapose, perennial herbs. Furthermore, Pteronia is yellow-rayed, while the other genera include species with rays mostly white to purple. Pteronia is further noted for opposite phyllotaxy; Felicia, Amellus, and Olearia taxa are variably alternate to opposite; Commidendron bears terminal clusters of leaves; and Chiliotrichum and Oritrophium produce alternate leaves. Also, while Amellus and Chiliotrichum have paleaceous receptacles, those of the other genera are naked. In addition, disc florets in Oritrophium are sterile; in the other genera they are fertile. These taxa, as a group, had not previously been identified as basal Astereae, although a chloroplast ndhF gene sequence analysis for Asteraceae (Kim and Jansen, 1995
) shows a basal Felicia, with 86% bootstrap support, relative to other Astereae (including Erigeron, Conyza, Baccharis, and Bellis). Four of seven taxa in the basal group are shrubs or trees, which suggests that the Astereae may have arisen from woody ancestors and that herbaceous taxa are derived. It is also significant that basal taxa occur in Australia, Africa, and South America. This distribution supports Nesom's (1994b)
hypothesis that the origin of the Astereae may be ancient, perhaps dating to a time when dispersal among the Southern Hemisphere continents would have been more likely.
Limited higher level comparisons between ITS and chloroplast restriction site phylogenies (Lane et al., 1996
) reveal striking discordance, in regard to the position of several major Astereae groups. Basal Astereae in the outgroup-rooted analysis include Erigeron, Conyza, Baccharis, and taxa representing five other North American genera. These occur basal to an unresolved polytomy that includes Felicia, Bellis, and four large North American groups. This is in contrast to ITS results (Fig. 3) that support a basal status for Felicia, placement of Baccharis and Bellis in the intermediate Southern Hemisphere grade, and a highly derived position for Erigeron and Conyza. Thus, the level of discordance between the two phylogenies involves differences in topologies of all three major groups recognized in the ITS analysis. Curiously, the chloroplast ndhF gene sequence analysis for Asteraceae (Kim and Jansen, 1995
), mentioned above, by showing a basal Felicia relative to other Astereae, supports the ITS topology over the chloroplast restriction site phylogeny (Lane et al., 1996
). This is perplexing because, as both chloroplast restriction and sequence data sets represent samples of variation from the same molecule, they should reflect the same underlying phylogenetic pattern. An analysis of Astereae chloroplast sequence variation currently underway, which will include all of the taxa for which ITS sequence was obtained, should help to clarify the sources of conflict among these three studies.
Origin of North American Astereae
According to the classification of Nesom (1994b)
, the core group of North American Astereae comprises four subtribes, the Chrysopsidinae, Machaerantherinae, Solidagininae, and Symphyotrichinae (Fig. 1). Twelve other small genera, endemic to North America, were considered to be more closely related to non-North American taxa, and, based on morphology, were consequently classified into subtribes with principal distributions on other continents—Boltonia (Asterinae: Asia); Aphanostephus, Astranthium, Dichaetophora, Geissolepis, Townsendia (Brachycominae: Australia); Chaetopappa, Monoptilon, Pentachaeta, Rigiopappus, Tracyina (Feliciinae: Africa); Ericameria (Hinterhuberinae: South America). In addition, the Conyzinae, represented in North America by numerous species of Erigeron and Conyza, were classified with Southern Hemisphere subtribes. It was hypothesized that ancestors of these taxa dispersed to North America from their respective continents and are not closely related to the North American core subtribes. The characters upon which these hypotheses were based include achene trichome types, achene morphology, receptacle morphology, pappus form, and shape and morphology of phyllaries, among others. This hypothesis is notable for being the first to provide a comprehensive overview of North American Astereae and for making detailed, rationalized hypotheses to explain the origin of the divergent elements in the flora.
The ITS phylogeny (Fig. 3) provides no evidence either for a monophyletic North American core clade (sensu Nesom) or for the dispersal of any taxon to North America from the Southern Hemisphere or Asia. Instead, the North American core genera, together with most Conyzinae (discussed below) and all 12 putatively dispersed genera, form a large and heterogeneous, but well-supported, monophyletic group. ITS does support a close relationship among members within many of the putatively migrant groups. For example, Tracyina, Rigiopappus, and Pentachaeta (Feliciinae: "Pentachaeta group") form a strongly supported, monophyletic group, as do Chaetopappa and Monoptilon (Feliciinae: "Monoptilon group"). One notable exception to this trend is Aphanostephus (Brachycominae), which unexpectedly occurs within North American Conyzinae (Fig. 3) and which does not appear to be closely related to other North American Brachycominae. Aphanostephus (four spp., southern United States and adjacent Mexico) has unique achenes and pappus that differ from those of typical Conyzinae. Furthermore, while Erigeron taxa have a base chromosome number of x = 9, Aphanostephus species are x = 3, 4, 5. In a more comprehensive ITS analysis of Conyzinae (Noyes and Rieseberg, 1996
), Aphanostephus appears to share a close relationship with a group of Erigeron species classified as E. sect. Olygotrichium (Nesom, 1989
), which occur in the same geographic region.
One completely unanticipated clade, among North American genera, unites Ericameria (Hinterhuberinae) and Tracyina, Rigiopappus, Pentachaeta (Feliciinae). The former is a genus of shrubs segregated in part from Haplopappus s.l. (Nesom, 1990a
) and currently includes several taxa traditionally recognized as Chrysothamnus (Nesom and Baird, 1993
, 1995
). The latter three genera are all annuals. All four genera are characterized by yellow rays and linear to oblanceolate entire leaves; however, the two groups differ markedly in achene and pappus characters. Interestingly, Rigiopappus, until recently, was considered to be a member of Tribe Helenieae. The morphological similarities of Rigiopappus to Tracyina and Pentachaeta were detailed by Robinson and Brettell (1973)
, who consequently transferred the genus to the Astereae.
The Conyzineae are represented in this study by Hysterionica, Erigeron (five species, three continents), and Conyza (three species, two continents). A group of six Conyzinae taxa form a strongly supported monophyletic clade that is sister to Chrysopsidinae and Geissolepis within the North American clade (Fig. 3). The derived position of Conyzinae within North American Astereae supports the contention of Cronquist (1947)
that Erigeron is a young and actively evolving group and not an old, primitive lineage within the tribe as hypothesized by others (e.g., Xiang and Semple, 1996
). Basal taxa in this Conyzinae clade are Erigeron species from southwestern United States and adjacent Mexico, in this and in more detailed analyses (Noyes and Rieseberg, 1996
; Noyes and Nesom, unpublished data). Conyzinae of South America (Hysterionica) and Europe (Erigeron uniflorus), as well as North American Conyza, all appear to be derived from basal North American Erigeron species. Four other Astereae subtribes also have North American origins and disjunct distributions in South America (Nesom, 1994b
): Machaerantherinae (Haplopappus s.s. and Grindelia species); Chrysopsidinae (Noticastrum DC.); Solidagininae (Solidago and Gutierrezia Lag. species); and Symphyotrichinae (Psilactis A. Gray species).
The data indicate that the Conyzinae, sensu Nesom (1994b)
, is polyphyletic. First, Erigeron byei appears to be more closely related to Boltonia, rather than to other North American Conyzinae. This species was described only recently by Sundberg and Nesom (1990)
, and its position within the genus had been considered to be uncertain (Nesom et al., 1991
). The ITS data support an hypothesis (Sundberg and Nesom, 1990
) that E. byei may be closely related to the genus Chloracantha Nesom, Suh, Morgan, Sundberg, & Simpson, a species of Erigeron that was recently segregated based on chloroplast restriction site data, and Boltonia (reported in Nesom et al., 1991
; unpublished data). Second, African Conyza occurs with Southern Hemisphere taxa rather than with North American Conyza. Conyza, historically, like Haplopappus and Aster, has served as a repository of superficially similar taxa. Recent efforts that have led to the establishment of a more natural Conyza include the transfer of several species to other genera (e.g., Zardini, 1981
; Nesom and Zanowiak, 1994
). Approximately 60 species occurring in Africa have been classified as Conyza primarily on the basis of habit and floral characters. Nesom (1990b)
suggested that African Conyza may represent several divergent lineages, independent of New World Conyza in the New World, based on morphological evidence. African species C. pyrrhopappa and C. gouanii both occur among Southern Hemisphere taxa (Fig. 3), distinct from New World Conyzinae, unequivocally supporting Nesom's hypothesis. An expanded analysis of 11 African species of Conyza (Noyes, unpublished data) establishes that the species in Africa constitute at least three independent clades. Excluding African Conyza and Erigeron byei, and with the addition of Aphanostephus, the Conyzinae appear to form a coherent group with its origin in North America, based on ITS evidence.
ITS data modestly support the chloroplast restriction site phylogeny of Lane et al. (1996)
. In general, both studies concur in defining North American taxa as several unresolved large clades. In Lane et al. (1996)
, North American Astereae form five distinct clades, four of which are unresolved, relative to a basal group. Both analyses also support relationships between Heterotheca and Chrysopsis, Astranthium and Townsendia, and Grindelia and Machaeranthera. Both analyses also show Aphanostephus and Conyza nested within Erigeron. Among the major disagreements, the ITS data support relationships between Symphyotrichum and the Grindelia–Machaeranthera clade, and between Chrysopsis–Heterotheca and Conyzinae, neither of which are supported by the restriction site analysis. Comparisons beyond these are tentative, and the inadequate overlap in taxa makes many comparisons impossible.
Our results are equivocal regarding the identity of putative ancestors and geographical origin of North American Astereae. Trees (e.g., Fig. 3) consistently place Podocoma notobellidiastrum, Laennecia sophiifolia, and Lagenifera panamensis closest to the base of the North American clade, supporting a possible South American origin for North American Astereae; Podocoma (nine species) occurs in southern South America (Nesom and Zanowiak, 1994
), and Laennecia sophiifolia represents a northern extension of a primarily South American genus. Interestingly, Laennecia Cass. is one of only two Astereae genera with single species distributed more or less continuously between North and South America (Nesom, 1994b
). Lagenifera Cass., in contrast, is principally a Southeast Asian genus. Lagenifera panamensis, examined here, is thought to represent a radiation of the genus in South and Central America (Nesom, 1994b
). Conclusions regarding the affinities of these three putative ancestors at this time is unwarranted, as bootstrap support for the nodes below the North American clade is uniformly below 50%. The loose alliance of Doellingeria with other North American taxa (node 2, Fig. 3, Table 2) is also curious, as the genus occurs in both Asia and North America. It is possible that further investigation of Doellingeria and its relatives will lend insight into the identity of the ancestors of North American Astereae.
Aster s.l.
ITS data indicate that Eurasian Aster s.l. representatives A. amellus, Kalimeris, and Crinitaria (Asterinae, sensu Nesom, 1994b
), are members of the ITS Southern Hemisphere grade. In contrast, Aster s.l. sampled from North America (Doellingeria, Oreostemma, Sericocarpus, and Symphyotrichum [Symphyotrichinae, sensu Nesom, 1994b
]) occur in the North American clade. The distinction of Eurasian and North American Aster supports Nesom's (1994b
, c
) hypothesis that Aster s.s. is restricted to Eurasia and not closely related to North American segregate genera and does not support the alternative hypothesis of Xiang and Semple (1996)
, based on chloroplast restriction site data, that Aster taxa on the two continents are intimately related. If ITS data had supported Xiang and Semple (1996)
, A. amellus would have appeared among North American taxa, presumably near to Sericocarpus. As noted above, the affinity of Asterinae (sensu Nesom) with Southern Hemisphere taxa, rather than with North American plants, is also supported by achene morphology; and it is further notable that Aster s.s. shares with other Eurasian Asterinae a specific NOR (nucleolar organizer region) chromosome morphology that is distinct from that observed for North American species of Aster s.l. (Huziwara, 1967
). In addition, ITS data shows that Aster s.s. and Kalimeris are closely related, as predicted by crossing and limited phylogenetic studies (Tara, 1979
; Ito et al., 1995
). Morphological similarities between the two genera are detailed by Gu and Hoch (1997)
. It is also interesting that Aster s.s. and Kalimeris occur separately from Crinitaria in the ITS Southern Hemisphere grade. This suggests that Old World Aster is polyphyletic, with independent lineages derived from possibly distant Southern Hemisphere ancestors. It is unclear why the ITS results differ so markedly from the chloroplast results (Xiang and Semple, 1996
) in the positioning of Aster amellus relative to North American taxa. Regardless, the conflict between the data sets suggests that the precise relationship of Old World to New World Aster species should be further investigated. We hope that these results encourage detailed molecular phylogenetic studies that include numerous worldwide representatives of the group, so that evolutionary relationships within Aster can be robustly determined.
The ITS data support the contention of both Xiang and Semple (1996)
and Nesom (1994b
, c
) that North American Aster s.l. is polyphyletic because the four representatives studied occupy three distinct clades within the North American clade (Fig. 3). Doellingeria occurs at the base of the North American taxa, Sericocarpus is sister to Solidago and Chrysothamnus, and Oreostemma and Symphyotrichum occur basal to Grindelia and Machaeranthera. Although the specific relationships among North American Aster supported in the ITS phylogenies differ from those proposed by Xiang and Semple (1996)
, meaningful comparisons should await analyses that include more comparably sampled taxon data sets.
Prospectus
The results presented here for Astereae appear to provide yet another example of macromolecules yielding powerful insight into geographically based phylogenetic issues (e.g., Baldwin, 1992
, 1997
; Chase et al., 1993
; Knox and Palmer, 1995
; Ballard, Carlquist, and Sytsma, 1996
). Because of the presumed high levels of morphological uniformity and high levels of parallelism (as in ray color) in the Astereae, understanding of the interrelationships among the major groups had been subject to speculation for the most part. Analysis of ITS sequence variation reveals considerable geographically based structure within the tribe. North American Astereae, in particular, appear to be closely interrelated and well separated from principal Southern Hemisphere and Eurasian lineages. The results also suggest that ancestral Astereae may have been shrubs or trees and that the tribe originated in the Southern Hemisphere. Clearly, the results here are only preliminary. The 55 taxa included in this analysis represent <2% of all Astereae species, and many species with tenuous tribal membership were not included. In particular, detailed studies of large and poorly known Southern Hemisphere Astereae (ongoing, T. Lowrey, University of New Mexico, personal communication) will almost certainly bring to light many evolutionary trends for that part of the tribe and may help to characterize ancestral taxa.
Many of the lower nodes in the ITS North American clade are poorly resolved, and thus the interrelationships among the several groups of taxa are unclear. Poor phylogenetic resolution and support for a common origin among a set of morphologically diverse taxa are consistent with patterns commonly interpreted as evidence for adaptive radiation (e.g., Hodges and Arnold, 1994
; Francisco-Ortega, Jansen, and Santos-Guerra, 1996
; Kim et al., 1996
; Baldwin, 1997
). On the other hand, the high genetic distance among North American taxa relative to Southern Hemisphere grade plants and skewed maximum parsimony phylograms, evidently resulting from long branches within some groups (Fig. 4), suggest an increase in the rate of molecular evolution for North American Astereae. Coincidentally, most shrub and tree Astereae apparently belong to the basal group and Southern Hemisphere grade; North American taxa are predominantly perennial herbs and annuals. Therefore, it is possible that an increase in the rate of molecular evolution among North American plants could, in part, be explained by reduction in generation time. While it is conceivable that the observed ITS patterns reflect the influence of both adaptive radiation and increased rate of molecular evolution, factors with profound phylogenetic effects, such as hybridization (e.g., McDade, 1992
; Rieseberg and Morefield, 1995
) or taxon sampling, have yet to be evaluated. Without additional data, only speculation is possible. Therefore, work is currently ongoing to develop a chloroplast sequence data set that will serve to test the phylogenetic patterns revealed by ITS, help resolve conflicts between the ITS and other published data sets, and, hopefully, lend further insight into evolutionary patterns in the tribe.
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