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Systematics and Phytogeography |
2Jepson Herbarium and Department of Integrative Biology, University of California, Berkeley, California 94720-2465 USA; 3Laboratory of Molecular Systematics, Swedish Museum of Natural History, Stockholm, Sweden
Received for publication May 14, 2002. Accepted for publication October 31, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: internal/external transcribed spacers • Malvaceae • nrDNA • phylogeny • phylogeography • Sidalcea
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Hitchcock, 1957
; Hill, 1993
) because of complex patterns of morphological variation and putative hybridization. Although most Sidalcea species are diploid (2n = 20), the occurrence of different ploidy levels (tetraploids and hexaploids) in the genus could potentially explain at least part of the confusing morphological variation within the group. Variation in vegetative morphology causes special problems for species delimitation in Sidalcea. The leaves are palmately veined, as is common for Malvaceae, but blade shapes vary depending on leaf position along the stems (proximal cauline leaves are often less deeply lobed than leaves on the distal stems). Indumentum can be highly variable within species, both in density and type (but usually includes stellate hairs, as is common in Malvaceae). Flowers and inflorescences of Sidalcea are less variable than vegetative characteristics but floral dimorphism due to gynodioecy (in which pistillate flowers are smaller than perfect flowers; see Ashman, 1994
; Graff, 1999
; Marshall and Ganders, 2001
) has been a source of confusion for delimiting species. Sidalcea flowers are protandrous and, like other Malvaceae, the filaments of the stamens are united around the style. What is probably unique for Sidalcea is that the filaments are fused into two groups near their tips.
Roush (1931)
postulated evolutionary trends for morphological characters of Sidalcea and also proposed a historical biogeographic hypothesis for the genus. She divided Sidalcea into three subgenera: "Eusidalcea" (comprising sections Annuae and Perennes), Malvastralcea, and Hesperalcea. Subgenus Hesperalcea, with the single species S. malachroides, was considered to be the most "primitive" group, retaining characteristics of the ancestor of Sidalcea such as suffrutescence, leafy branches, and only slightly lobed leaves. Subgenus Malvastralcea also contained only one species, S. hickmanii, which was suggested to be a "slightly more recent branch" than subgenus Hesperalcea and which had retained most "primitive" characteristics except for more wrinkled carpels, slight changes in stamens, and larger pollen. Section Perennes in subgenus "Eusidalcea" contained the rest of the perennial species known at that time (S. stipularis was described later, in 1974, by Howell and True).
Roush (1931)
suggested that the ancestor to modern species of Sidalcea spread northward from Mexico along two major routes, one through the Rocky Mountains and one through the Sierra Nevada foothills in California. Sidalcea candida and S. neomexicana, both of which occur in the Rocky Mountains, were proposed to have descended from an ancestor that moved along the eastern route after the divergence of lineages that gave rise to S. malachroides and S. hickmanii. The other species of subgenus Perennes were suggested to constitute a much more recent group, whose ancestor dispersed along the route through California. Section Annuae, which occurs only in California, was considered to have all advanced characteristics of the genus and hence to be a very recent offshoot from section Perennes. Hitchcock (1957)
agreed that the annuals constitute a single lineage and excluded them from his study of Sidalcea.
Sidalcea has been treated within tribe Malveae (Malvaceae) since its description (e.g., Gray, 1849
; Bentham and Hooker, 1862
; Schumann, 1895
; Fryxell, 1988
). Malveae are a diverse tribe of approximate 70 genera with equivocal generic interrelationships. Traditionally, the tribe has been divided into subtribes, with Sidalcea in subtribe Malvinae, usually together with Alcea, Callirhoë;, Lavatera, Malva, and Napaea (e.g., Bentham and Hooker, 1862
; Schumann, 1895
). Bates (1968)
abandoned the subtribal taxonomy because he thought that too much emphasis had been placed on the occurrence of filiform style branches and stigmatic position, characteristics that had been used to group Sidalcea with the genera of Malveae-Malvinae. Bates (1968)
and Bates and Blanchard (1970)
took into account the chromosome numbers in the tribe and suggested informal groupings (alliances) within Malveae. Bates (1968)
separated Sidalcea (n = 10, 20, 30) and Napaea (n = 15) from the genera of the Malveae-Malvinae and put them in the Sidalcea alliance. Later, Bates and Blanchard (1970)
transferred Napaea to an alliance of its own, based on new chromosomal data and the hypothesized evolution of chromosome numbers. They also pointed to a possible relationship of Sidalcea to Eremalche (n = 10, 20) and Urocarpidium (n = 10, 15) of the Sphaeralcea (n = 5, 10, 15, 25) alliance, although they retained the Sidalcea alliance with Sidalcea as the sole member. Fryxell (1988
, 1997)
associated Sidalcea, Callirhoë;, and Napaea in the Sidalcea alliance.
Published phylogenetic analyses of Malvaceae have included only a few taxa of Malveae but sampled members of the tribe have consistently formed a strongly supported monophyletic group (La Duke and Doebley, 1995
; Judd and Manchester, 1997
). Within the tribe, phylogenetic results have indicated problems with circumscriptions of the proposed alliances and subtribes. In a chloroplast DNA restriction site analysis of Malvaceae sensu stricto that included 14 genera of Malveae, La Duke and Doebley (1995)
showed that the Sphaeralcea alliance was not monophyletic and did not find support for other subgroups previously proposed within Malveae.
Considering the highly variable and difficult morphology of Sidalcea, we suspected that examining rapidly evolving nuclear DNA regions would shed new light on historical relationships in the group. Internal and external transcribed spacers (ITS and ETS) of 18S–26S nuclear ribosomal DNA (rDNA) are often sufficiently variable to be useful for resolving phylogenetic questions at low taxonomic levels (see e.g., Baldwin et al., 1995
; Baldwin and Markos, 1998
; Andreasen and Baldwin, 2001
). Accordingly, rDNA transcribed spacers were sequenced and analyzed to estimate the phylogeny of Sidalcea and to evaluate earlier proposed hypotheses of relationships and historical biogeography of the group.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Judd and Manchester, 1997
). Outgroup taxa (four sequences, each representing the genera Sphaeralcea, Napaea, Eremalche, and Callirhoë;) were chosen from Malveae based on morphology and on suggested close relationships of these taxa to Sidalcea (Kearney, 1951
; Bates and Blanchard, 1970
; Fryxell, 1988
, 1997
). We also attempted to use more distantly related genera, from other Malvaceae tribes (from e.g., Hibisceae and Gossypieae) as outgroup taxa.
Within the ingroup, we sampled from 24 species of Sidalcea sensu Roush (1931)
, Hitchcock (1957)
, and Hill (1993)
and from as many intraspecific taxa as possible (taxa and voucher information has been archived at the Botanical Society of America website http://ajbsupp.botany.org/v90). For many of the taxa more than one population was included to permit investigation of potential geographic and interpopulational differences. Taxa represented in the analyses by more than one population have been distinguished by arabic numerals in text and figures and can be identified with their source at http://ajbsupp.botany.org/v90.
Laboratory procedures
DNA was extracted from fresh, recently pressed, or silica-gel dried plant material or from herbarium specimens. Total DNAs were isolated using a modification of the cetyltrimethylammonium bromide (CTAB) procedure (Doyle and Doyle, 1987
) with phenol extraction and ethanol precipitation. When possible, fresh material from 10 or more plants per population was collected and pooled in the extractions to capture potential intrapopulational variation.
The ITS region was amplified with primers ITS-I (ITS-leu.1) and ITS4 and sequenced with ITS4 and ITS5-A, and for some polymerase chain reaction (PCR) products, ITS2 and/or ITS3 (White et al., 1990
; Downie and Katz-Downie, 1996
; Andreasen et al., 1999
; Urbatsch et al., 2000
) using standard procedures (see Andreasen and Baldwin, 2001
). The primer Sid-F (Andreasen and Baldwin, 2001
), approximately 550 base pairs (bp) upstream from 18S, was used together with 18S-E (Baldwin and Markos, 1998
) to amplify an ETS fragment of approximately 540 bp with the same PCR conditions as for ITS, except that the annealing temperature used was 50°C instead of 48°C. Primer 18S-E and Sid-F also were used in the ETS sequencing reactions. The ITS and ETS amplification products were cycle sequenced with BigDye (Applied Biosystems, Foster City, California, USA) or Thermo Sequenase (Amersham Pharmacia Biotech, Piscataway, New Jersey, USA) terminator cycle sequencing kits and resolved on polyacrylamide gels using an ABI Prism 377 Automated Sequencer (Applied Biosystems).
Four Sidalcea species, i.e., S. ranunculacea, S. reptans, S. campestris, and S. cusickii, were not included in the present analyses because they displayed complex patterns of different ITS and ETS copy types and of recombinants between copy types. These species, together with other polymorphic Sidalcea samples, are the subjects of a separate investigation (K. Andreasen and B. G. Baldwin, unpublished data). The exclusion of these species does not alter the conclusions presented below.
Data analysis
Resolved sequences were aligned using the Clustal method or Goroh-Myers' comparative alignment option as implemented in the software Sequence Navigator (Applied Biosystems) and adjusted by eye. Gaps were coded as missing data and each potentially phylogenetically informative indel (regardless of length) was recoded as one binary character. Identical sequences were identified and only one sequence from each set of identical sequences was included in the analyses. PAUP* 4.0b2–8 (Swofford, 1998
) was used for the parsimony analyses. Cladistic analyses of the combined and separate ITS and ETS aligned-sequence matrices were conducted using heuristic searches with at least 1000 random taxon-addition replicates, tree bisection-reconnection (TBR) branch swapping, and MULPARS option in effect. To estimate support for clades, jackknife (Farris et al., 1996
) support was estimated with 10 000 replicates, five random TBR replicates, and MULPARS off. For the combined analysis, Bremer/decay support (Bremer, 1994
) analysis was carried out using PAUP*, with batch files generated by the program Autodecay 4.0 (Eriksson, 1999
) and the same settings as for the heuristic searches above. The incongruence length difference test (ILD; Farris et al., 1994
) was performed using the programs Xarn (Farris, 1991
; Farris et al., 1994
) or PAUP* (10 000 replicates, 10 randomly selected addition sequences with TBR branch swapping) to compare the ITS and ETS matrices. Only phylogenetically informative characters were included in the test (see Cunningham, 1997
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) and close to the upper end of the range for other sequenced Malvaceae genera (the whole ITS region, 588–689; ITS-1, 218–297; ITS-2, 206–229 [Seelanan et al., 1997
]). Most length variation in Sidalcea was due to a 34-bp deletion in the middle of ITS-1 in S. calycosa and deletions at the 5' end of ITS-2 in S. hartwegii (20 bp) and S. stipularis (14 bp). Sidalcea and outgroups exhibited no extreme values in guanine and cytosine (GC) content. The GC values varied between 49% and 54% in the ingroup and between 50% and 55% in the outgroup.
The alignment of sequences from the ITS region of Sidalcea and the outgroup was straightforward and required the insertion of few gaps (EMBL accession number ALIGN_000457 for the aligned sequences; for the individual sequences see http://ajbsupp.botany.org/v90). Attempts to include more distantly related Malvaceae genera (e.g., Hibiscus and Gossypium sequences from GenBank) resulted in too much alignment uncertainty to allow their use as members of the outgroup. (Note: Seelanan et al. [1997
] suggested that the ITS region is too variable to use for family-wide studies in Malvaceae.) The aligned sequences with gaps yielded a matrix of 692 bp.
Pairwise distances (HKY85 corrected) between the sequences varied between 0 and 11% within the ingroup and up to 15% between the most divergent pair of ingroup and outgroup taxa (S. diploscypha and Callirhoë digitata). Within Sidalcea, 154 (22%) of the characters were variable, and 98 (14%) potentially phylogenetically informative (plus 13 recoded gap characters). Eighty-two of the variable characters and 53 (18% of the ITS-1 positions) of the potentially phylogenetically informative characters (plus eight recoded gap characters) were found in ITS-1. ITS-2 contributed 68 variable and 42 (19% of the ITS-2 positions) potentially phylogenetically informative characters (plus five gap characters). Only three characters (2% of the 5.8S positions) were potentially phylogenetically informative in the 5.8S gene (all of them point mutations).
ETS sequences
The 3' region of the universal ETS (upstream from the 18S subunit) was highly uniform in length between the different Sidalcea accessions; all sequences were 540 bp except for those of the two S. hartwegii accessions, which were 539 bp. Length of the sequenced ETS region in the outgroup varied from 540 to 544 bp. The number of positions in the aligned matrix (see EMBL accession number ALIGN_000458 for the aligned sequences; for the individual sequences see http://ajbsupp.botany.org/v90) including the outgroups was 550 bp. Of these positions, 148 (27%) were variable within the ingroup and 101 (18%) were potentially phylogenetically informative (plus four recoded gap characters). Variation in the sequenced ETS region was not evenly distributed; a region about 100 bp long, starting approximately 390 bp from 18S instead seems to be more conservative and less variable than the rest of the region (Fig. 1). The GC content was 47–50% for Sidalcea and 48–52% for the outgroups. Pairwise distances (HKY85 corrected) varied from 0% to 13% within the ingroup and up to 21% between the most divergent pair of ingroup and outgroup taxa (S. calycosa and Callirhoë digitata).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Brassicaceae, Bennett and Smith, 1991
; Solanaceae, Volkov et al., 1996
; Fabaceae, Bena et al., 1998
; Chandler et al., 2001
; Myrtaceae, Wright et al., 2001
). In Calycadenia (Asteraceae) the ETS region approximately 1–700 bp upstream from the 18S subunit was found to contain about 1.5 times the number of variable and potentially informative sites found in ITS-1 and ITS-2 combined (Baldwin and Markos, 1998
). In the present study the number of variable and potentially informative sites were similar for ETS and ITS. The ratio of average pairwise distances between Osmadenia (the sister-group of Calycadenia) and each taxon in Calycadenia was 1.4 (ETS) to 1 (ITS) (Baldwin and Markos, 1998
). The ETS/ITS ratio for pairwise distances (HK85 corrected) in Sidalcea was on average one for comparisons between Eremalche and representatives of the different clades in Sidalcea. It thus appears that the variation of the 3' portion of the universal ETS region in Sidalcea, in contrast to Calycadenia and various other plant groups (e.g., Medicago [Fabaceae], Bena et al., 1998
; Argyranthemum, Asteriscus, and Helianthus [Asteraceae], Linder et al., 2000
; Lessingia [Asteraceae], Markos and Baldwin, 2001
), is not greater than in the internal transcribed spacers. Similar results, showing the same general ETS region to be equally or even less variable than ITS, have been found in some investigated genera of Asteraceae, i.e., Arnica (K. Andreasen and B. G. Baldwin, unpublished data), Lasthenia (Chan et al., 2001
), Cirsium, (D. Kelch, University of California, Berkeley, personal communication), and Stephanomeria (Lee et al., 2002
). Regardless of the relative levels of variation in ETS and ITS sequences, combining data from both sources has been shown to enhance tree support and resolution and is thus a useful strategy for investigating phylogeny of young groups such as Sidalcea.
Even more so than in the ITS region (see also Baldwin et al., 1995
) variation in the sequenced ETS region is predominantly present as point mutations rather than indels in Sidalcea. The few ETS indels that were present were found in sequences of S. diploscypha and S. keckii and were only one or two base pairs long. In contrast to the conserved length of the sequenced ETS region, certain parts of the ITS-1 and ITS-2 seem to have been under relaxed evolutionary constraints, e.g., at the 5' end of ITS-2, where long (nonhomologous) deletions occur both in S. hartwegii and S. stipularis. These ITS-2 deletions occur in regions that are highly variable among angiosperms (Hershkovitz and Zimmer, 1996
) and are likely to be under less functional constraints than are other regions of the spacer.
Outgroups and incongruence
No higher-level phylogenetic analysis of Malvaceae has included Sidalcea; consequently, the sister group to Sidalcea is unknown. For the outgroup, we used taxa that have been suggested to be closely related to Sidalcea based on morphology. The outgroup attached at different branches of the ingroup in the ITS and ETS analyses. In the combined-data analysis and in the analysis of ITS alone, the outgroup attached at the branch separating S. malachroides, S. hickmanii, and S. stipularis from other taxa of Sidalcea; in the ETS analysis the root was positioned at the S. malachroides branch. The different positions of the outgroup in the ETS and ITS trees and the significant result in the ILD test only when the outgroup taxa were included may be an indication that the outgroup taxa introduced problematical homoplasy into the analysis. This potentially disruptive homoplasy was not so extensive, however, as to make sequence alignment difficult or to result in a different ingroup topology when the outgroup taxa were added to the analysis.
Clade support and unequal evolutionary rates
Lack of support for many clades of Sidalcea may reflect rapid radiation of lineages without sufficient time between branching events for mutations to accumulate. Annual clades are marked by many more mutations than are most perennial clades, as is reflected by the high support for these branches (Figs. 2, 3, and 4) relative to branches leading to perennial species. Comparisons of evolutionary rates between perennial and annual lineages in Sidalcea established that both ITS and ETS have evolved significantly faster in the annuals than in the perennials (Andreasen and Baldwin, 2001
).
Phylogeny and polyploidy in Sidalcea
The molecular evidence for monophyly of checker mallows (100% jk; Fig. 4), given the outgroup taxa sampled, supports the interpretation that the most conspicuous morphological feature unique to Sidalcea, i.e., the numerous stamens forming two distinct groups (an inner and an outer whorl), is synapomorphic for the group. The occurrence of different ploidy levels (tetraploids and hexaploids) in different clades in the trees (see Fig. 4) demonstrates multiple origins of polyploidy in Sidalcea. If a polyploid taxon displays heterogenous sequences, one possible explanation is allopolyploidy. As mentioned above, heterogenous sequences were excluded from the analyses and the samples were cloned and sequenced to investigate the origin of the polymorphisms (K. Andreasen and B. G. Baldwin, unpublished data). When nuclear sequences (such as ITS and ETS) from polyploids are not heterogenous, the plants may be autopolyploid. Alternatively, biased concerted evolution may have homogenized divergent parental ITS and ETS copies, with only one copy type remaining (or PCR drift or selection may have occurred). To investigate these possibilities further, additional data from an independent DNA region, nuclear or chloroplast, are needed.
Basal perennials
Roush's (1931)
hypothesis that the perennial species S. hickmanii and S. malachroides represent basally divergent groups within Sidalcea is upheld by our rDNA trees (the "basal perennials"; jk 76% in Fig. 4). Hitchcock (1957)
also supported the view that these two taxa were more closely related to each other than to other Sidalcea species. Sidalcea stipularis, the third species in the "basal perennials," was discovered subsequent to Roush's and Hitchcock's studies (Howell and True, 1974
) and represents an additional basally divergent lineage. Until now, no phylogenetic placement has been suggested for the morphologically distinct S. stipularis, but the beakless carpels, the tribracteolate calyx (ebracteolate or sometimes uni- or bi-bracteolate in S. malachroides), and the similarity of lower and upper leaves are characteristics otherwise found within Sidalcea only in S. hickmanii and S. malachroides.
Annuals
The suggested monophyly of the annual species of Sidalcea (Roush, 1931
; Hitchcock, 1957
) is not consistent with the results presented here (Figs. 2, 3, and 4; annuals are indicated by asterisks in Fig. 4); neither is the idea that the annuals represent young lineages relative to the perennials (Roush, 1931
). Based on the rDNA data, we conclude that the annual habit arose at least four times, probably as an adaptation to seasonally dry habitats. Alternately, if the annual habit is interpreted as plesiomorphic (the reconstruction of the ancestral state within Sidalcea is equivocal), then the annuals constitute a paraphyletic group.
The malviflora clade
The seven subspecies of S. malviflora (subspp. californica, dolosa, laciniata, malviflora, patula, purpurea, and sparsifolia) form a clade along with S. covillei, S. pedata, and S. neomexicana (the malviflora clade; jk 91%, Fig. 4), although relationships among the taxa are not well supported. The subspecies of S. malviflora are of coastal or southern distribution in California and S. neomexicana and S. pedata occur in the same areas as some of the southern subspecies. Sidalcea neomexicana is the most widely distributed species of Sidalcea. Besides occurring in southern California, S. neomexicana occurs in Oregon, Nevada, Utah, Colorado, New Mexico, and Arizona in the U.S.A. and in northern Mexico. Sidalcea covillei, on the other hand, is a rare species, occurring only in Owens Valley, Inyo County, California. The affinity between S. malviflora and S. neomexicana was suggested earlier (Roush, 1931
; Hitchcock, 1957
) but, according to Hitchcock, S. covillei and S. pedata are closer to the "S. oregana-spicata complex" than to S. malviflora. Roush (1931)
, however, recognized an affinity among S. covillei, S. malviflora, and S. neomexicana, but treated S. covillei as a variety of S. neomexicana. The position of S. malviflora subsp. sparsifolia basally to the clade (jk 76%) consisting of the other subspecies of S. malviflora plus S. pedata and S. neomexicana, provides evidence for paraphyly of S. malviflora and may justify treatment of S. malviflora subsp. sparsifolia as a separate species. The position of S. malviflora subsp. californica is not resolved in the combined analysis but it is sister to the majority of the subspecies of S. malviflora in the ITS analysis.
The asprella clade
Hitchcock's (1957)
treatment of S. asprella as a subspecies of S. malviflora resulted in a polyphyletic S. malviflora based on our analyses. In contrast to the subspecies of S. malviflora recognized here (in the malviflora clade), S. asprella has a more inland distribution and also occurs at higher altitudes, in the Sierra Nevada and northward to northwestern Oregon. Sidalcea asprella forms the informal asprella clade (jk 100%; Fig. 4), together with S. sp. nov. ("gigantea"), S. hirtipes, S. maxima, and a proposed hybrid plant (S. asprella x S. oregana).
The glaucescens clade
Taxa in the asprella clade are part of a larger group, the glaucescens clade (jk 100%; Fig. 4). The glaucescens clade also contains S. glaucescens, S. multifida, S. robusta, and S. asprella population 3 [= S. malviflora (DC.) Benth. subsp. nana (Jeps.) C. L Hitchc.], which all have inland and largely overlapping distributions. Sidalcea asprella 3 represents a taxon that was described by Hitchcock (1957)
as S. malviflora subsp. nana but was later treated by Hill (1993)
as a synonym of S. malviflora subsp. asprella, as was subsp. elegans (= S. asprella 2 in our analysis).Various taxa in the glaucescens clade are characterized by, or at least have a tendency towards, glaucous stems and leaves. Sidalcea glaucescens has the widest distribution, occurring in mountain meadows of the Sierra Nevada; S. robusta is the most narrowly distributed member of the group, found only in Butte County. The glaucescens clade corresponds to a group recognized by Roush (1931)
as the "S. asprella-S. glaucescens affinity." According to her, S. asprella, S. glaucescens, S. multifida, and S. robusta all belong to that group. Sidalcea glaucescens and S. multifida are morphologically similar, e.g., in glaucescence and leaf shape, and have been noted to intergrade (Hitchcock, 1957
; Hill, 1993
), with a "striking transition" from pedately dissected leaves to those typical of S. glaucescens (Hitchcock, 1957
). Hitchcock even suggested that S. multifida might be treated as a subspecies of S. glaucescens; the near identity of ITS and ETS sequences of the two taxa does not detract from his proposal. Sidalcea glaucescens has been suggested to interbreed with S. asprella and S. multifida (Hitchcock, 1957
; Hill, 1993
) and Hitchcock (1957)
pointed to the affinity of S. glaucescens to "S. malviflora subsp. nana" and of "subsp. asprella" sensu Hitchcock to S. robusta. Our results illustrate that the taxa in question are so closely related and minimally divergent from one another that hybridization or incomplete lineage sorting could account for mosaic variation among them.
The oregana clade
Another well-supported group is the oregana clade (jk 99%; Fig. 4), which consists of S. oregana subspp. spicata, hydrophila, and valida, and also the endangered S. nelsoniana, from Oregon. Sidalcea nelsoniana has been considered to be "not greatly unlike" S. oregana subsp. spicata (Hitchcock, 1957
, p. 65), as is also true for ETS and ITS sequences of the two taxa. Although the oregana clade is strongly supported, rDNA sequences of the component taxa are minimally divergent and no phylogenetic resolution was obtained within the group, in accord with a hypothesis of recent diversification from a common ancestor.
Historical biogeography of Sidalcea
Roush (1931)
suggested that the ancestor to the modern species of Sidalcea spread northward from Mexico along two major routes: one through the Rocky Mountains and one through the Sierra Nevada foothills. Sidalcea candida and S. neomexicana, in the Rocky Mountains, supposedly descended from an ancestor that moved along the eastern (Rocky Mountain) route after the divergence of lineages that gave rise to S. malachroides and S. hickmanii. Our results leave open the possibility that this scenario may apply to S. candida, except for the positions of the "basal annuals" and S. hartwegii, which also branch off before S. candida in our trees. Sidalcea neomexicana, however, is well nested within the malviflora clade and must represent a lineage of relatively recent descent. The simplest biogeographic interpretation of our results is that S. neomexicana originated in California and subsequently became established widely in southwestern North America and northern Mexico.
Conclusions
Improved phylogenetic resolution and support in rDNA trees of Sidalcea that include ETS and ITS sequences compared to trees based on ITS data alone confirm the phylogenetic utility of the 3' portion of the universal ETS for a young plant lineage in Malvaceae. Our results provide no support for Roush's (1931)
sections Annuae and Perennes and indicate instead that life-form evolution in Sidalcea has been highly dynamic. Our results are inconsistent with Roush's (1931)
biogeographic scenario of an early divergence of the Rocky Mountain species S. neomexicana but are consistent with subgenera Hesperalcea and Malvastralcea being basally divergent lineages within Sidalcea. Three of the five major, strongly supported clades of Sidalcea resolved in our analyses displayed low levels of sequence variation (and phylogenetic resolution) that are consistent with recent radiation of each group. Future phylogenetic studies in Sidalcea probably would benefit from analysis of rapidly evolving low-copy nuclear DNA regions, both to evaluate the present results from nrDNA and, if possible, to enhance phylogenetic resolution and support, especially within the most recently diverged clades. Phylogenetic understanding of Sidalcea should prove to be valuable for resolving evolutionary changes in sexual system in the genus, upon comprehensive characterization of patterns of sexual expression throughout the (often gynodioecious) checker mallows (see Ashman, 1994
; Graff, 1999
; Marshall and Ganders, 2001
).
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FOOTNOTES |
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4 Current address: Department of Systematic Botany, Evolutionary Biology Centre, Uppsala University, Sweden
5 Author for reprint requests (Katarina.Andreasen{at}ebc.uu.se
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Ashman T.-L. 1994 Reproductive allocation in hermaphrodite and female plants of Sidalcea oregana ssp. spicata (Malvaceae) using four currencies. American Journal of Botany 81: 433-438[CrossRef][ISI]
Baldwin B. G. S. Markos 1998 Phylogenetic utility of the external transcribed spacer (ETS) of 18S–26S rDNA: congruence of ETS and ITS trees of Calycadenia (Compositae). Molecular Phylogenetics and Evolution 10: 449-463[CrossRef][ISI][Medline]
Baldwin B. G. M. J. Sanderson J. M. Porter M. F. Wojciechowski C. S. Campbell M. J. Donoghue 1995 The ITS region of nuclear ribosomal DNA: a valuable source of evidence on angiosperm phylogeny. Annals of Missouri Botanical Garden 82: 247-277[CrossRef][ISI]
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