Ploidal Levels Research Articles

Population genetic variation, geographic structure, and multiple origins of autopolyploidy in Galax urceolata.

Whereas population genetic studies have examined allopolyploids, comparable studies of naturally occurring autopolyploids remain rare. To address fundamental questions regarding autopolyploidy, we undertook a detailed population genetic study of one of the classic examples of autopolyploidy, Galax urceolata (Diapensiaceae), which comprises diploid, triploid, and autotetraploid cytotypes. Galax is endemic to the Appalachian Mountains, the adjacent piedmont, sandhills, and coastal plain and represents perhaps the most widely known example of autopolyploidy in nature. Flow cytometry was used to diagnose ploidal level of ∼1000 individuals across 71 populations. We used 10 microsatellite markers to examine genetic variation across the geographic range of Galax and assessed multiple origins though comparisons of diploid, triploid, and tetraploid accessions using multiple analytical approaches. Tetraploids had higher levels of heterozygosity than diploids did. Genetic variation in diploid and tetraploid Galax is geographically structured among the ecoregions of the southeastern United States. Autotetraploidy in Galax urceolata has occurred independently at least 46 times, with triploidy having occurred a minimum of 31 times. Genetic differentiation among ecoregions suggests historical patterns of local adaptation. The numerous independent origins of tetraploid Galax reported here are among the highest frequencies of independent polyploidizations ever reported for any polyploid (auto- or allopolyploid).

Assignment of homoeologs to parental genomes in allopolyploids for species tree inference, with an example from Fumaria (papaveraceae).

There is a rising awareness that species trees are best inferred from multiple loci while taking into account processes affecting individual gene trees, such as substitution model error (failure of the model to account for the complexity of the data) and coalescent stochasticity (presence of incomplete lineage sorting [ILS]). Although most studies have been carried out in the context of dichotomous species trees, these processes operate also in more complex evolutionary histories involving multiple hybridizations and polyploidy. Recently, methods have been developed that accurately handle ILS in allopolyploids, but they are thus far restricted to networks of diploids and tetraploids. We propose a procedure that improves on this limitation by designing a workflow that assigns homoeologs to hypothetical diploid ancestral genomes prior to genome tree construction. Conflicting assignment hypotheses are evaluated against substitution model error and coalescent stochasticity. Incongruence that cannot be explained by stochastic mechanisms needs to be explained by other processes (e.g., homoploid hybridization or paralogy). The data can then be filtered to build multilabeled genome phylogenies using inference methods that can recover species trees, either in the face of substitution model error and coalescent stochasticity alone, or while simultaneously accounting for hybridization. Methods are already available for folding the resulting multilabeled genome phylogeny into a network. We apply the workflow to the reconstruction of the reticulate phylogeny of the plant genus Fumaria (Papaveraceae) with ploidal levels ranging from 2[Formula: see text] to 14[Formula: see text]. We describe the challenges in recovering nuclear NRPB2 homoeologs in high ploidy species while combining in vivo cloning and direct sequencing techniques. Using parametric bootstrapping simulations we assign nuclear homoeologs and chloroplast sequences (four concatenated loci) to their common hypothetical diploid ancestral genomes. As these assignments hinge on effective population size assumptions, we investigate how varying these assumptions impacts the recovered multilabeled genome phylogeny.

Morphometric analysis of the North American creosote bush (Larrea tridentata, Zygophyllaceae) and the microspatial distribution of its chromosome races

Polyploidy is a major mechanism of chromosome evolution and speciation in flowering plants. Delineation of polyploid populations as species or subspecies is complicated because of the uncertainties of distinguishing closely related diploids and polyploids in field conditions. Here we evaluate the practical identification of polyploids—using geographic distributions and morphological features—in the North American creosote bush (Larrea tridentata, Zygophyllaceae). Regarded as a classical autopolyploid complex, L. tridentata comprises diploids, tetraploids, and hexaploids distributed throughout the Chihuahuan, Sonoran, and Mojave Deserts. Transect sampling on cytotype range boundaries revealed strong spatial structuring of the chromosome races, and infrequent sympatry, over small spatial scales (<50 km). Inter-cytotype hybrids were rare, with putative triploids and pentaploids comprising <1 % of the sampled plants (N = 1,197). In morphometric analyses of stem, flower, and leaf characteristics (N = 14 traits), we found significant ploidy effects in several cytotype comparisons; reproductive structures and foliage characteristics were particularly discriminatory. Significant transect and ploidy × transect effects for most traits suggest, however, that environmental and genic factors influence plant architecture and morphology. Nonetheless, discriminant function analysis with a combined morphometric data set correctly assigned 68.9 % of plants to ploidal level. Pollen diameters increased significantly with ploidal level, providing another potentially informative trait for comparisons of reproductive plants. Taken together, the spatial distribution and morphometric data presented here suggest that the majority of L. tridentata plants could, in principle, be assigned to cytotype in the field. However, because of potential misclassifications, we suggest recognition of the L. tridentata cytotypes as subspecies.

Dihaploidy yields diploid apomicts and parthenogens in Erigeron (Asteraceae)

• Dihaploids result when tetraploids produce reduced eggs that develop without fertilization into diploid progeny. This process is useful for reducing genome complexity and studying trait expression at different ploidal levels. In this study we evaluated genetic inheritance and expression of diplospory (D) and parthenogenesis (P) in a population of dihaploids produced by tetraploid apomictic Erigeron• From 400 mostly maternal (tetraploid) progeny, we identified 64 (16%) dihaploids with 2n = 18 (53 plants) or 2n = 19 (11 plants). Differential interference contrast (DIC) imaging of ovules was used to evaluate megasporogenesis (meiosis vs. diplospory) and capacity for parthenogenetic embryo development. Seed production was estimated as the proportion of filled seeds.• For 60 analyzed dihaploids, diplospory vs. meiosis segregated approximately 1: 1 (P = 0.44) while all exhibited parthenogenetic embryo development. Parthenogenesis for meiotic progeny (n = 27) was observed in approximately 50% of ovules. Apomictic dihaploids (combining D and P; n = 33) produced seeds with mean 24.8% (range 1.3-74.4%) of total flowers.• The dihaploid population consisted of half apomicts (D + P) and half parthenogens (P only). We infer that formation of dihaploid seeds requires the parthenogenesis locus. The highest seed values obtained for diploid apomicts are comparable to those recorded for wild type polyploid apomicts. This is one of the first reports of diploid apomixis in the Asteraceae and it demonstrates that both diplospory and parthenogenesis can be transmitted and expressed at a high level in the diploid condition.

Miscellaneous Chromosome Numbers in Opuntieae Dc. (Cactaceae) with a Compilation of Counts for the Group

Abstract: Chromosome counts of members of the Opuntieae were carried out to further our understanding of ploidal levels, species boundaries, and evolutionary patterns within this group of stem succulents, which has been well-studied cytologically and is well known for hybridization and polyploidy. Here we counted chromosomes of 53 taxa in 4 genera (Consolea Lem., Nopalea Salm-Dyck, Opuntia Mill., and Tacinga Britton & Rose). Thirty of these counts are the first for a given taxon, and six counts represent a different ploidy for a taxon than has been reported previously. We also present a review of chromosome counts reported for Opuntieae. Ploidy in these taxa ranged from diploid, 2n = 2x = 22, to 20-ploid, 2n = 20x = 220. Of the 164 species in the Opuntieae for which chromosome counts have been carried out, including our new counts, 26.2% are diploid, 13.4% are both diploid and polyploid, and 60.4% are polyploid reiterating that the frequency of genome duplication in the group is far more common than diploidy.

A phylogenetic monograph of the Sphagnum subsecundum complex (Sphagnaceae) in eastern North America

Abstract A taxonomic treatment based on field studies, examination of herbarium collections, and previously published molecular data is provided for the North American species of the Sphagnum subsecundum complex. Sphagnum platyphyllum, S. contortum, S. lescurii, and S. subsecundum are gametophytically haploid. Sphagnum missouricum and S. carolinianum are gametophytically allodiploid. A single allodiploid population of S. platyphyllum and a homoploid (haploid) hybrid between S. contortum and S. subscundum, previously documented genetically, are described and illustrated (but neither distinguished taxonomically). Epitypes are designated from (haploid) S. lescurii and (allodiploid) S. missouricum because the ploidal level of the original type material cannot be determined unambiguously. In North America, S. subsecundum is restricted to the eastern part of the continent, as western plants named this species are not conspecific. North American plants named S. inundatum are considered synonymous with either S. ...

Morphological diversity of pollen from selected species of the genus Taraxacum, according to their ploidal level

The study of the morphology of pollen concerned eight species of the genus &lt;i&gt;Taraxacum&lt;/i&gt;. Seven types were chosen with a different number of chromosomes belonging to the section &lt;i&gt;Palustria&lt;/i&gt;: &lt;i&gt;Taraxacum paucilobum&lt;/i&gt; 2n = 3x = 24, &lt;i&gt;T. subdolum&lt;/i&gt; 2n = 3x =24, &lt;i&gt;T. subpolonicum&lt;/i&gt; 2n =3x = 24, &lt;i&gt;T. vindobonense&lt;/i&gt; 2n = 4x = 32, &lt;i&gt;T. trilobifolium&lt;/i&gt; 2n = 4x = 32, &lt;i&gt;T. mendax&lt;/i&gt; 2n = 5x = 40, &lt;i&gt;T. portentosum&lt;/i&gt; 2n =? and one type from the section &lt;i&gt;Obliqua&lt;/i&gt; - &lt;i&gt;T. platyglossum&lt;/i&gt; 2n =?. Pollen from all the examined species was observed by SEM microscopy. A distinct relation exists between the morphology of pollen and the way of reproduction. Pollen of triploidal species, being compulsory apomicts, is characterized by a great variability of the size and a high percentage of deformed pollen grains. Tetraploids, being optional apomicts, produce regular pollen with a relatively even size. The regular type of pollen in pentaploidal &lt;i&gt;Taraxacum mendax&lt;/i&gt; and in species with an unknown number of chromosomes (&lt;i&gt;Taraxacum portentosum&lt;/i&gt; and &lt;i&gt;T. platyglossum&lt;/i&gt;) suggests that these taxa are optional apomicts.

The more the better? The role of polyploidy in facilitating plant invasions.

Biological invasions are a major ecological and socio-economic problem in many parts of the world. Despite an explosion of research in recent decades, much remains to be understood about why some species become invasive whereas others do not. Recently, polyploidy (whole genome duplication) has been proposed as an important determinant of invasiveness in plants. Genome duplication has played a major role in plant evolution and can drastically alter a plant's genetic make-up, morphology, physiology and ecology within only one or a few generations. This may allow some polyploids to succeed in strongly fluctuating environments and/or effectively colonize new habitats and, thus, increase their potential to be invasive. We synthesize current knowledge on the importance of polyploidy for the invasion (i.e. spread) of introduced plants. We first aim to elucidate general mechanisms that are involved in the success of polyploid plants and translate this to that of plant invaders. Secondly, we provide an overview of ploidal levels in selected invasive alien plants and explain how ploidy might have contributed to their success. Polyploidy can be an important factor in species invasion success through a combination of (1) 'pre-adaptation', whereby polyploid lineages are predisposed to conditions in the new range and, therefore, have higher survival rates and fitness in the earliest establishment phase; and (2) the possibility for subsequent adaptation due to a larger genetic diversity that may assist the 'evolution of invasiveness'. Alternatively, polyploidization may play an important role by (3) restoring sexual reproduction following hybridization or, conversely, (4) asexual reproduction in the absence of suitable mates. We, therefore, encourage invasion biologists to incorporate assessments of ploidy in their studies of invasive alien species.

Polyploidy and ecological transfiguration inAchillea

Polyploidy is a prominent chromosomal/genetical feature in most flowering plant groups (1, 2). Change in the number of chromosome sets (and thus gene dosage) is an important factor in the genesis of new species, in part because plants with higher ploidal levels often have ecological tolerances that differ from those of their progenitors. Plant evolutionists have been very interested in the origin of such differences, which may arise in two distinctive ways (3, 4). In the absence of hybridization, ecological divergence may be a simple by-product of change in ploidal level. Ecological divergence also may arise from natural selection acting on polyploid populations. It is impossible to distinguish between these alternatives from studies of two established entities that may have diverged in ploidal level thousands of years ago. In PNAS, Ramsey (5) demonstrates that a ploidal increase alone indeed may allow the invasion of a habitat that was previously unassailable by a long-established species. This study also demonstrates that long-standing polyploids are better adapted to this new habitat than are newly emergent ones.

Allopolyploid speciation and ongoing backcrossing between diploid progenitor and tetraploid progeny lineages in the Achillea millefoliumspecies complex: analyses of single-copy nuclear genes and genomic AFLP

BackgroundIn the flowering plants, many polyploid species complexes display evolutionary radiation. This could be facilitated by gene flow between otherwise separate evolutionary lineages in contact zones. Achillea collina is a widespread tetraploid species within the Achillea millefolium polyploid complex (Asteraceae-Anthemideae). It is morphologically intermediate between the relic diploids, A. setacea-2x in xeric and A. asplenifolia-2x in humid habitats, and often grows in close contact with either of them. By analyzing DNA sequences of two single-copy nuclear genes and the genomic AFLP data, we assess the allopolyploid origin of A. collina-4x from ancestors corresponding to A. setacea-2x and A. asplenifolia-2x, and the ongoing backcross introgression between these diploid progenitor and tetraploid progeny lineages.ResultsIn both the ncpGS and the PgiC gene tree, haplotype sequences of the diploid A. setacea-2x and A. asplenifolia-2x group into two clades corresponding to the two species, though lineage sorting seems incomplete for the PgiC gene. In contrast, A. collina-4x and its suspected backcross plants show homeologous gene copies: sequences from the same tetraploid individual plant are placed in both diploid clades. Semi-congruent splits of an AFLP Neighbor Net link not only A. collina-4x to both diploid species, but some 4x individuals in a polymorphic population with mixed ploidy levels to A. setacea-2x on one hand and to A. collina-4x on the other, indicating allopolyploid speciation as well as hybridization across ploidal levels.ConclusionsThe findings of this study clearly demonstrate the hybrid origin of Achillea collina-4x, the ongoing backcrossing between the diploid progenitor and their tetraploid progeny lineages. Such repeated hybridizations are likely the cause of the great genetic and phenotypic variation and ecological differentiation of the polyploid taxa in Achillea millefolium agg.

Allopolyploidy and homoploid hybridization in the Sphagnum subsecundum complex (Sphagnaceae: Bryophyta)

Several complexes of species in Sphagnum (peat mosses) originated through hybridization and allopolyploidy, suggesting that these processes have played a major evolutionary role in this genus. The Sphagnum subsecundum complex includes gametophytically haploid and diploid species in North America. Analyses of 12 microsatellite loci and sequences from two plastid DNA markers show that the evolutionary history of this group is substantially more complex than previously thought. Two taxonomic species, Sphagnum lescurii and Sphagnum inundatum, include both haploid and diploid populations. Within each ploidal level, S. lescurii and S. inundatum are not genetically differentiated. The diploid taxa show patterns of fixed heterozygosity for the microsatellite markers, consistent with an allopolyploid origin. Diploid S. lescurii is an allopolyploid between haploid S. lescurii and (haploid) S. subsecundum. Sphagnum carolinianum is an allopolyploid between haploid S. lescurii and an unknown parent. We detected homoploid hybridization between the haploids Sphagnum contortum and S. subsecundum. Finally, we report three samples of diploid Sphagnum platyphyllum (otherwise haploid) that have an allopolyploid origin involving north-eastern haploid S. platyphyllum and an unidentified taxon. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 135–151.

Genetic Diversity and Population Structure of Teucrium polium (Lamiaceae) in Tunisia

Random amplified polymorphic DNA markers were used to assess the genetic diversity within and among seven Tunisian diploid and polyploid populations of Teucrium polium L. from five bioclimatic areas. Out of the 141 bands generated from eight selected primers, 124 were polymorphic. The genetic diversity within a population (Shannon's index) was high and varied according both the ploidal levels and bioclimatic zones. The genetic differentiation among populations assessed by G (ST) and Phi(ST) statistics was high, suggesting a low level of gene flow among them. The major proportion of the variation was attributable to individual differences within populations. The UPGMA analysis based on Nei and Li's coefficient showed that individuals from each population clustered together. In a dendrogram using the Phi(ST) distance matrix, population grouping is concordant with bioclimates and cytotypes. Conservation strategies should take into account the level of the genetic diversity of the populations according to their bioclimate and ploidal levels.

Three‐genome mosses: complex double allopolyploid origins for triploid gametophytes in Sphagnum

This paper documents the occurrence of allotriploidy (having three differentiated genomes) in gametophytes of two Southern Hemisphere Sphagnum species (S. australe, S. falcatulum). The pattern of microsatellite alleles indicates that both species are composed of a complex of allodiploid and allotriploid gametophytes, with the latter resulting from two allopolyploidization events. No haploid (n = x) gametophytes were found for either species. The ploidal levels suggested by the pattern of microsatellite alleles were confirmed by flow cytometry and Feulgen DNA image densitometry. For both S. australe and S. falcatulum, the respective allodiploid plants (or their ancestors) are one of the parent species of the allotriploid plants. This is the first report of triploidy in Sphagnum gametophytes occurring in nature and also the first report of the presence of three differentiated genomes in any bryophyte. It is also the first report of intersectional allopolyploidy in Sphagnum, with S. australe appearing to have parental species from Sphagnum sections Rigida and Sphagnum, and S. falcatulum having parental species from Sphagnum sections Cuspidata and Subsecunda. In both species, the allotriploid cytotypes were the most prevalent cytotype on the South Island of New Zealand. The pattern of microsatellite alleles shows the presence of two genetically distinct populations of allodiploid S. australe, possibly indicating multiple origins of polyploidy for that allodiploid cytotype. Morphological evidence is also highly indicative of recurrent polyploidy in the allotriploid cytotype of S. falcatulum. Allopolyploidy has clearly played a major evolutionary role in these two Southern Hemisphere taxa. This study, in conjunction with other recent research, indicates that allopolyploidy is a common, if not the predominant, form of polyploidy in Sphagnum.

Microsatellites resolve the taxonomy of the polyploid Cardamine digitata aggregate (Brassicaceae)

AbstractPolyploidy, probably the single most important mode of sympatric speciation in plants, tends to result in complicated evolutionary patterns. The Cardamine digitata aggregate is a species complex where polyploidy has resulted in taxonomic and nomenclatural controversies. Two basic chromosome numbers are found (x = 7 and x = 8), and all plants studied so far are tetra‐ to dodecaploids. We used six microsatellite loci originally developed for the Arabidopsis genome to identify evolutionary and taxonomic units within the C. digitata aggregate, obtaining 102 polymorphic markers. Using different analysis methods (PCO, CVA, STRUCTURE, parsimony), we recognised four approximately equidistant units corresponding in morphology with the four described species: C. blaisdellii, C. digitata, C. microphylla, and C. purpurea. All taxa include at least two ploidal levels; thus recurrent taxonomic autopolyploidy is indicated.

Correction of chlorophyll-defective, male-sterile winter oilseed rape (Brassica napus) through organelle exchange: Phenotypic evaluation of progeny

Introduction of male-sterile Raphanus sativus cytoplasm into Brassica napus results in a chlorophyll deficiency in the backcrossed male-sterile B. napus line. We describe the apparent correction of this defect by protoplast fusion. enabling somatic recombination and/or elimination of organelles. Methods for fusion of male-sferile and male-fertile protoplasts, and for regeneration of plants are described. Potential recombining ability in somatic fusions is shown to be influenced by the genotypes, with one combination of genotypes resulting in five times more plants than the least successful combination. Of 87 plants that were regenerated. 44 have been analyzed for ploidal level and organellar composition. Two of the 44 plants were found to be male sterile with a green phenotype. The behaviour of these plants was followed during two subsequent seed generations.

Intraspecific morphological and genetic differentiation in Scrophularia grayana (Scrophulariaceae)

Scrophularia grayana, which is distributed throughout northern Japan and Sakhalin, and its locally endemic variety var. grayanoides, have been examined morphometrically and genetically. Principal-component analysis using a total of 26 morphological characteristics revealed that these taxa are morphologically differentiated, but that the difference is not distinct. These two taxa have the same number of chromosomes in the somatic cells, 2n = 94, suggesting that ploidal level difference is not relevant to their divergence. The distributions of the taxa are adjoining in the north of Japanese mainland Honshu. Nevertheless, principal-coordinate analysis using putative 112 ISSR loci indicated they are genetically very distinct. Many taxon-specific alleles were found, and many of the alleles were fixed in each taxon. This genetic information suggests that a relatively long time has passed since the taxa became differentiated and that gene flow has rarely occurred between them, although morphological similarity has been maintained, probably because of natural selective forces.

ON THE ABUNDANCE OF POLYPLOIDS IN FLOWERING PLANTS

The wide distribution of polyploidy among plants has led to a variety of theories for the evolutionary advantages of polyploidy. Here we claim that the abundance of polyploidy may be the result of a simple ratcheting process that does not require evolutionary advantages due to the biological properties of organisms. The evolution of polyploidy is a one-way process in which chromosome number can increase but not decrease. Using a simple mathematical model, we show that average ploidal level within a plant lineage can continually increase to the levels observed today, even if there are ecological or physiological disadvantages to higher ploidy. The model allowed us to estimate the average net speciation and polyploidy rates for ten angiosperm genera. Based on these estimates, the model predicts distributions of ploidal levels statistically similar to those observed in nine of the 10 genera.

Interploidal gene flow and introgression in bryophytes: Sphagnum girgensohnii × S. russowii, a case of spontaneous neotriploidy

Hybrid zones between female sporophyte-bearing haploid Sphagnum girgensohnii and male allodiploid S. russowii were studied. The existence of hybrids was initially hypothesized based on incidence of conspicuously large sporophytes in S. girgensohnii in the presence of male S. russowii. Measurements of nuclear DNA content, morphology and isozymes provide the first evidence of spontaneous hybridization across ploidal levels in bryophytes. Viable spores from the hybrid capsules yielded triploid protonemata and juvenile gametophores in culture. Isozyme profiles of the triploid gametophores displayed unbalanced heterozygous patterns containing two alleles. As S. russowii is an allodiploid species with S. girgensohnii and S. rubellum as progenitors, isozyme banding patterns of the triploids are interpreted as consisting of a single allele copy from the S. rubellum genome in S. russowii, and two copies of another allele at the same locus inherited from S. girgensohnii. The triploid sporophytes can be considered as F2 hybrids formed by introgression. The allotriploid gametophores are formed through diplospory or syndiploidy in the hybrid capsules. Comparison of hybrid sporophyte morphology to S. girgensohnii and S. russowii confirmed that hybrid capsules are larger and with an abnormally swollen sporophyte foot. The spore sizes of the hybrid capsules were significantly larger than spores from normal, intraspecific S. girgensohnii capsules. The percentage of germinated spores was <5% in cultures from hybrid sporophytes, which is much lower than in intraspecific sporophytes. The new knowledge opens the way for re-evaluation of polyploid evolution in bryophytes with emphasis on gametic non-reduction, introgression and two-step models of polyploid formation.

ON THE ABUNDANCE OF POLYPLOIDS IN FLOWERING PLANTS

The wide distribution of polyploidy among plants has led to a variety of theories for the evolutionary advantages of polyploidy. Here we claim that the abundance of polyploidy may be the result of a simple ratcheting process that does not require evolutionary advantages due to the biological properties of organisms. The evolution of polyploidy is a one-way process in which chromosome number can increase but not decrease. Using a simple mathematical model, we show that average ploidal level within a plant lineage can continually increase to the levels observed today, even if there are ecological or physiological disadvantages to higher ploidy. The model allowed us to estimate the average net speciation and polyploidy rates for ten angiosperm genera. Based on these estimates, the model predicts distributions of ploidal levels statistically similar to those observed in nine of the 10 genera.

The Saxifraga rivularis complex in Svalbard: Molecules, ploidy and morphology

In many arctic plant groups, reticulate histories involving hybridization and polyploidization have resulted in problems with species delimitation and phylogeny reconstruction. The Saxifraga rivularis complex is a typical example. Two taxa, S. rivularis and S. hyperborea, have been reported from the arctic archipelago of Svalbard (‘Spitsbergen’), but their delimitation, relationship, and taxonomic status are uncertain. We analyzed variation in chromosome number and DNA content (using flow cytometry), two types of molecular markers (RAPDs and isozymes), and 59 morphological characters along transects in two large, mixed Svalbard populations. We also included presumedly ‘pure’ reference populations from other sites to address whether the tentative taxa are distinct and whether they hybridize in mixed populations. In the random amplified polymorphic DNA (RAPD) analysis, we identified two very distinct groups of multilocus phenotypes, corresponding to the tentative species. These groups also differed in DNA content, chromosome number ( S. hyperborea: 2 n = ca . 26; S. rivularis: 2 n = ca . 52; but endopolyploidy was observed in several plants), some morphological characters, and isozyme phenotypes (with a few exceptions). The cytosolic Gpi locus was duplicated also at the 2 n = 26 level and fixed heterozygosity was observed in all populations, suggesting that S. hyperborea and S. rivularis may be secondary allotetraploid and allooctoploid, respectively, rather than diploid and tetraploid as traditionally assumed. We conclude that the Svalbard populations represent two distinct species at different ploidal levels and occurring in partly overlapping habitats, and that extensive hybridization presently does not occur between them even in mixed populations. Most molecular markers observed in S. hyperborea formed a subset of the S. rivularis markers, consistent with a hypothesis that S. rivularis is an allopolyploid lineage with the S. hyperborea lineage as one of its progenitors.