Tales of the unexpected: Phylogeography of the arctic-alpine model plant Saxifraga oppositifolia (Saxifragaceae) revisited

Arctic-alpine biota occupy enormous areas in the Arctic and the northern hemisphere mountain ranges and have undergone major range shifts during their comparatively short history. The origins of individual arctic-alpine species remain largely unknown. In the case of the Purple saxifrage, Saxifraga oppositifolia, an important model for arctic-alpine plants, phylogeographic studies have remained inconclusive about early stages of the species' spatiotemporal diversification but have provided evidence for long-range colonization out of a presumed Beringian origin to cover today's circumpolar range. We re-evaluated the species' large-scale range dynamics based on a geographically extended sampling including crucial areas such as Central Asia and the (south-)eastern European mountain ranges and employing up-to-date phylogeographic analyses of a plastid sequence data set and a more restricted AFLP data set. In accordance with previous studies, we detected two major plastid DNA lineages also reflected in AFLP divergence, suggesting a long and independent vicariant history. Although we were unable to determine the species' area of origin, our results point to Europe (probably the Alps) and Central Asia, respectively, as the likely ancestral areas of the two main lineages. AFLP data suggested that contact areas between the two clades in the Carpathians, Northern Siberia and western Greenland were secondary. In marked contrast to high levels of diversity revealed in previous studies, populations from the major arctic refugium Beringia did not exhibit any plastid sequence polymorphism. Our study shows that adequate sampling of the southern, refugial populations is crucial for understanding the range dynamics of arctic-alpine species.     

Effect of elevation on sexual reproduction in alpine populations of Saxifraga oppositifolia (Saxifragaceae)

Self-compatibility in high arctic and alpine areas is regarded as an adaptation to low pollinator abundance. However, high genetic variability as a consequence of outcrossing is, with regard to population persistence, favorable in highly stochastic environments such as tundra habitats. To evaluate these contradictory scenarios, I performed in situ pollination experiments to examine the breeding system of the predominant outcrosser Saxifraga oppositifolia in ten populations at two different elevations in the Swiss Alps. Pollinator limitation was detected at both elevations, but fruit set in naturally pollinated flowers was only slightly less at the higher elevation. Increased pollinator limitation at high compared with low elevation thus could not be demonstrated in this experiment. Hand-crossings yielded equal mean proportion seed set at both elevations, and so did hand-selfings. This constant pattern of the breeding system in S. oppositifolia indicates selective factors that lead to the maintenance of a high level of outcrossing even in high-elevation populations. Based on sex allocation models, it was expected that a high ovule number should be selectively advantageous in a plant-pollinator system where chance visitation or selfing play important roles. However, female reproductive offer in terms of ovule number per flower did not change from low to high elevation. Since neither increased pollinator limitation nor increased seed set in selfed flowers was found at high compared with low elevation, the prerequisites for testing the hypothesis were not given. This study contradicts the hypothesis that inimical environmental factors in alpine or arctic habitats necessarily select for increased selfing rates in a preferentially outcrossing species like S. oppositifolia. Received: 28 April 1997 / Accepted: 20 October 1997     

Variation within some Nordic Saxifraga species (Saxifragaceae)

By means of chemometric and morphometric analyses, the North and South Norwegian populations of Saxifraga paniculata are shown to be different from each other. No difference could be found between one North Norwegian and Icelandic populations of S. paniculata ; the name var. neogaea should not be used for the Icelandic plants. The enigmatic S . × blyttii from Western Norway is most probably an aberrant form of S. paniculata. S. hyperborea and S. rivularis were compared by means of morphometric analyses and a study of pollen size. Presence/ absence of stolons and pollen size are apparently sufficient as differential characters. No Icelandic material could be determined as S. hyperborea . By means of morphometric analyses, S × opdalensis was compared to its putative parents, S. cernua and S. rivularis , and shown to be well separated from them.     

A phylogenetic study ofSaxifraga sect.Saxifraga (Saxifragaceae) based on nrDNA ITS sequences

Parsimony analyses of 54 nrDNA ITS (Internal Transcribed Spacer) sequences ofSaxifraga sect.Saxifraga were performed. In addition to some unresolved clades, there is strong disagreement between the ITS phylogeny and previous classifications based primarily on morphology. The extensive cytological instability of sect.Saxifraga prevents previous cytotaxonomical results from resolving the incongruence between molecular and morphological data. Dissimilar topologies between chloroplast (matK) and nuclear (ITS) trees for eight species of sect.Saxifraga suggest that gene trees and the true species tree are not coincident. Recent and mid-term reticulation is proposed as an explanation for the incongruence between morphological, cytological, organellar, and nuclear data. Homogenization in multigene families, such as the ITS region, via concerted evolution may be the key to the interpretation of results based on ITS sequences within sect.Saxifraga. The use of organellar genes in a larger sample should help to determine whether extensive reticulation occurs in sect.Saxifraga, as has been documented in various genera of Saxifragaceae.     

The identity of some taxa of Schizophragma (Saxifragaceae)

After examining a large number of type and authoritative specimens of Schizophragma Siebold & Zucc., S. elliptifolium C. F. Wei and S. integrifolium var. glaucescens Rehder were reduced to synonyms of S. hypoglaucum Rehder and S. integrifolium f. cordatum S. Y. Wang was reduced to synonym of S. corylifolium Chun. Schizophragma crassum var. ellipticum J. Anthony was treated as the correct name of S. crassum var. hsitaoanum (Chun) C. F. Wei. The name S. integrifolium var. minus Rehder is reinstated. The taxonomic treat-ment is supported by leaf epidermal characters.     

Phylogeography of the genus Cardiandra based on genetic variation in cpDNA sequences

We investigated the phylogenetic relationships within the genus Cardiandra based on plastid DNA sequences. The phylogenetic tree showed that Cardiandra populations from the Ryukyu Islands (Japan) and Taiwan were monophyletic (Ryukyu–Taiwan clade), whereas taxa from China and mainland Japan were sisters to this clade. The divergence time between the Ryukyu–Taiwan clade and the other species was estimated to be 0.082 MYA, i.e., the late Pleistocene. The infrageneric and/or infraspecific differentiation of Cardiandra is estimated to have depended largely on allopatric differentiation caused by the presence or division of the past landbridge of the Ryukyu Islands, which connected mainland Japan to the Asian Continent during the Quaternary.     

matK and rbcL gene sequence data indicate that Saxifraga (Saxifragaceae) is polyphyletic

The large genus Saxifraga, which consists of ≈400 morphologically and cytologically diverse species, has long been considered taxonomically complex. Phylogenetic analysis of over 2500 bp of chloroplast sequence data derived from matK and rbcL was employed to examine relationships among sections of Saxifraga, the segregate genera Zahlbrucknera, Saxifragopsis, and Cascadia, and the relationships of these taxa to other Saxifragaceae sensu stricto. Phylogenetic trees resulting from separate analyses of the matK and rbcL sequences were highly congruent; phylogenetic analysis of a combined matK–rbcL data matrix was therefore also conducted. Our analyses indicate that Saxifraga is polyphyletic, comprising two well-differentiated clades. One clade, Saxifraga sensu stricto, is the sister to the remainder of the family and consists of Saxifraga sections Irregulares, Heterisia, Trachyphyllum, Cymbalaria, Mesogyne, Xanthizoon, Porphyrion, Ciliatae, Cotylea, Ligulatae, Saxifraga, and Gymnopera. With the exception of Gymnopera, the species-rich sections of this clade are monophyletic. Also part of this clade is the problematic Zahlbrucknera paradoxa, which is allied with members of section Saxifraga. A second major clade of Saxifraga species, Micranthes sensu lato, comprises the large section Micranthes, as well as the segregate genus Cascadia, and S. tolmiei of section Merkianae. This clade is allied with the Heuchera, Darmera, and Chrysosplenium-Peltoboykinia groups of genera. The segregate genus Saxifragopsis is only distantly related to species of Saxifraga, and is instead the sister to Astilbe. The monotypic Oresitrophe is confirmed as a member of the Darmera group of genera. These results suggest that the floral features used to define Saxifraga may simply be symplesiomorphic in these well-separated Saxifraga lineages. Furthermore, the enormous cytological diversity encompassed by Saxifraga likely represents two independent instances of extensive aneuploidy and polyploidy in Saxifragaceae.     

虎耳草属异叶虎耳草复合群的数量分类学研究

山羊臭组（Saxifraga sect. Ciliatae Haworth）隶属于虎耳草属（Saxifraga Tourn. ex L.）,具有极高的物种丰富度,但山羊臭组内部的系统发育关系一直都未能很好解决。唐古拉亚组（S. subsect. Hirculoideae Engl.）是山羊臭组中最大的亚组,主要分布于青藏高原及其周边地区。根据形态特征,可将唐古拉亚组划分为3个复合群,其中叶背边缘有突起叶脉的物种被归入异叶虎耳草复合群（S. diversifolia complex）。本研究通过标本查阅,选取异叶虎耳草复合群24个物种、2变种共657份标本,对其24个定性性状信息进行主成分分析和聚类分析;获取443条地理分布信息,构建复合群的分布式样。特征值大于1的前7个主成分的累计贡献率较低,仅为67.748%。“茎生叶是否具柄”“中下部茎生叶形态”“花序类型”“中下部茎生叶大小”“萼片脉纹于先端汇合”“叶基心形”等性状对前3个主成分的贡献值大,可作为异叶虎耳草复合群物种分类与鉴定的关键性状。基于形态聚类结果和地理分布式样,可将异叶虎耳草复合群划分为3个分支：喜马拉雅山南坡分支、环四川盆地山区分支以及横断山分支（包含一个广布种S. egregia）。主成分分析结果支持将异叶虎耳草复合群划分为3个分支。     

Trichome anatomy and taxonomy of Saxifraga (Saxifragaceae)

Trichomes on flower pedicels and leaves were examined in 213 species of Saxifraga L. and in the monotypic, segregate genera Zahlbrucknera Reichenb., Saxifragella Engl., Saxifragodes D. M. Moore and Saxifragopsis Small. Pedicel trichomes were found to be better taxonomic characters than foliar ones. Six main trichome types were identified: i) multiseriate, glandular; ii) uniseriate, glandular; iii) sessile, multicellular, glandular; iv) multiseriate, eglandular; v) uniseriate, eglandular; vi) unicellular, eglandular. The different types were distributed (except in certain species) on pedicels as follows. Multiseriate, glandular types occur in four series of Saxifraga sect. Micranthes (Haw.) D. Don, part of sect. Gymnopera D. Don, and in sects Ciliatae Haw., Trachyphyllum Gaudin, Xanthizoon Griseb., Ligulatae Haw., Porophyllum Gaudin, Porphyrion Tausch and Irregulares Haw. They also occur in the genus Saxifragopsis. Uniseriate, glandular types occur in five series of Saxifraga sect. Micranthes , part of sect. Gymnopera and Irregulares , and in sects. Miscopetalum (Haw.) Sternb., Cymbalaria , Griseb. Tridactylites (Haw.) Engl., Saxifraga and Stoloniferae Haw. They also occur in the genus Zahlbrucknera. Eglandular, multicellular trichomes occur occasionally in Saxifraga sects Micranthes and Gymnopera , more frequently in sects Ciliatae and Ligulatae , and are ubiquitous in sects Xanthizoon, Porophyllum and Porphyrion. Eglandular, unicellular trichomes occur only in Saxifragopsis and Saxifragodes. Sessile, glandular trichomes occur in a group of species from Saxifraga sect. Stoloniferae. Saxifragella is glabrous. This taxonomic distribution of trichome types generally supports Engler's classification of the genus, and is compatible with other evidence from palynology, embryology and seed morphology. The taxonomic disposition of the few exceptional species is discussed.     

Frequency of local, regional, and long-distance dispersal of diploid and tetraploid Saxifraga oppositifolia (Saxifragaceae) to Arctic glacier forelands

? Premise of the Study: Climate change forces many species to migrate. Empirical small-scale data on migration and colonization in the Arctic are scarce. Retreating glaciers provide new territory for cold-adapted plant species, but the genetic consequences depend on dispersal distances and frequencies. We estimated local, regional, and long-distance dispersal frequencies, as well as their effect on levels of genetic diversity, in diploid and tetraploid individuals of Saxifraga oppositifolia. ? Methods: Samples were collected in four aged moraines in each of three glacier forelands, in surrounding areas and reference populations in the Arctic archipelago Svalbard. These samples were analyzed for neutral amplified fragment length polymorphisms (AFLPs, n = 707) and ploidy levels (n = 30). ? Key Results: Genetic clustering and ploidy analyses revealed two distinct genetic groups representing diploids and tetraploids, with few intermediate triploids. The groups were intermixed in most sampled populations. No differences in genetic diversity were found between tetraploids and diploids, or between established and glacier foreland populations. Seeds were dispersed over local, regional, and long distances, with the highest proportions of seeds originating from close sources. A minimum of 4-15 founding individuals from several source populations had initially established in each glacier foreland. ? Conclusions: Our data suggest that S. oppositifolia can rapidly colonize new deglaciated areas without losing genetic diversity. Thus, glacier forelands can be alternative habitats for cold-adapted vascular plants tracking their climatic niche. Our data show no difference in colonization success between diploid and tetraploid individuals.     

Phylogenetic and evolutionary insights into the Saxifraga pentadactylis complex (Saxifragaceae): variation in nrITS sequences

Variation in nrlTS sequences among seven members of a seemingly natural group of Saxifraga (the S. pentadactylis complex; series Pentadactyles ) is phylogenetically analyzed. The two most parsimonious trees indicate that the S. pentadactylis complex is not a monophyletic group, because 5. intricata is excluded and is most closely related to S. exarata (series Cespitosae ). A well-defined lineage includes, however, six of the seven members of the S. pentadactylis complex. The tree topology suggests that this remaining group of six taxa may have diverged in the northeastern part of the Iberian Peninsula. Two sublineages can be identified from the ITS phylogenetic reconstruction: S. losae-S.moncayensis and the three subspecies of S. pentadactylis (subspp. pentadactylis, willkommiana , and almanzorii ). The relationships between S. losae and S. moncayensis provide an example of north-south speciation along the Sistema Ibérico mountain range. The three subspecies of S. pentadactylis form a monophyletic group resolved into one clade containing subspp. willkommiana and almanzorii , sister to subsp. pentadactylis. The molecular evidence agrees with chorological, cytotaxonomical, chemosystematic, and reproductive data indicating two differentiation patterns within S. pentadactylis. Transition to predominant autogamy in subsp. almanzorii probably resulted in rapid reproductive isolation from the outcrossing subsp. willkommiana in part of the Sistema Central mountain range, as suggested by their two distinct floral types but similar ITS sequences. Allopatric separation of populations of subsp. willkommiana from subsp. pentadactylis (the Pyrenees) has resulted in molecular divergence and some morphological differentiation.     

Foliar crystals in Saxifraga and segregate genera (Saxifragaceae)

In the genera Saxifraga, Saxifragodes, Saxifragella, Saxifragopsis and Zahlbrucknera foliar crystals were found to be present only in Saxifragodes and Saxifraga sections Micranthes and Irregulares . The crystals occur as stellate druses, except in Saxifraga nipponica and S. cortusifolia (section Irregulares ) where they occur as single needles. This finding indicates a close relationship between these two sections of Saxifraga . The monotypic Saxifragodes , from southern South America, does not appear to be related to Andean species of Saxifraga , which lack crystals, but rather to species of Saxifraga section Micranthes in North America. The taxonomic affinities of five species in section Micranthes that lack crystals are discussed.     

Chloroplast phylogenomics and the taxonomy of Saxifraga section Ciliatae (Saxifragaceae)

Comprising ca. 200 species, Saxifraga sect. Ciliatae is the most species-rich section of Saxifraga s.str., whose center of diversity is in the Tibeto-Himalayan region. The infra-sectional classification of sect. Ciliatae is still in debate due to the high level of species richness, as well as remarkable variations of habitat, morphology, physiology and life cycles. Subdivisions of this section proposed in various taxonomic systems have not been adequately tested in previous phylogenetic studies, partly due to low taxonomic sampling density, but also to the use of few DNA markers. In order to achieve a more robust infra-sectional classification of sect. Ciliatae, complete chloroplast genomes of 94 taxa from this section were analyzed, of which 93 were newly sequenced, assembled and annotated. The length of the 94 plastomes of sect. Ciliatae taxa range from 143,479 to 159,938 bp, encoding 75 to 79 unique protein-coding genes (PCGs). Analyses of the 94 plastomes revealed high conservation in structural organization, gene arrangement, and gene content. Gene loss and changes of IR boundaries were detected but in extremely low frequency. The molecular phylogenetic tree from concatenated PCGs and complete chloroplast genome sequences exhibited high resolution and support values and confirms that sect. Ciliatae is monophyletic. Three well-supported clades were revealed within the section that agree relatively well with the subsectional taxonomy of Gornall (1987), but some minor modifications should be made. Firstly, the monotypic subsection Cinerascentes should be abandoned and its constituent species, S. cinerascens, assigned to subsect. Gemmiparae. Secondly, subsections Rosulares and Serpyllifoliae should be merged and become subsect. Rosulares. Section Ciliatae thus comprises: subsect. Hirculoideae Engl. & Irmsch.; subsect. Rosulares Gornall; subsect. Gemmiparae Engl. & Irmsch.; subsect. Flagellares (C. B. Clarke) Engl. & Irmsch. and subsect. Hemisphaericae (Engl. & Irmsch.) Gornall.     

Chemotaxonomic importance of diarylheptanoids and phenylpropanoids in Saxifraga tangutica (Saxifragaceae)

A phytochemical investigation of Saxifraga tangutica led to the isolation of 11 compounds, including eight diarylheptanoids (1–6, 10 and 11) and three phenylpropanoids (7–9). The chemical structures were established by extensive analysis of their MS and NMR spectroscopic data or comparison with literature data. In the present research, we report the isolated compounds 1–11, for the first time, in the species S. tangutica. Moreover, compounds 1, 2 and 4–11 have not been reported from any species in Saxifragaceae family. Furthermore, we discuss the chemotaxonomic significance of the isolated compounds.     

Chloroplast DNA phylogeography of the arctic-montane species Saxifraga hirculus (Saxifragaceae)

The genetic structure of populations of an arctic-montane herb, Saxifraga hirculus (Saxifragaceae), was analysed by means of chloroplast restriction fragment-length polymorphism. Sampled populations were distributed across Europe and North America (Alaska and Colorado). There was no evidence for geographically structured genetically divergent lineages, and although no haplotypes were shared between North America and Europe, the haplotypes from different continents were intermixed on a minimum spanning tree. European populations were much more highly differentiated and had much lower levels of haplotype diversity than their Alaskan counterparts. Centres of haplotype diversity were concentrated in those Alaskan populations located outside the limits of the last (Wisconsin) glaciation, suggesting that they may have acted as refugia during the Pleistocene. It was not possible to identify putative migration routes or corresponding refugia in the European genepool. One British population, from the Pentland Hills, was genetically very distant from all the others, for reasons that are as yet unknown.     

Lack of reproductive plasticity in alpine Saxifraga rhomboidea (Saxifragaceae)

Phenotypic plasticity is predicted to be highly advantageous to alpine plants because of the extreme spatial and temporal heterogeneity of the alpine environment. Alpine plants are constrained in their vegetative plasticity by developmental preformation, however, they have the potential for reproductive plasticity via abortion of reproductive structures. Saxifraga rhomboidea Greene (Saxifragaceae) is a perennial of alpine dry meadows. It lacks a mechanism for vegetative reproduction, therefore, seed production is particularly important in maintaining populations of this species. Using long-term fertilized plots at the Niwot Ridge Long-Term Ecological Research Site, we ask if fertilization has changed the abundance of S. rhomboidea , and if S. rhomboidea has shown a plastic reproductive response to nutrient additions. Despite a pronounced increase in community biomass, we found that abundance of S. rhomboidea was not affected by fertilization. Moreover, S. rhomboidea showed no plastic response in seed to ovule ratio or other reproductive measures. Other arctic and alpine species have shown the potential for plasticity of maternal investment; S. rhomboidea appears to lack this reproductive plasticity. We suggest that morphological constraints may limit plasticity in this species.     

On the gynodioecious polymorphism in Saxifraga granulata L. (Saxifragaceae)

Sexual and vegetative fitness components in hermaphrodite and female plants of the self-compatible, perennial herb Saxifraga granulata are compared using material derived from a gynodioecious population in northern England.
Females produced only 57% as many seeds as hermaphrodites, but their ovule offspring were 1.28 times as fit as those of hermaphrodites, and females were more vegetatively vigorous. The advantages to females in ovule offspring quality and in vegetative reproduction counteract their disadvantages in pollen and seed production and therefore probably play a role in the maintenance of the gynodioecious polymorphism. Pollination ecology, resource reallocation and inbreeding depression all appear to contribute to the observed sex differences in fitness.     

Phylogeny of Saxifraga section Saxifraga subsection Arachnoideae (Saxifragaceae) and the origin of low elevation shade-dwelling species

Saxifraga section Saxifraga subsection Arachnoideae is a lineage of 12 species distributed mainly in the European Alps. It is unusual in terms of ecological diversification by containing both high elevation species from exposed alpine habitats and low elevation species from shady habitats such as overhanging rocks and cave entrances. Our aims are to explore which of these habitat types is ancestral, and to identify the possible drivers of this remarkable ecological diversification. Using a Hybseq DNA-sequencing approach and a complete species sample we reconstructed and dated the phylogeny of subsection Arachnoideae. Using Landolt indicator values, this phylogenetic tree was used for the reconstruction of the evolution of temperature, light and soil pH requirements in this lineage. Diversification of subsection Arachnoideae started in the late Pliocene and continued through the Pleistocene. Both diversification among and within clades was largely allopatric, and species from shady habitats with low light requirements are distributed in well-known refugia. We hypothesize that low light requirements evolved when species persisting in cold-stage refugia were forced into marginal habitats by more competitive warm-stage vegetation. While we do not claim that such competition resulted in speciation, it very likely resulted in adaptive evolution.