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Systematics |
2Department of Biology and Wildlife, Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775-6100 USA; 3Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland; 4National Centre for Biosystematics/Botanical Garden, Natural History Museum and Botanical Garden, University of Oslo, P.O. Box 1172, 1172 Blindern, N-0318 Oslo, Norway
Received for publication January 7, 2003. Accepted for publication May 1, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Alaska, USA • arctic plant species complex • Fabaceae • ITS • morphological characters • Oxytropis • Pleistocene glaciations • RAPDs
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Yurtsev, 1999
). The Alaskan species of Oxytropis occur in the interior and arctic areas of the state, and most of them are assigned to two major polyploid complexes, the O. arctica and O. campestris complexes, both belonging to section Orobia (Yurtsev, 1999
). Taxonomic consensus on these complexes is lacking. Only few morphological characters, both discrete and continuous, have been used to separate these two complexes: flower color (blue vs. white), flower size (small to large), number of flowers per inflorescence (few to many), and plant size and habit (tall or erect to small or procumbent) (Porsild, 1951
; Barneby, 1952
; Wiggins and Thomas, 1962
; Welsh, 1967
, 1968
, 1974
, 1991
; Hultén, 1968
; Elisens and Packerd, 1980
; Porsild and Cody, 1980
; Yurtsev, 1993
, 1999
; Cody, 1996
; Tables 1, 2). A certain degree of overlap among alternative states for these four key characters has generated this taxonomic disagreement on species' boundaries in these two complexes (see Table 2).
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, 1974
) emphasized flower size over flower color. For simplicity, the nomenclature proposed by Welsh (1991
; Table 1) is used to identify operational taxonomic units (OTUs) throughout the text. In the O. arctica complex, flowers are larger than in the O. campestris complex (see Table 2 and references therein). For example, Welsh (1968)
placed O. arctica var. barnebyana, characterized by white to fading yellow corollas, within the otherwise blue-flowered O. arctica complex because of its large flowers, and the predominantly blue-flowered O. roaldi in the otherwise yellow-flowered O. campestris complex (Welsh, 1991
). In contrast, Yurtsev (1999)
argued that Welsh (1991)
underestimated the diagnostic value of flower color. He proposed that O. arctica var. barnebyana (O. sordida subsp. barnebyana according to Yurtsev's nomenclature; Yurtsev, 1999
), with white to fading yellow corollas, is most closely related to the Eurasian yellow-flowered O. sordida (Table 1). Oxytropis arctica var. barnebyana had been included in the yellow-flowered O. campestris complex by Barneby (1952)
.
Several other Alaskan taxa are morphologically similar to the O. arctica and O. campestris complexes, but have not been considered part of them, e.g., the blue-flowered, endemic O. kobukensis, which is suggested to have affinities to O. arctica (Welsh, 1967
), and the white-flowered endemic O. tananensis, which is considered to be morphologically most similar to O. campestris (Yurtsev, 1999
).
The taxonomic confusion within and between the O. arctica and O. campestris complexes has important implications for conservation. Oxytropis arctica var. barnebyana is listed in the Alaska rare-plant field guide (Lipkin and Murray, 1997
) and is treated as rare by the Conservation of Arctic Flora and Fauna Program (CAFF; Talbot et al., 1999
). In 1996, a conservation agreement between the U.S. Fish and Wildlife Service and the U.S. Air Force was signed to cooperatively conserve populations of O. arctica var. barnebyana occurring in the vicinity of Kotzebue, Alaska (Moran, 1997
). This is the locus classicus for O. arctica var. barnebyana (Welsh, 1968
). It is therefore reasonable to ask whether O. arctica var. barnebyana really is a distinct taxon deserving protection. Similar conservation questions were addressed, for example, in the midwestern North American taxa Aconitum noveboracense (Cole and Kuchenreuther, 2001
) and Sedum integrifolium subsp. leedyi (Olfelt et al., 2001
), and the Californian Eriastrum densifolium (Brunell and Whitkus, 1997
) by using random amplified polymorphic DNA (RAPD) analysis.
Alaska was partially covered by glaciers during Pleistocene glaciations, but unglaciated areas remained in its central-western and northern portions (Dyke and Prest, 1987
; Fig. 1). Populations of Oxytropis might have survived in different North American refugia, as suggested for other arctic species such as Dryas octopetala (Tremblay and Schoen, 1999
) and Saxifraga oppositifolia (Abbott et al., 2000
). A scenario proposed to explain the likely consequences of Pleistocene glaciations on the genetic differentiation of extant species involves (1) fragmentation of formerly continuous species' distribution ranges during glacial maxima, causing differentiation among gene pools due to restricted gene flow or genetic drift, and (2) opportunistic recolonization of newly ice-free land and perhaps hybridization between formerly isolated and diverged populations (Hewitt, 1996
; Stehlik et al., 2001
, 2002
; Stehlik, 2002
). In this context, the morphologically similar, hence perhaps recently diverged taxa of the O. arctica and O. campestris complexes offer an interesting case study of the consequences of Pleistocene glaciations on the evolution of arctic taxa.
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), Saxifraga sect. Ligulatae (Conti et al., 1999
), and Primula (Conti et al., 2000
); and (2) RAPDs, which have also been used to resolve relationships among closely related taxa (Landergott et al., 2001
). Additionally, RAPD markers were used to search for genetic polymorphisms within and among Alaskan populations of the O. arctica and O. campestris complexes, with special emphasis on populations of the endangered O. arctica var. barnebyana.
The aims of our study were (1) to test whether molecular data for the Alaskan populations provide support for any of the two alternative taxonomic treatments of the O. arctica and O. campestris complexes (Welsh, 1991
; Yurtsev, 1999
), (2) to assess the relationships and taxonomic status of the threatened O. arctica var. barnebyana (Lipkin and Murray, 1997
; Talbot et al., 1999
), and (3) to infer whether the distribution of genetic variation in the O. arctica and O. campestris complexes in Alaska has a geographic component, possibly influenced by the Pleistocene glaciations.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Table 1). Taxonomically equivocal plants from NON and NOR (see Table 3 for population abbreviations) were identified as O. arctica var. barnebyana by S. L. Welsh (Moran and Meyers, 1996
).
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DNA cut with a double digest of HindIII and EcoRI).
The ITS sequences were generated in the DNA laboratory at the University of Alaska in Fairbanks. The ITS regions were amplified in 50-µL reactions by adding 1–2 ng of genomic DNA to a reaction mix that contained 3 mmol/L MgCl2, 10x polymerase chain reaction (PCR) buffer (PE Biosystems, Foster City, California, USA) containing 15 mmol/L MgCl2, 0.2 µmol/L of each dNTP, 1.25 units AmpliTaq Gold DNA polymerase (PE Biosystems), and 1 mmol/L of each amplification primer. The ITS region was amplified using primers ITS5 (5'-GGAAGGAGAAGTCGTAACAAGG-3') and C26A (5'-GTTTCTTTTCGTCCGCT-3'; Yokota et al., 1989
). The thermocycler (PE Biosystems, 2400 series) was programmed for 5 min at 95°C, followed by three cycles of 30 s at 94°C, 60 s at 55°C, and 45 s at 72°C; three cycles of 30 s at 94°C, 60 s at 53°C, and 45 s at 72°C; 20 cycles of 30 s at 94°C, 30 s at 51°C, and 45 s at 72°C, with a final extension for 10 min at 72°C. The PCR products were separated on 1% agarose gels and visualized with ethidium bromide. Amplifications were cleaned using a QIAquick PCR Purification Kit (QIAGEN, Chatsworth, California, USA) following the protocol from the manufacturer. Sequencing reactions with a volume of 10 µL were performed using the primers ITS1F (5'-TCCGTAGGTGAACCTGCGG-3'), ITS2R (5'-GCTRCGTTCTTCCATCGATAC-3'), ITS3F (5'-GCATCGATGAAGAACGYAGC-3'), and ITS4R (5'-TCCTCCGCTTATTGATATGC-3'; Baldwin, 1992
). Each reaction included 4 µL sequencing master mix (Dye Terminator Cycle Sequence Ready Reaction with AmpliTaq DNA; PE Biosystems), 5.0 µL amplified DNA template, and 1.0 µL primer with a concentration of 3.2 pmol/µL. Sequencing reactions were performed in the thermocycler with an initial denaturing step at 80°C for 4 s, followed by 25 cycles of 10 s at 96°C, 5 s at 50°C, and a final extension step of 4 min at 60°C. Samples were sequenced on a Perkin Elmer 373 automated sequencer (PE Biosystems). Sequences were manually aligned.
The analysis of RAPD markers was performed in the DNA laboratory at the Department of Biology, University of Oslo. To simplify selection of primers in this study, we used a list of 39 primers (among a total of 143) displaying reliable banding patterns in previous studies conducted in this laboratory (summarized in Fjellheim et al., 2001
). Different DNA concentrations (0.01–2.0 ng per reaction) were tested to identify repeatable PCR products. The following six primers (Operon Technologies, Alameda, California, USA) produced distinct and reproducible bands and were used for amplifications of the total set of DNA samples: A-11 (5'-CAATCGCCGT-3'), C-05 (5'-GATGACCGCC-3'), C-15 (5'-GACGGATCAG-3'), D-03 (5'-GTCGCCGTCA-3'), D-07 (5'-TTGGCACGGG-3'), and D-08 (5'-GTGTGCCCCA-3'). Amplifications were performed following Gabrielsen et al. (1997)
, with 3 min at 94°C, 35 cycles of 15 s at 94°C, 30 s at 39°C, 1 min at 72°C, and 5 min at 74°C on a Perkin Elmer GeneAMP PCR System 9600 thermal cycler (PE Biosystems). Amplification products were separated by size on 1.4% agarose gels, stained with ethidium bromide, and visualized under UV light at 302 nm. Pictures were taken with an Electrophoresis Systems Photo Documentation Camera (Fisher Scientific, Suwanee, Georgia, USA). The gels were scored conservatively, i.e., only the most reliable bands were scored (as present or absent). The reproducibility of all initially scored bands was rechecked by comparing banding patterns of individuals plants that were rerun several times: in a first and second primer test, in the main analysis, and in final reruns to check the cross-comparability among gels.
Analysis of ITS and RAPD data
Phylogenetic analyses of ITS sequence data were performed in PAUP* 4.02b2 (Swofford, 1999
) using maximum parsimony (MP) and maximum likelihood (ML) as optimization criteria. Heuristic searches under both MP and ML were conducted by randomizing the order of taxon addition 100 times, tree bisection-reconnection (TBR) branch swapping, and MULTREES option in effect. Resulting trees were rooted with O. nigrescens subsp. bryophila. Nested likelihood ratio tests conducted in the software MODELTEST (Posada and Crandall, 1998
) were used to choose the best model of nucleotide substitution for the ITS sequence data. The parameters corresponding to the selected model were then used to conduct an ML heuristic search. Statistical support for different clades in both MP and ML analyses was estimated by 1000 bootstrap replications using the fast-heuristic option in PAUP (Felsenstein, 1985
).
For the analysis of RAPD markers, the binary matrix was transformed to a similarity matrix using Dice similarity (NTSYS-PC 2.0; Rohlf, 1998
). Dice's coefficient assigns weights to matches rather than to mismatches and does not take shared absences of bands into account (Sneath and Sokal, 1973
). The similarity matrix was subjected to a clustering analysis using the unweighted pair group method with arithmetic means (UPGMA; NTSYS-PC 2.0; Rohlf, 1998
). We also analyzed the RAPDs matrix using the neighbor-joining (N-J) method (Saitou and Nei, 1987
) and evaluated statistical support for the clusters recovered both in the UPGMA and N-J trees by generating 1000 bootstrap pseudoreplicates.
We tested whether the grouping of OTUs that reflected either of the two taxonomical classifications (Welsh, 1974
; Yurtsev, 1999
) or rather the basal split in the two molecular trees (see Results and Figs. 3, 4) better explained genetic variation in the ITS and RAPD data sets. Therefore, we performed an analysis of molecular variance (AMOVA) based on untransformed sequences for the ITS data set and on squared Euclidean distances among individuals for the RAPD data set by grouping terminals according to the three criteria mentioned above: (1) classification according to Welsh (1974)
, based on corolla size; (2) classification according to Yurtsev (1999)
, based on corolla color; and (3) basal split in the ITS and RAPD trees, respectively (ARLEQUIN 2.0; Excoffier et al., 1992
, 2000
). We excluded the populations VIC and PEA from the AMOVA analysis of the RAPD data set, as these populations were represented by only one individual each and not investigated in the ITS analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The sequence divergence among taxa was very low and ranged from 0 to 1.48%. The greatest divergence was found between O. kobukensis and O. tananensis. Sequence divergence within the O. arctica complex ranged from 0% to 1.41%. Within O. arctica var. koyukukensis, the divergence ranged from 0 to 0.94%, within O. arctica var. arctica from 0.46 to 1.41%, and within the O. campestris complex from 0 to 1.40%. No sequence divergence was observed within O. campestris subsp. gracilis. The ITS sequences provided 14 variable characters, of which only 10 were parsimony informative. Intra-individual polymorphisms were detected in seven out of the 10 parsimony-informative positions and in 14 out of the 26 individuals where the ITS regions were sequenced.
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Nested likelihood ratio tests performed in MODELTEST showed that the best model of nucleotide substitution for ITS sequences was the Jukes-Cantor model (all rates equal, equal frequencies, proportion of invariable sites of 0 with gamma shape distribution 
= 0.006). These parameters were used in a heuristic search that produced three trees with a log likelihood score of –738.4506 and bootstrap values slightly higher than those from MP (Fig. 3). The ML tree showed no congruence between clades and traditionally recognized taxa, which appeared to be para- or polyphyletic. The basal split in the tree (Fig. 3), although poorly supported (bootstrap <50%), reflected geographic origin rather than taxonomic relationships; one of the two main clades in the tree contained the populations from northeastern Alaska (IBP, SAG, VAB) together with two populations from interior Alaska (CLI, MTH), and the other main group contained the populations located in western Alaska together with the remaining ones from interior Alaska.
RAPD
The UPGMA analysis based on 23 polymorphic RAPD markers scored in 89 individuals produced a phenogram in which no cluster corresponded to Welsh' or Yurtsev's taxonomical treatment (i.e., according to corolla length or color; Welsh, 1974
; Yurtsev, 1999
; Fig. 4). Different populations assigned to the same taxon always clustered with populations assigned to another taxon, except for O. tananensis. In most cases, samples from the same population clustered together.
As in the ITS analyses, populations from northeastern Alaska (IBP, SAG, VAB) grouped into one of the two major clusters identified in the RAPD-based UPGMA tree, and those from western Alaska (KOT, KUG, NOR, NON, TOR) grouped into the other cluster (Fig. 4). The two individuals from northwestern Canada (PEA, VIC) clustered together and within the group from western Alaska. The interior Alaskan CLI and MIN clustered with the northeastern populations (Fig. 4). Only five clusters were recovered in more than 500 of the UPGMA trees generated from the 1000 bootstrap replications: NOR20 and NOR24, with a bootstrap support (BS) of 52%; VAB86 and VAB89 (BS = 76%); CLI72, CLI73, and CLI74 (BS = 68%); CLI72 and CLI73 (BS = 56%); and CLI75 and CLI76 (BS = 64%).
The topology of the N-J tree was almost identical to that of the UPGMA tree, except for a few minor differences at the tips of the tree (results not shown). Most importantly, the clusters recovered in more than 500 of the N-J trees generated from 1000 bootstrap replications were identical to those supported in the UPGMA bootstrap consensus tree: NOR20 and NOR24 (BS = 52%); VAB86 and VAB89 (BS = 70%); CLI72, CLI73, and CLI74 (BS = 65%); CLI72 and CLI73 (BS = 57%); and CLI75 and CLI76 (BS = 53%).
AMOVAs
In AMOVAs performed on ITS sequences with groupings that reflected the two types of taxonomic classification (Welsh, 1974
; Yurtsev, 1999
), i.e., using corolla color and corolla size, only 2.51% (corolla size) and 0.03% (corolla color; nonsignificant; Table 4b) of the genetic variation was found between groups. Conversely, more than half of the variance (55.03%) was attributable to the between-group component of the two main clades in the ML tree, reflecting geographic origin rather than taxonomic assignment (Table 4c; Fig. 3; populations from the northern and western part of Alaska [SAG, IBP, VAB, CLI, and MTH] vs. all other populations; see Figs. 1, 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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); within the alpine Saxifraga, sect. Ligulatae (Saxifragaceae), ITS sequence divergence ranged between 0.4 and 15.6% and 247 positions were parsimony-informative (Conti et al., 1999
); the corresponding values were 0.5–17% and 238 in Iris series Californicae (Iridaceae) of the North American Pacific Coast (Wilson, 2003
) and 0–0.13% and 165 in the small Californian genus Lasthenia (Asteraceae: Desrochers and Dodge, 2003
; see also Baldwin et al., 1995
, for a review of ITS variation in angiosperms). However, the number of parsimony-informative positions in our data set is comparable to that found in the circumboreal species Saxifraga oppositifolia, where only nine parsimony-informative characters were detected (Holderegger and Abbott, 2003
).
Disconnection between genetic and morphological variation
The results of the AMOVAs suggest that genetic diversity and morphological variation, as partitioned in traditional taxonomic treatments, are disconnected within the O. arctica and O. campestris species complexes. The analysis of two types of genetic markers, ITS and RAPDs, did not support groups corresponding to the taxa proposed by Welsh (1974)
and Yurtsev (1999)
. Taxa with blue and white to fading yellow flowers were intermingled in the ITS cladogram and the RAPD phenogram (Yurtsev, 1999
), and the same was true for taxa with long and short corollas (Welsh, 1974
; Figs. 3, 4). With the exception of O. tananensis, none of the populations of traditionally recognized taxa was supported as monophyletic in the ITS and RAPD trees (Figs. 3, 4). The very low levels of genetic differentiation between traditional taxonomic units detected in the AMOVAs (Tables 4, 5) corroborated the lack of congruence between hierarchical structure in the ITS and RAPD trees and traditional taxonomic treatments (Figs. 3, 4). In the AMOVAs of ITS sequence data, corolla size and corolla color explained very low proportions of genetic variation (2.51% and a nonsignificant value of 0.03%, respectively; Table 4). Based on the RAPD data set, grouping according to corolla size (Welsh, 1974
) explained only 5.45% of the genetic variance, whereas grouping according to corolla color (Yurtsev, 1999
) explained 21.75% of the genetic variance (Table 5).
Recent and multiple origins of the Oxytropis arctica and O. campestris complexes
The grouping of populations representing the O. arctica and O. campestris complexes in the ITS and RAPD trees was more congruent with plant distribution than taxonomy. The northeastern Alaskan populations IBP, SAG, and VAB, and some interior Alaskan populations (CLI, MIN, MTH) grouped together, whereas all the populations from other North American geographic areas formed another group (including the northwestern Canadian PEA and VIC; Figs. 3, 4). There was no major discrepancy between the groupings identified in the ITS and RAPD trees. Although bootstrap support for the two basal clades of the ITS tree was below 50% (Fig. 3), geographic structure was also explained by very high levels of genetic variation among groups in the AMOVAs (55.3%), as compared to the low levels of genetic variation explained by taxonomic groupings (2.51 and 0.03%, respectively; Table 4). Similarly, the AMOVAs of the RAPD data set showed that groupings reflecting taxonomic treatments based on flower color and size explained only 5.45% and 21.75 of the genetic variation, respectively, while the basal split in the RAPD tree, reflecting geographic origins, explained 48.60% of the genetic variation (Table 5). Therefore, the results of our analyses supported neither Welsh's nor Yurtsev's classifications (Welsh, 1974
; Yurtsev, 1999
).
The proportion of genetic variation (Tables 4 and 5) explained by geographic origin in the O. arctica and O. campestris complexes is similar to or higher than that detected in other perennial plants of arctic/alpine habitats. For example, the alpine Phyteuma globulariifolium is characterized by a proportion (55%) of genetic variation between two major geographic groups (Schönswetter et al., 2002
) similar to what we found in our two Oxytropis complexes. However, in other cases, e.g., the alpine Eritrichium nanum (Stehlik et al., 2001
), Erinus alpinus (Stehlik et al., 2002
), Rumex nivalis (Stehlik, 2002
), Saponaria pumila (Tribsch et al., 2002
), and Saxifraga oppositifolia (Holderegger et al., 2002
), geography explains a much lower proportion of the genetic variation. As suggested for the arctic-alpine taxa mentioned earlier, the genetic structure of the O. arctica and O. campestris complexes has probably been influenced by the climatic changes of the Pleistocene. More specifically, our results suggest that the two major clusters in the ITS and RAPD trees comprise populations possibly representing refugial areas that remained ice-free during the Pleistocene glacial peaks. One refugium probably occurred northeast of the northern coastal ice shield of Alaska and included the ancestors of the IBP, SAG, and VAB populations (Fig. 1). Based on the present data and sampling, it is not possible either to infer the exact location of the refugium (possibly more than one) south of the northern coastal ice shield or the colonization influence of Canadian and Russian populations.
The advances of glaciers during the Pleistocene presumably led to retreat and fragmentation of formerly more continuous distributions of Oxytropis populations. Such fragmentation probably occurred, at different spatial scales, between and within the major refugia, as climatic conditions were more adverse even in regions where glaciers were absent (e.g., shorter growing seasons in colder climates). Random sorting of mutations among isolated populations probably resulted in genetic divergence of more uniform and coherent preglacial gene pools. Oscillations between warm and cold periods occurred repeatedly (Webb and Bartlein, 1992
), and, as a result, periods of isolation were followed by range expansion and secondary contact between diverged populations during interglacial warm periods.
Depending on the degree of divergence, formerly isolated taxa can be fully interfertile, whereas hybridization among genetically more distant gene pools is often accompanied by polyploidization. Such processes are known to have shaped the flora of the Arctic (McGraw, 1995
; Brochmann and Steen, 1999
; Abbott and Brochmann, 2003
) and have been demonstrated in detail for arctic species of Draba (Brochmann et al., 1992a
, b
, 1993
; Brochmann, 1993
; Scheen et al., 1999
). In the latter genus, gene flow across different ploidy levels has also been reported (Brochmann et al., 1992c
). Chromosome numbers between 16 and 96 were recorded in the Oxytropis complexes (O. arctica var. koyukukensis, O. campestris subsp. jordalii, O. kobukensis; Table 2). Although no crossing experiments have been conducted, thus leaving interspecific compatibilities unknown, the occurrence of variable and high ploidy levels and the polyphyly of most taxa in both the ITS and RAPD trees suggest a scenario of multiple formations of polyploids, possibly including hybridization among diverged Alaskan Oxytropis populations. The occurrence of intraindividual polymorphisms in the ITS sequences further suggests a potentially important role of hybridization in the evolutionary history of these species complexes. However, it is premature to draw any conclusions on the relative contributions of gene flow and homogenization among repeats to the molecular evolution of the ITS regions in Oxytropis, until evidence from multiple sequences of ITS clones and maternally inherited chloroplast DNA becomes available.
The geographic groups we observed in Oxytropis probably reflect the evolutionary role played by the northern coastal ice shield in shaping the current geographic structuring of genetic variation (Fig. 1). The close relationships between some of the interior Alaskan populations (MIN and MTH) and the northeasternmost populations may provide support for the existence of a suture zone between two major refugial areas, one north of the northern ice shield and one south and west of it.
Implications for conservation
The results of our molecular analyses have implications for the conservation and management of Oxytropis arctica var. barnebyana. This taxon is listed as rare and threatened in Alaska (Lipkin and Murray, 1997
; Talbot et al., 1999
), and there have been joint efforts to conserve the populations of its locus classicus in the vicinity of Kotzebue (KOT), Alaska (U.S. Fish and Wildlife Service and the U.S. Air Force; Moran, 1997
). Similar cases, where the conservation status of intraspecific taxa was at stake, have also been addressed using RAPD markers, e.g., in Sedum integrifolium subsp. leedyi (Olfelt et al., 2001
), Eriastrum densifolium subsp. sanctuorum (Brunell and Whitkus, 1997
), and Aconitum noveboracense (Cole and Kuchenreuther, 2001
). Sedum integrifolium subsp. leedyi was found to be clearly differentiated from its closest relatives, genetically as well as morphologically. Therefore, Olfelt et al. (2001)
recommended that this subspecies should be treated as a distinct taxonomic unit and that its populations deserved special conservation efforts. Conclusions were different for E. densifolium subsp. sanctuorum. The only known population of this taxon is genetically depauperated and neither divergent nor a member of a distinct population group (Brunell and Whitkus, 1997
); thus, no special recommendations were made on its conservation status. Similarly, levels of divergence detected by RAPDs between the rare Aconitum noveboracense and the common congener A. columbianum were so low that the treatment of both taxa as a single species was recommended (Cole and Kuchenreuther, 2001
). The results of our study suggest that O. arctica var. barnebyana is a polyphyletic taxon and that the population from the locus classicus KOT is not distinct from other populations (Figs. 3, 4). Thus, the genetic analyses performed to this point provide no support for any special conservation status for Oxytropis arctica var. barnebyana.
Summary
Neither morphological nor molecular characters can be used to establish an unambiguous taxonomy in the O. arctica and O. campestris complexes in Alaska (Table 2; Figs. 3, 4). In this group of closely related taxa, a discrete morphological character such as flower color apparently has little taxonomic value. A recent study of the molecular genetic basis of flower color in Ipomoea purpurea (Durbin et al., 2000
) showed that single mutations can cause significant changes in corolla color, suggesting that reliance on flower color as a diagnostic character might be misplaced. Studies of natural populations of Linanthus parryae with white- and blue-flowered individuals have demonstrated that flower color polymorphisms can persist under fluctuating selection both in time and space (Schemske and Bierzychudek, 2001
; Turelli et al., 2001
). The low degree of ITS differentiation we observed in the Oxytropis complexes suggests recent diversification, a pattern that has been well documented in other arctic plant lineages (Nordal and Razzhivin, 1999
; Abbott and Brochmann, 2003
; Holderegger and Abbott, 2003
). The recent origins of many arctic groups simply means that too little time has elapsed for the fixation of consistent characters that can be used to circumscribe taxa across their entire distribution areas. In these cases, traditional specific and subspecific categories may be inadequate to delimit evolutionary units that are still undergoing active diversification processes (Bachmann, 1998
).
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FOOTNOTES |
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5 Present address: Department of Botany, University of Toronto, 25 Willcocks Street, Toronto, Ontario, Canada M5S 3B2
6 Author for correspondence, present address: Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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