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Systematics |
Nationaal Herbarium Nederland, Universiteit Leiden, P.O. Box 9514, 2300 RA Leiden, The Netherlands
Received for publication October 30, 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: Asteraceae • nr ITS • sect. Jacobaea • Senecio • Senecioninae • systematics • trnK • trnL-F
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Monographs with subdivisions into subgenera or sections could be a good starting point to select such groups (Reznicek, 1990
). Bohs and Olmstead (1997)
, however, mention four problems with this strategy. Many large genera have no modern worldwide monographs and traditional taxonomic subdivisions often prove to be highly unnatural. Furthermore, many species in speciose genera are often not ascribed to subgeneric or sectional groups. Finally, even if the group under study is monophyletic, it is a very difficult task to select suitable outgroup species among the many other species in the genus, because phylogenetic relationships to other groups are generally unknown. Crins (1990)
, Oliver and Oliver (1991)
, Linne von Berg et al. (1996)
, and Miller and Bayer (2001)
reached similar conclusions for Carex, Erica, Allium, and Acacia, respectively.
Senecio L. (Asteraceae; Senecioneae) is one of about 50 plant genera comprising over 500 species (Mabberley, 1997
). Senecio has a worldwide distribution and many of its species are notorious for their poisonous pyrrolizidine alkaloids (PAs) that are responsible for killing more grazing livestock than all other poisonous plants together (Jeffrey, 1978
). The large size of the genus (between 1000 and 3000 species; Jeffrey et al., 1977
; Nordenstam, 1978
; Bremer, 1994
; Vincent, 1996
) has troubled attempts to make an infrageneric classification of Senecio, and therefore, the evolutionary history of this genus is still poorly known. A second problem is the remarkable amount of morphological variation in habit, leaf shape and texture, indument and flower color, both between and within species of Senecio (Barkley, 1978
). Moreover, interspecific hybridization is presumably widespread in this genus and other genera in the Senecionineae (Barkley, 1978
, 1988
; Bain and Jansen, 1995
; Comes and Abbott, 1999
). These problems have prevented attempts to make a modern worldwide monograph of Senecio and/or of its infrageneric groups. Furthermore, the majority of the approximately 150 sections of Senecio are not distinguished from one another by unique character states or syndromes of character states. This has resulted in quite a number of vague section circumscriptions and compositions that are extremely variable and not mutually exclusive. Sectional divisions in Senecio should therefore be regarded as informal groups of species that share a resemblance in their gross morphology rather than being putative monophyletic groups. Jeffrey tried to overcome these problems with the infrageneric classification of Senecio by selecting "modal species around which other species might reliably be clustered..." (Jeffrey et al., 1977
, p. 48). In this approach, groups of presumably related species were made on the basis ofmacro- and micromorphological characters and "some fifteen years' working knowledge of the group" (Jeffrey et al., 1977
, p. 48). These groups are supposedly suitable operational taxonomic units to use in following phylogenetic analyses (Jeffrey, 1992
). Such analyses were, however, never published and it thus remains to be tested if Jeffrey's classification reflects the evolutionary history of Senecio.
Although the phylogeny of the entire genus Senecio has never been studied, there are several studies of the phylogeny of selected groups within the tribe Senecioneae. These studies, based on both morphological and molecular data, show that Senecio is paraphyletic or even polyphyletic (Knox and Palmer, 1995
; Kadereit and Jeffrey, 1996
). Because classifications should be based on groups of evolutionary historical reality to be of universal value for biological research in general (Sanders and Judd, 2000)
, monophyletic satellite genera such as Kleinia, Packera, and Dendrosenecio are nowadays usually split off from Senecio sensu lato (s.l.) in the process to arrive at a monophyletic delimitation of Senecio sensu stricto (s.s.) (Jeffrey et al., 1977
; Jeffrey, 1979
; Bremer, 1994
). Until now, however, the generic classification in the Senecioneae is largely unresolved.
Because of its enormous size, studies on the evolutionary history of Senecio s.l. should be confined to infrageneric groups of this genus of which the phylogenetic history can be reconstructed within a relatively short period of time. The assemblage consisting of Senecio jacobaea and closely related species is a suitable candidate for such studies. Senecio jacobaea is a widespread herb, sometimes a noxious weed, with its main distribution in western Eurasia. This species has been introduced into North and South America, South Africa, Australia, and New Zealand (Harper and Wood, 1957
; Bain, 1991
). Senecio jacobaea is an important pest in many countries and is well known for its toxic PAs (Bain, 1991
). Therefore, information about the phylogeny of the group of S. jacobaea and closely related species can provide important insights into the evolution of PAs in this assemblage.
Senecio jacobaea is the type species of sect. Jacobaea (Mill.) Dumort. This section is usually circumscribed as a section of perennial or rarely biennial herbs, with a subglabrous to floccose indument. The leaves of these plants are more or less pinnately incised. Supplementary bracts are usually more than one-quarter as long as the involucre of the flower heads. The achenes are subcylindrical and hairy or glabrous (Chater and Walters, 1976
). Unfortunately, section descriptions of sect. Jacobaea by different authors (e.g., Chater and Walters, 1976
; Jeffrey, 1992
; Shishkin, 1995
) are well applicable to many species from other sections of Senecio s.s. This resulted in many different views of the species composition of sect. Jacobaea (see Table 1 for an overview of the main taxonomic subdivisions).
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to include the species of sect. Jacobaea in sect. Senecio s.l.
Recent studies using plastid restriction fragment length polymorphism (RFLP) data (Knox and Palmer, 1995
; Kadereit and Jeffrey, 1996
), however, suggest a remarkably different phylogenetic position of sect. Jacobaea than would be expected from morphological data. In these RFLP studies, sect. Jacobaea appears to be only distantly related to other mainly European sections of Senecio s.s. Relatively close allies seem to be the succulent sect. Rowleyani and the genera Delairea, Gynura, Kleinia, Packera, and Solanecio. Unfortunately, these relationships are only weakly supported and the result of studies with a very poor taxon sampling with respect to members of sect. Jacobaea, due to their aim to reconstruct the phylogeny of other taxonomic groups in the tribe Senecioneae (Dendrosenecio and Senecio sect. Senecio s.l., respectively).
The main aims of this study were to use phylogenetic analyses of DNA sequence data to investigate (1) the species composition and (2) the interspecific relationships of sect. Jacobaea and (3) to study the phylogenetic position of this section in the tribe Senecioneae. We used a broad taxon sampling strategy and DNA sequence data from the trnT-L intergenic spacer, the trnL intron, two parts of the trnK intron, flanking both sides of the matK gene (plastid genome), and the ITS1–5.8S-ITS2 region (nuclear genome). The trnT-L and ITS regions were selected because they have proven their usefulness in phylogenetic studies at the species and genus levels in Senecioneae (e.g., Bain and Golden, 2000
; Alvarez Fernandez et al., 2001
). The trnK region was chosen because this region has been used for phylogenetic reconstructions at a variety of taxonomic levels in angiosperms, among which is Asteraceae (Denda et al., 1999
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Blennosperma nanum was used as outgroup. This species belongs to the subtribe Blennospermatinae, which is assumed to be a sister group to the other two subtribes (Bremer, 1994
; Barkley, 1999
; but see Alvarez Fernandez et al., 2001
).
Sources of plant material
Plant material was obtained from various sources. Most plants were grown from seeds in the Hortus Botanicus Leiden. These seeds were acquired from seedbanks of botanical gardens and private collections. Some plants were collected in the field. Tissue samples of these plants were preserved on silicagel until DNA extraction. Other accessions included DNA or tissue samples kindly contributed by coworkers in Senecioneae and herbarium specimens from L and SOM (acronyms following Holmgren, Holmgren, and Barnett [1990
]). Voucher specimens are archived on the American Journal of Botany Supplementary Data Site (http://ajbsupp.botany.org/v89/pelser.xls).
DNA extraction, PCR amplification, and sequencing
Total genomic DNA was extracted from tissue samples according to the method of Doyle and Doyle (1987)
or using DNeasy Plant Mini Kit (QIAGEN, Leusden, The Netherlands). In total, four plastid regions (the trnT-L intergenic spacer, the trnL intron, and two regions of the trnK intron) and three nuclear regions (ITS1, 5.8S, and ITS2) were sequenced.
Polymerase chain reaction (PCR) and sequencing amplification of the trnT-L intergenic spacer and trnL intron were performed with primers a, b, c, and d, designed by Taberlet et al. (1991)
. In total 36 cycles of PCR amplification were carried out under the following conditions: denaturation for 45 s at 94°C, annealing for 45 s at 48°C, and extension for 2 min at 72°C. The trnT-L intergenic spacer was only sequenced for 37 taxa due to PCR amplification problems.
The 5' and 3' trnK intron (flanking sequences of matK) were partially sequenced using primers 39F (5'-TGCGGCTAGGATCTTTTACACA-3'), 546R (5'-TTTTTCAACCCAATCGCTCTTT-3'), 1023F (5'-GATTTGGGCCGATTTCTC-3'), and 1559R (5'-GCACACGGCTTTCCCTCTG-3'), designed by the authors. The conditions were as follows: denaturation for 1 min at 94°C, annealing for 30 s at 52°C, and extension for 1 min at 72°C. Both regions were PCR amplified in 29 cycles.
Internal transcribed spacer (ITS) regions 1 and 2 and the 5.8S gene were PCR amplified and sequenced with primers ITS4 and ITS5 (White et al., 1990
). The PCR amplification conditions were: 36 cycles, denaturation for 1 min at 94°C, annealing for 1 min at 52°C, and extension for 1 min at 72°C.
The sequenced samples were run on an ABI 377 automated sequencer (Applied Biosystems, Nieuwerkerk a/d IJssel, The Netherlands) using standard dye-terminator chemistry following the manufacturer's protocol. All DNA sequences were submitted to GenBank (see http://ajbsupp.botany.org/v89/pelser.xls for accession numbers) and Treebase.
DNA sequence alignment
DNA sequences were aligned using the "Clustal" option in Megalign 4.03 (DNAstar, 1999, Madison, Wisconsin, USA) and by subsequent manual adjustment where necessary. The aligned data sets are available from the first author. Sequence limits of the trnL intron, ITS1, 5.8S, and ITS2 were determined with known sequences of trnL, 18S, 5.8S, and 25S (Shinozaki et al., 1986
; Baldwin, 1992
; Kim and Jansen, 1994
). Pairwise sequence divergence values were calculated with PAUP* version 4.0b8 (Swofford, 2001
).
Phylogenetic analyses
Phylogenetic analyses were performed with PAUP*, using the maximum parsimony algorithm with the option "Heuristic search." Regions of the alignment in which homology could not be assessed reliably were excluded from the analyses (Table 2). Transitions and transversions were equally weighted. The data matrix for the phylogenetic analyses was composed of the DNA sequence alignment in which insertion/deletion events were treated as missing data, and an additional matrix in which gaps were binary coded for their presence or absence. Initial trees in the heuristic search were built ten times with a random addition sequence of the taxa. The options tree bisection-reconnection (TBR) branch-swapping, ACCTRAN optimization, and MULPARS were invoked. Bootstrap support (BS) for monophyletic groups (Felsenstein, 1985
) was calculated with 5000 bootstrap replicates, using the same settings as for the general heuristic search analyses, but holding only ten trees per replicate. Decay values (Bremer, 1988
; Donoghue et al., 1992
) were calculated with AutoDecay 4.0.2 (Eriksson, 1998
) and PAUP* using 100 addition sequence replicates for each run. All seven DNA regions were analyzed separately and combined. Partition homogeneity tests (1000 replications, saving ten trees per replicate; Farris et al., 1995
), as is implemented in PAUP*, were used to test for incongruence between the phylogenetic signal of individual data sets.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Phylogenetic analyses
Partition homogeneity analyses showed that all sequenced plastid regions (the trnT-L intergenic spacer, the trnL intron, and the two parts of the trnK intron) are congruent (P = 0.38), as is also true for the nuclear partitions (ITS1, 5.8S, and ITS2; P = 0.25). There is, however, significant incongruence between the plastid data set and the nuclear dataset (P = 0.0001).
Parsimony analysis of the combined plastid regions yielded more than 200 000 most parsimonious trees (MPTs) with a length of 519 steps, a consistency index (CI) of 0.78 (0.63 excluding uninformative characters) and a retention index (RI) of 0.82. Analysis of the combined nuclear data resulted in 640 MPTs with a length of 1591 steps, a CI of 0.46 (0.42 excluding uninformative characters) and an RI of 0.68 (Table 3; cladograms not shown).
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) resulted in a total of 18 MPTs. These trees require 2168 character state changes and have a CI of 0.53 (0.44 excluding uninformative characters) and an RI of 0.69 (Table 3). A strict consensus tree accompanied by BS values, decay indices, and branch lengths is given in Fig. 2. This cladogram shows that the clade S. abrotanifolius–S. adonidifolius–S. incanus–S. minutus is the most basal group in sect. Jacobaea. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Our findings of the percentage of VP and PIP and the CI and RI values are in accordance with the results of other studies in Asteraceae using nr ITS and/or trnL-F sequences (e.g., Baldwin, 1992
; Kim and Jansen, 1994
; Panero et al., 1999
; Bain and Golden, 2000
; Francisco-Ortega et al., 2001
). Alvarez Fernandez et al. (2001)
found similar differences in the number of PIP and the extent of homoplasy between nr ITS and trnL-F sequences in their study of Doronicum (Senecioneae).
Species composition of sect. Jacobaea
A total of 14 taxa included in our study prove to be closely related to S. jacobaea. Two of these species, together with S. jacobaea, were already generally considered to form the core of sect. Jacobaea (Table 1). These two species are S. aquaticus and S. erucifolius, which have a Eurasian distribution. The other 12 taxa were previously ascribed to other sections of Senecio s.s. or have not been consistently placed in sect. Jacobaea. These taxa include S. alpinus, S. pancicii, and S. subalpinus, three morphologically similar taxa growing in central and east European mountainous regions. All three species have been ascribed to sect. Jacobaea, but S. alpinus and S. subalpinus have also been placed in other sections (Table 1). Senecio abrotanifolius and S. adonidifolius are two European species that are frequently ascribed to sect. Jacobaea (Table 1). These species prove to be closely related to S. jacobaea as well.
Senecio minutus is the only annual species in this group of close relatives of S. jacobaea. This species grows in south and central Spain and was previously placed in sect. Senecio s.l. (De Candolle, 1838
) and sect. Delphinifolius (Chater and Walters, 1976
) (Table 1). Senecio minutus shares the finely pinnatisect leaf shape of S. abrotanifolius and S. adonidifolius. This character is also present in S. delphinifolius, a species that was ascribed by Muschler (1909)
and Konechnaya (1981)
to sect. Jacobaea and that could not be included in our analyses. Senecio cannabifolius grows in northeastern Asia and was placed by De Candolle (1838)
, Kitamura (1942)
, and Koyama (1969)
in sect. Jacobaea (Table 1).
Senecio paludosus has been listed in sect. Jacobaea (Dumortier, 1827
; Kitamura, 1942
; Koyama, 1969
), but is more commonly regarded as a member of sect. Doria s.l. (including. sect. Crociserides, Crociseris, Oliganthi, Pseudo-Oliganthi, Sarracenici, and Umbrosae; Table 1), because of its entire leaves (Boissier, 1875
; Hoffmann, 1890–1894
; Chater and Walters, 1976
; Jeffrey, 1992
; Shishkin, 1995
). This species has a Eurasian distribution.
Senecio chrysanthemoides is a species that grows in the Himalayas. De Candolle (1838)
placed this taxon in his series Indici because of its geographic distribution. Hooker (1881)
and Koyama (1969)
placed S. chrysanthemoides in sect. Jacobaea (Table 1).
Finally, three species formerly ascribed to sect. Incani are grouped together with core members of sect. Jacobaea. Senecio incanus from the central and east European mountain areas is the type species of sect. Incani. Senecio incanus was already ascribed by Reichenbach (1831–1832)
, Rouy (1903)
, and Jeffrey (1992)
to sect. Jacobaea (Table 1). Konechnaya (1981)
placed S. carniolicus in sect. Jacobaea. This taxon is closely related to S. incanus and sometimes regarded to be a subspecies of S. incanus (e.g., Chater and Walters, 1976
). Senecio cineraria is usually ascribed to sect. Incani as well (Kitamura, 1942
; Chater and Walters, 1976
; Shishkin, 1995
), but Boissier (1875)
, Rouy (1903)
, Konechnaya (1981)
, and Jeffrey (1992)
suggested that this species is better placed in sect. Jacobaea (Table 1). Senecio ambiguus is morphologically very similar to S. cineraria and was ascribed by Jeffrey (1992)
to sect. Jacobaea (Table 1). Both species have a Mediterranean distribution. Senecio cineraria is also locally naturalized elsewhere. Our results show that sect. Incani is not monophyletic. This section is characterized by the tomentose, grey indument of the leaves, stems, and capitula, a character that proves to be homoplasious when plotted on the molecular phylogeny presented here.
We propose here to include the 12 species mentioned above in sect. Jacobaea. These species are closely related to the core species of sect. Jacobaea and form together a strongly supported monophyletic group (99% BS).
Unfortunately, due to a lack of plant material of some species and the large number of species, not all European taxa of Senecio could be included in our analyses. Because of this, and the fact that macromorphological synapomorphies could not be found for sect. Jacobaea in the present study, additional members of this section might be identified in future studies.
Phylogeny of sect. Jacobaea
The typical element of sect. Jacobaea is formed by eight very closely related taxa: S. alpinus, S. ambiguus, S. aquaticus, S. chrysanthemoides, S. cineraria, S. jacobaea, S. pancicii, and S. subalpinus. Phylogenetic relationships between these taxa could not be resolved satisfactorily, due to a high degree of genetic uniformity in both the plastid and nuclear DNA regions studied (average pairwise sequence divergence between 0.1 and 0.8%). Senecio erucifolius is placed basal to this clade of closely related species.
There are two alternative hypotheses about the most basal clade of sect. Jacobaea. The analysis of the plastid data set (trnT-L intergenic spacer, the trnL intron, and two parts of the trnK intron) suggests that the clade composed of S. cannabifolius and S. paludosus is the sister group of all other species in sect. Jacobaea. The nuclear ITS data, however, indicate that the clade formed by S. abrotanifolius, S. adonidifolius, S. incanus, and S. minutus is the most basal clade of the section. Both competing hypotheses are supported with moderate to high BS (79 and 95%, respectively). Separate parsimony analysis of the plastid data set yielded more than 200 000 MPTs and could not be swapped to completion. A heuristic search through the nuclear data set resulted in 640 MPTs. A combined data set of plastid and nuclear data supports the basal placement of the S. abrotanifolius–S. adonidifolius–S. incanus–S. minutus clade with a BS of 91%. This combined analysis results in fewer MPTs (only 18), which are more resolved, and the resulting strict consensus tree also has a better resolution, when compared with analyses of the individual data sets. Furthermore, such a combined analysis gives a higher overall BS (Table 3; Figs. 1–2). We are therefore in favor of combining both data sets, despite the results of the partition homogeneity test (Soltis et al., 1998
; Yoder, Irwin, and Payseur, 2001
).
Detailed morphological studies have not been performed in the present study and macromorphological synapomorphies for sect. Jacobaea have not been identified. Nonetheless, species resembling each other in general habit are grouped together (e.g., S. bicolor and S. ambiguus). More elaborate studies need to be performed to get a better view of the systematic value of morphological characters in sect. Jacobaea.
All species in sect. Jacobaea, except for S. cannabifolius and S. chrysanthemoides, have their main distribution in Europe. Phylogeographic patterns in this group are, however, not distinguishable.
Phylogenetic position of sect. Jacobaea within the subtribe Senecioninae
The results of the present study are not conclusive about the sister group relationships of sect. Jacobaea. The genera Emilia, Packera, and Pseudogynoxys form the sister clade of sect. Jacobaea in the strict consensus cladogram of the combined plastid and nuclear data (Fig. 2), but this relationship is only weakly supported (<50% BS).
Knox and Palmer (1995)
, using plastid RFLP data, identified a sister group relationship between sect. Jacobaea and a succulent group of Senecioninae (Senecio sect. Rowleyani and the genus Delairea). This last group formed together with species of Gynura, Kleinia, Packera, and Solanecio a paraphyletic group in which sect. Jacobaea is nested. Our data, however, support the monophyly of the clade composed of Gynura, Kleinia, Solanecio (termed the "Gynuroid complex" by Jeffrey [1986
]), and the succulent Senecio s.s. sections Delairea, Peltati, and Rowleyani. The supposed close relationship between this clade and sect. Jacobaea cannot be confirmed here.
Kadereit and Jeffrey (1996)
also included Emilia, Kleinia, and Pseudogynoxys in their plastid RFLP analysis. In their study, Kleinia groups together with members of sect. Senecio s.l. and sect. Doria s.l. Emilia and Pseudogynoxys are placed basal to this clade and the clade formed by members of sect. Jacobaea. Their analysis is, however, not accompanied by branch support values, so that we have no indication of the statistical support for these findings.
In the most recent phylogenetic survey of the genus Packera, Bain and Golden (2000)
found that their accession of S. jacobaea is sister to Packera and Pericallis. Although this relationship is well supported (87% BS), taxon sampling of Old World Senecio s.l. species was confined to S. jacobaea and Pericallis cruenta in their study. According to the results of our analyses, Pericallis groups together with Cineraria, Dendrosenecio, S. lineatus, and S. scandens, and this clade is placed much more basal in the Senecioninae. These findings are, however, only weakly supported (<50% BS). Pericallis and Packera share a "helianthoid" pollen type (Skvarla et al., 1977
; Bain and Walker, 1995
) in which the exine is filled with small holes or internal foramina. This character is more typical for species from the subtribe Tussilagininae, as representatives of the Senecioninae typically have "senecioid" pollen without these holes (Bain, Tyson, and Bray, 1997
; Bain and Golden, 2000
). Determination of the pollen type of species of sect. Jacobaea might provide additional information about the phylogenetic position of this section. A close relationship between sect. Jacobaea and Packera is also suggested by the delimitation of sect. Jacobaea by Chater and Walters (1976)
. They included Packera cymbalaria under its synonym S. resedifolius in sect. Jacobaea. Phylogenetic analyses based on nuclear ITS sequence data (Bain and Jansen, 1995
; Bain and Golden, 2000
), however, show that Packera cymbalaria is nested within Packera.
In contrast to the overall macromorphological resemblance between species of sect. Jacobaea and sect. Senecio s.l. (Alexander, 1979
), molecular data presented here and by Knox and Palmer (1995)
and Kadereit and Jeffrey (1996)
clearly indicate that both sections are only distantly related. Our molecular data show that sect. Senecio s.l., represented in this paper by S. squalidus, S. sylvaticus, S. viscosus, and S. vulgaris, is more closely related to representatives of sections from Australia, North America, and Africa. Senecio squalidus and S. nebrodensis (the latter one not included in our analyses) are two examples of species that were previously included in sect. Jacobaea, but now prove to be nested within sect. Senecio s.l. based on molecular data. Senecio squalidus was included in sect. Jacobaea by Dumortier (1827)
and Chater and Walters (1976)
. Chater and Walters (1976)
also placed several taxa in sect. Jacobaea that are supposed to be closely related to S. squalidus (S. aethnensis, S. cambrensis, S. fruticulosus, and S. siculus; Ashton and Abbott, 1992
; Harris and Ingram, 1992a
, b
; Abbott et al., 2000
). Hybrids between S. jacobaea and S. squalidus have been recorded (Harper and Wood, 1957
), but Benoit, Crisp, and Jones (1975)
concluded that these observations are erroneous. The results of the present study and those of, e.g., Kadereit and Jeffrey (1996)
and Comes and Abbott (1999)
indicate that S. squalidus and its close allies are better placed in sect. Senecio s.l. Senecio nebrodensis from the mountains of Spain is another species that was erroneously ascribed to sect. Jacobaea by De Candolle (1838)
, Boissier (1875)
, Hoffmann (1890–1894)
, Muschler (1909)
, and Chater and Walters (1976)
. This species is, however, closely related to S. viscosus (Emig and Kadereit, 1993
; Kadereit et al., 1995
; Purps and Kadereit, 1998
; Comes and Abbott, 1999
) and firmly nested within sect. Senecio s.l. (Kadereit and Jeffrey, 1996
; Comes and Abbott, 1999
).
Also, many species of the Eurasian sect. Doria s.l., which are generally characterized by their undivided leaves and the low number of ligules, have been associated with species of sect. Jacobaea (Table 1). It is not clear to us why species of sect. Doria s.l. have been regarded to be close allies of S. jacobaea, since both groups are very different in their overall morphology. Results of our analysis and those of Knox and Palmer (1995)
and Kadereit and Jeffrey (1996)
confirm that the sections are not closely related.
Phylogenetic surveys of the Senecioneae (Bremer, 1994
; Knox and Palmer, 1995
; Kadereit and Jeffrey, 1996
; Panero et al., 1999
; Bain and Golden, 2000
, and this study) clearly show the problems of tackling speciose genera. The enormous size of the Senecioneae makes it very difficult to reach representative sampling sizes for studies into the evolutionary history of this group. Additionally, and partly because of this, the generic classification in the Senecioneae is still largely unresolved. It is obvious that Senecio s.s., as currently circumscribed, is a polyphyletic assemblage (Bremer, 1994
; Knox and Palmer, 1995
; Kadereit and Jeffrey, 1996
; Vincent, 1996
). Because only monophyletic groups of species have evolutionary historical reality (Sanders and Judd, 2000)
, a monophyletic delimitation of Senecio is of utmost importance. There are two ways to arrive at a monophyletic Senecio. One way is to transfer monophyletic groups of species from Senecio s.l. to other genera, to reinstate genera, or to raise new genera. This has been the prevailing strategy in the last three decades (e.g., Robinson and Brettell, 1973a
, b
, c
, d
, 1974
; Robinson, 1974
; Nordenstam, 1978
; Jeffrey and Chen, 1984
; Jeffrey, 1986
, 1992
). Another option is to abolish genera nested within Senecio s.l. and to transfer their species to Senecio. In either way, many nomenclatural changes are necessary, but these should only be performed when the phylogeny of the Senecioneae is sufficiently known. The present study is again another indication that a classification of the Senecioneae is only possible after a well-supported framework of the phylogeny of this group is reconstructed (Knox and Palmer, 1995
).
The results of the present study are a significant contribution to the knowledge about the evolutionary history of sect. Jacobaea and Senecio in general. (1) First, it has become clear what species are members of sect. Jacobaea, apart from the three core species that have been consistently ascribed to this section. (2) In addition to this, phylogenetic relationships within sect. Jacobaea have been clarified, although evolutionary relationships between S. jacobaea and seven of its closest relatives are not well resolved. (3) Finally, we have gained more knowledge about the phylogenetic position of sect. Jacobaea in the tribe Senecioneae.
In future studies we will examine the currently unresolved evolutionary relationships among S. jacobaea and its closest relatives and the taxonomic status of several taxa in this group in more detail, using faster evolving molecular markers (e.g., amplified fragment length polymorphisms [AFLPs]). In addition to this we will use the results of our phylogenetic analyses to gain more knowledge about the evolution of the diversity of PA types found by M. Macel (Institute of Evolutionary and Ecological Sciences, The Netherlands, unpublished data) in sect. Jacobaea. Furthermore, we will search for morphological synapomorphies for sect. Jacobaea as circumscribed here, to find additional support for this section. These characters might also contribute to a better understanding of the phylogenetic position of sect. Jacobaea and the phylogenetic value of morphological characters that are frequently used in classifications of Senecio s.l.
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FOOTNOTES |
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2 Author for reprint requests (pelser{at}nhn.leidenuniv.nl
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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