Journal of Heredity Advance Access originally published online on April 13, 2006
Journal of Heredity 2006 97(3):296-302; doi:10.1093/jhered/esj029
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Brief Communications |
Electrophoretic Evidence for Disomic Inheritance and Allopolyploid Origin of the Octoploid Cerastium alpinum (Caryophyllaceae)
From the Department of Natural Sciences, Mid Sweden University, S-851 70 Sundsvall, Sweden (Nyberg Berglund); Department of Molecular Biology, Umeå University, S-901 87 Umeå, Sweden (Saura); and Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden (Westerbergh)
Address correspondence to A.-B. Nyberg Berglund at the address above, or e-mail: Anna-Britt.Nyberg-Berglund{at}miun.se.
The mode of inheritance of six enzyme markers in the octoploid alpine plant Cerastium alpinum was analyzed. Offspring from crosses between heterozygotes showed fixed heterozygosity at malate dehydrogenase-2, phosphoglucoisomerase-2, triosephosphate isomerase-2, and triosephosphate isomerase-3. Phosphoglucomutase-1 also showed fixed heterozygosity except in offspring from one cross. Fixed heterozygosity in five enzyme systems suggests that C. alpinum has originated through at least some allopolyploidization. Offspring from plants heterozygous for two alleles at the menadione reductase-1 (Mr-1) locus did not deviate significantly from a 1:2:1 ratio. The large proportion of homozygotes suggests disomic inheritance because any kind of polysomic inheritance would result in a substantially increased proportion of heterozygotes relative to disomic inheritance. Assuming a diploid model for Mr-1, this locus was used to analyze the population genetic structure within C. alpinum populations. Inbreeding was found in many alpine populations. This may help explain the large genetic distances found among alpine populations in a previous study. The analysis is only based on one segregating locus, and the results should therefore be treated with caution. However, by establishing the mode of inheritance through crosses, we have been able to use a codominant marker in population genetic analysis of an octoploid plant.
Polyploidy has played an important role in the evolution of higher plants, and new reproductively isolated plant taxa can emerge almost instantaneously through genome duplication (Otto and Whitton 2000; Soltis and Soltis 2000). More than 70% of all angiosperms have experienced one or more episodes of polyploidization (Masterson 1994). The frequency of polyploid angiosperms increases with latitude (e.g., Stebbins 1950, 1984). This is reflected by the high number of polyploid plants in the Fennoscandian flora (Löve and Löve 1975). A recent study of many circumarctic plants supports the hypothesis that polyploids have been more successful than diploids in colonizing deglaciated areas exposed at the end of the most recent ice age (Brochmann et al. 2004).
Cerastium alpinum (2n = 8x = 72; Löve and Löve 1975) is a typical representative of an arctic-alpine polyploid. It is morphologically variable in northern Europe with three recognized subspecies: ssp. alpinum, ssp. glabratum, and ssp. lanatum (Jonsell 2001). C. alpinum is hermaphroditic and reproduces through both selfing and outbreeding (Grundt et al. 1999; Totland and Schulte-Herbrüggen 2003). It is common and abundant in the alpine region in the Scandinavian mountain range, where it grows on alpine heaths and in serpentine soils rich in heavy metals (Hultén 1956; Rune 1953, 1957). C. alpinum also has a scattered distribution on open habitats such as serpentine, dolomite, riverbanks, and screes in the subalpine and boreal region of Fennoscandia (Hultén 1956; Rune 1953).
Previous enzyme phenotype analyses and amplified fragment length polymorphism analysis of populations on different soil types suggest that C. alpinum has colonized Fennoscandia through at least two postglacial immigration events, seen as an eastern and a western lineage which meet in a contact zone in northern Norway and Sweden (Nyberg Berglund and Westerbergh 2001; Nyberg Berglund et al. 2001; Nyberg Berglund AB and Westerbergh A, unpublished). Large genetic differences among populations within each lineage were also found, suggesting restricted gene flow. The genetic differentiation among populations may result from limited pollinator flight between populations or a high degree of selfing and inbreeding. To differentiate between these alternatives, we need to know the genetic structure within populations. Comparing observed and expected heterozygosity based on enzyme markers is, however, complicated in a polyploid with multiple isozyme bands. Getting a good view of evolutionary processes in polyploid plant populations also is difficult because there are typically few, if any, segregating loci in polyploids, presumably due to the bottlenecks associated with polyploid formation (e.g., Arnold 1997; Hedrick 1987; Müntzing 1938).
Polyploids have been classified as either allopolyploids that originate from a combination of nuclear genomes from two or more ancestral species or autopolyploids that evolve through doubling of genetically identical chromosome sets (Stebbins 1950). In autopolyploids, both bivalents and multivalents between chromosomes may be formed during meiosis because each chromosome will find more than one potential pairing partner. This gives rise to polysomic inheritance with both homo- and heteroallelic gametes and a high proportion of heterozygotes in the offspring (Haldane 1930). The segregation of autopolyploids is dependent on the meiotic configuration, that is, the proportion of bivalent and multivalent formation where the latter configuration also involves the possibility of double reduction (two sister chromatids are inherited together), which increases the proportion of homoallelic gametes (Bever and Felber 1992). In allopolyploids, parental genomes may be so different that pairing and bivalent formation only occur between chromosomes that originated from each parental genome. Alleles of each parental genome segregate as if they were included in a diploid after disomic inheritance. If the parental genomes are homozygous for different alleles, all gametes will be heteroallelic and all offspring will be heterozygous, a condition called fixed heterozygosity (e.g., Carson 1967). Some polyploid taxa may behave as intermediates between auto- and allopolyploids (segmental allopolyploids; Stebbins 1950). Wu et al. (2001) formulated a general polyploid model for gene segregation that combines meiotic behavior of both bivalent and multivalent pairings. In order to study the population genetic structure, we have first analyzed the segregation patterns of different enzyme markers in the polyploid C. alpinum through controlled crosses.
 |
Materials and Methods |
|---|
 Top Materials and Methods Results and DiscussionReferences |
|---|
Plant Material and Crosses
C. alpinum plants with known enzyme electrophoretic phenotypes (Nyberg Berglund and Westerbergh 2001) were planted outdoors at Nordvik Garden, Sweden (62°51'50''N, 18°00'40''E), in 1999 and crossed during 2000 and 2001. Flower buds were isolated prior to crossings. In crossings between individuals, stamens were removed from the female parent. Pollen was then transferred by brushing the stamens cut off from the donor flower (male parent) against the stigmas of the female parent. To facilitate selfing, stigmas were brushed with stamens from the same flower or other isolated flowers on the same plant. Seeds were first stored dry at +20°C for 1 month and then at +8°C for 2 months. They were then kept on constantly moist filter paper for germination. The seedlings were transplanted into pots with standard low-nutrient soil:sand (1:1) and cultivated at +20°C with 16 h lights (400 W sodium vapor lamps).
Enzyme Electrophoresis
Fresh leaves were homogenized in 30 µl extraction buffer (0.10 M Tris-HCl extraction buffer pH 7.5) with 4% PVP-40000 (polyvinylpyrrolidone) and 15 mM dithiothreitol (modified from Soltis et al. 1983). The plant extracts were analyzed for the enzymes malate dehydrogenase (MDH), menadione reductase (MR), phosphoglucoisomerase (PGI), phosphoglucomutase (PGM), and triosephosphate isomerase (TPI) as described by Nyberg Berglund and Westerbergh (2001). Enzyme phenotypes were named from the observed enzyme bands that were numbered so that the slowest migrating band was given the highest number (Figure 1).
|
Data Analysis
The observed pattern of segregation among offspring from individuals heterozygous at the Mr-1 locus was tested with chi-square tests performed in MINITAB (version 14) against two models: disomic inheritance with a 1:2:1 segregation and a model for tetrasomic inheritance. We chose to test against a tetrasomic model of inheritance with the highest proportion of double reduction (
= 2/7; Bever and Felber 1992), which yields the highest expected proportion of homozygous offspring, that is, a 1:19:1 segregation of two different homozygotes and one pooled class of multiple-stage heterozygous individuals. The Weir and Cockerham (1984) estimates of FIS within populations and the significance test of deviation from Hardy-Weinberg expectations were performed using the probability test in the 3.3 version of the software GENEPOP (Raymond and Rousset 1995). Analyses of F statistics were performed in POPGENE (version 1.31) according to Nei (1987) to elucidate the genetic structure among populations surrounded by forest and alpine heaths. |
Results and Discussion |
|---|
|
TopMaterials and MethodsResults and DiscussionReferences |
|---|
Inheritance of Enzyme Markers
Overall, crosses between plants resulted in fewer seeds than selfed plants. Cutting stamens away from plants used as females in the outcrosses may have had a negative effect on the seed set.
Fixed Heterozygous Systems
For the dimeric enzymes PGI and TPI, Nyberg Berglund and Westerbergh (2001) found two and three areas of activity, respectively. PGI-1 and TPI-1 were monomorphic, while PGI-2, TPI-2, and TPI-3 were polymorphic. Offspring from crosses between heterozygote phenotypes at PGI-2, TPI-2, or TPI-3 did not segregate (Figure 1, Table 1). The appearance of fixed polymorphic patterns suggests that the different isozymes are coded by independent loci. For dimeric enzymes, the fixed three-banded heterozygous enzyme phenotypes result from cytoplasmic interaction of the products of two different monomorphic loci (Carson 1967). Fixed heterozygosity has been found for many other polyploid plants, e.g., Polygala palustris (Lack and Kay 1986), Draba cacuminum (Brochmann et al. 1992a), and Agrostis palustris (Warnke et al. 1998) but is also known in diploids (e.g., Ford and Gottlieb 1999; Goodman et al. 1980; Werth et al. 1993). In the latter case, duplications of genes may have arisen after translocations between nonhomologous chromosomes or unequal crossing over. The finding of fixed heterozygosity in many enzyme systems in a polyploid is commonly used as evidence for allopolyploidy. On the other hand, the lack of fixed heterozygosity may strengthen the evidence for autopolyploidy (Soltis and Rieseberg 1986). The fixed heterozygosity in PGI-2, TPI-2, and TPI-3 suggests that C. alpinum has at least some allopolyploid origin.
|
Two different fixed PGI phenotypes were found (Figure 1), 95100 and 100120. This may suggest at least two different origins of polyploid C. alpinum. In fact, most polyploids seem to have multiple origins (e.g., Brochmann et al. 1992b; Soltis and Soltis 2000). The cross between plants with either of the two fixed PGI-2 phenotypes resulted in offspring with a "hybrid phenotype" (95100120) characterized by lower intensity of the slowest migrating enzyme band (Figure 1).
Nyberg Berglund and Westerbergh (2001) found two areas of activity for monomeric PGM: PGM-1, which was polymorphic, and PGM-2, which was monomorphic. Offspring from selfed plants with phenotype 40506080 showed only the 40506080 phenotype (Figure 1, Table 1). This suggests that the four bands at PGM-1 are coded by four monomorphic and independent loci rather than four alleles at one locus. This is another example of fixed heterozygosity. The cross between the phenotypes 40506080 and 4050 yielded only 40506080 phenotypes. Selfed plants with the phenotype 4050 (Figure 1) gave only 4050 phenotypes in the offspring, except for the plant Grå 34 (Table 1). This selfed plant gave 10 offspring with 4050 and 28 with 40506080. In addition, for the outcross between 4050 and 4050, we found only 4050 phenotypes in the offspring except for crosses involving Grå 34. In these crosses, one half of the offspring had the 4050 phenotype and the other half 40506080. Offspring from four separate flowers of Grå 34 showed the same segregating pattern so that the deviating segregation in crosses involving Grå 34 is caused by foreign pollen or that the selfed plant would accidentally consist of more than one genotype are therefore not likely. The phenotype 4050 in Grå 34 might be hypothesized to result from silencing of two loci that are expressed in phenotype 40506080. Given that phenotype 4050 is coded by the genotype 4040505060*60*80*80* (* for a silenced gene), Grå 34 with phenotype 4050 will produce two types of gametes. One half of the gametes has two genes silenced (as in the Grå 34 parent), while the other half of the gametes has lost the silencing. In the offspring from the selfed Grå 34, this would result in 1:2:1 segregation of the genotypes 4040505060*60*80*80*, 404050506060*8080*, and 4040505060608080, respectively. Because the two latter genotypes will show the enzyme phenotype 40506080, we will observe a segregation of 1:3 for phenotype 4050 and 40506080, respectively. This agrees with the proportions observed in offspring from the selfed Grå 34.
Unbalanced Fixed Heterozygous System
Nyberg Berglund and Westerbergh (2001) found three areas of activity for dimeric MDH. MDH-1 and MDH-3 were monomorphic, while MDH-2 showed polymorphism expressed as three-banded enzyme phenotypes differing only in the relative intensity of the enzyme bands. Most of the offspring from selfed plants with phenotype 80100 (higher intensity of the faster migrating enzyme; Figure 1) and almost all of the offspring from selfed plants with phenotype 80100 (higher intensity of the slower migrating enzyme) had phenotypes identical to the selfed plant (Table 1). Crosses between 80100 plants resulted in offspring mainly with the 80100 phenotype. Because no homozygote offspring showing 80 or 100 phenotypes were observed in crosses with 80100 heterozygotes, MDH-2 illustrates an additional system of fixed heterozygosity. Brochmann et al. (1992a) also found fixed heterozygous patterns in D. cacuminum that differed only in having higher intensity on either the faster or slower migrating enzymes. These were referred to as unbalanced fixed heterozygous patterns, which may also be a useful term for the patterns we have observed for MDH. Fixed unbalanced heterozygosity is possible for polyploids. For example, if AABBCCDD represents four genomes contributing to an octoploid—A, B, and C genomes each carry allele 80 and D carries allele 100—then the genotype would be unbalanced and fixed, that is, no segregation in intensity differences would be expected in the offspring. However, about 20% of the offspring from selfed 80100 plants resulted in 80100 phenotypes. This observed segregation is difficult to explain without invoking a hypothesis involving reversible gene silencing. Epigenetic effects such as methylation have been shown to be important in allopolyploids (Adams et al. 2004). The evolutionary success of allopolyploids may at least in part be attributable to epigenetic phenomena such as gene silencing (Chen et al. 1998; Kashkush et al. 2002).
Disomic-Inherited Marker with Segregation
The tetrameric MR gave clearly stained enzyme bands in one area on the gels. In addition, three levels of slower migrating enzymes were shown as weak and diffuse bands (Figure 1). Nyberg Berglund and Westerbergh (2001) found several heterozygous patterns including enzyme band intensity differences coded by the Mr-1 locus. Offspring from crosses between homozygous plants with phenotypes 70 and 100, respectively, showed not only a single heterozygous phenotype but several with differences in enzyme band intensity. These differences may be a result of differences in kinetic activity or partial gene silencing. We have here considered all individuals with heterozygous banding pattern as 70100 phenotypes, irrespective of differences in band intensity.
Selfed plants with enzyme phenotype 70100 resulted in 45 offspring with phenotype 70, 27 with 100, and 52 with the heterozygous phenotype 70100 (Table 1). The large proportion of homozygotes suggests disomic inheritance because any kind of polysomic inheritance would result in a substantially increased proportion of heterozygotes relative to disomic inheritance. In addition, even though the result suggested a segregation pattern of 2:2:1 of the phenotypes 70, 70100, and 100, respectively, it did not deviate significantly from the 1:2:1 segregation expected with disomic inheritance (chi-square test, P = .155). It did, however, significantly deviate from what is expected with tetrasomic inheritance (with the highest proportion of double reduction, that is, a 1:19:1 segregation of two different homozygotes and one pooled class of multiple-stage heterozygous individuals; chi-square test, P = .000). Crosses between individuals with phenotype 70100 gave many fewer offspring than the selfed 70100 plants. The two reciprocal crosses resulted in five offspring with phenotype 70, five with 100, and 11 with 70100, showing a 1:2:1 segregation pattern expected with disomic inheritance. Distorted segregation at the Mr-1 locus obtained by selfing may be due to linkage of recessive deleterious genes but has to be studied further. Distorted segregation for the MR enzyme has been found for other plants as well (e.g., Fallour et al. 2001). In the cross 100 x 70100 (Table 1), one may expect at least some offspring to be heterozygous. However, all 13 offspring were homozygous for the 100 allele. Assuming a diploid model for Mr-1, this locus was used to analyze the genetic structure within C. alpinum populations based on enzyme data of Nyberg Berglund and Westerbergh (2001).
Genetic Structure of Populations
The overall FST value of 0.526 for the Mr-1 locus (Table 2) adds further evidence for large genetic differences among C. alpium populations in Scandinavia (Nyberg Berglund and Westerbergh 2001). Eighteen of 31 populations of C. alpinum studied in Nyberg Berglund and Westerbergh (2001) were surrounded by dense forests (Table 2), eight of which represented an eastern postglacial immigration lineage (populations 24 through 31), four represented a western lineage (populations 9, 12, and 14), and six represented a contact zone between the two immigration lineages (populations 10, 11, 16 through 18 and 20). These forest populations showed an FST value of 0.629 for the Mr-1 locus (Table 2). Thirteen populations were located in open alpine areas, 10 of which represented the western lineage (populations 1 through 8, 13, and 19) and 3 populations were located within the contact zone (populations 21 through 23), showing an FST value of 0.321. Only two of the alpine populations showed fixed homozygous patterns for Mr-1, while more than half of the forest populations were fixed for one allele (using the 95% criterion), in most cases (80%) for allele 100. Some geographically adjacent forest populations were fixed for different alleles (e.g., populations 30 and 31), showing that the surrounding vegetation affects gene flow. The isolating effect of the surrounding vegetation may explain the large genetic differences found among forest populations in both the eastern and western immigration lineages as well as in the southern part of the contact zone. The importance of the surrounding vegetation in influencing gene flow patterns among populations has also been shown for the related and dioecious Silene dioica in the Scandinavian mountains (Westerbergh and Saura 1992, 1994).
|
The genetic differentiation among populations may also be a result from a high degree of selfing and inbreeding, which would be reflected as a deficiency of heterozygous individuals within populations. Overall, we found a small positive FIS value (Table 2). However, when dividing the populations into a group of alpine populations and a group of forest populations, the alpine populations showed a large positive FIS value, while the forest populations showed a large negative FIS value. The negative FIS value is to a great extent caused by populations 10 and 20, which harbored only heterozygous individuals (Table 2). Even though the FIS values also varied among alpine populations, most of the nonfixed populations showed large positive FIS. These populations are located in all alpine regions (populations 1, 2, and 3 in western Norway; populations 7 and 8 in central Norway; population 13 in western Sweden; and population 21 in northern Norway; Table 2). In addition, three of these populations (1, 2, and 21) showed significant deviation from Hardy-Weinberg expectations. Accordingly, the Mr-1 locus showed genetic structuring within at least one population in each of the alpine regions. The deviation from random mating could be explained by subdivision of populations into restricted neighborhood groups caused by a high degree of selfing and/or restricted pollinator flight and limited seed dispersal. Inbreeding may therefore help explain the large genetic distances found in the alpine region. Polyploid plants are expected to tolerate high degrees of selfing because they maintain genetic variation through fixed heterozygosity (Hedrick 1987; Soltis and Soltis 2000).
In conclusion, fixed heterozygosity was found in five enzyme systems (MDH-2, PGM-1, PGI-2, TPI-2, and TPI-3), which suggests that C. alpinum has originated at least through some allopolyploidization. There could be a combination of allo- and autopolyploidy, although the autopolyploidization cannot be expected to have contributed to any additional segregating variation. Strong evidence for polyploid origins can only be found when putative progenitor species are included (e.g., Spiranthes, Arft and Ranker 1998; Sun 1996). The progenitor species of C. alpinum are unknown and were not studied here.
The segregation pattern for Mr-1 suggests disomic inheritance in C. alpinum. Assuming a diploid model for Mr-1, inbreeding was found in many alpine populations of C. alpinum. This may explain the large genetic distances found among populations within the western immigration lineage. Populations fixed for one allele were mainly found in the forest populations, showing that forests effectively isolate C. alpinum populations. The results should be treated with caution because the analysis is only based on one segregating locus. However, by establishing the mode of inheritance and phenotypic expression through controlled crosses, we have been able to use a codominant marker in the population genetic analysis of a polyploid plant.
|
Acknowledgments |
|---|
This work was supported by the Royal Physiographic Society in Lund, the Natural Science Research Council of Sweden (NFR), and Mid Sweden University. We would also like to acknowledge two anonymous reviewers for their constructive criticism and valuable comments.
|
Footnotes |
|---|
Corresponding Editor: James Hamrick
Received July 6, 2005
Accepted January 24, 2006
|
References |
|---|
|
TopMaterials and MethodsResults and DiscussionReferences |
|---|
-
Adams KL, Percifield R, Wendel JF. (2004) Organ-specific silencing of duplicated genes in a newly synthesized cotton allotetraploid. Genetics 168:2217–2226.
Arft AM and Ranker TA. (1998) Allopolyploid origin and population genetics of the rare orchid Spiranthes diluvialis. Am J Bot 85:110–122.[Abstract]
Arnold ML. (1997) Natural hybridization and evolution. (Oxford University Press, New York).
Bever JD and Felber F. (1992) The theoretical population genetics of autopolyploidy. Oxf Surv Evol Biol 8:185–217.
Brochmann C, Brysting AK, Alsos IG, Borgen L, Grundt HH, Scheen AC, Elven R. (2004) Polyploidy in arctic plants. Biol J Linn Soc 82:521–536.[CrossRef]
Brochmann C, Soltis PS, Soltis DE. (1992a) Multiple origins of the octoploid Scandinavian endemic Draba cacuminum: electrophoretic and morphological evidence. Nord J Bot 12:257–272.
Brochmann C, Soltis PS, Soltis DE. (1992b) Recurrent formation and polyphyly of nordic polyploids in Draba (Brassicaceae). Am J Bot 79:673–688.[CrossRef][Web of Science]
Carson HL. (1967) Permanent heterozygosity. Evol Biol 1:143–168.
Chen ZJ, Comai L, Pikaard CS. (1998) Gene dosage and stochastic effects determine the severity and direction of uniparental rRNA gene silencing (nucleolar dominance) in Arabidopsis allopolyploids. Proc Natl Acad Sci USA 95:14891–14896.
Fallour D, Fady B, Lefèvre F. (2001) Evidence of variation in segregation patterns within a Cedrus population. J Hered 92:260–266.
Ford VS and Gottlieb LD. (1999) Molecular characterization of PgiC in a tetraploid plant and its diploid relatives. Evolution 53:1060–1067.[CrossRef][Web of Science]
Goodman MM, Stuber CW, Lee CN, Johnson FM. (1980) Genetic control of malate dehydrogenase isozymes in maize. Genetics 94:153–168.
Grundt HH, Borgen L, Elven R. (1999) Aspects of reproduction in Cerastium alpinum L. on calcic and ultramafic soils in Central Norway. Nord J Bot 19:447–453.
Haldane JBS. (1930) Theoretical genetics of autopolyploids. J Genet 22:359–372.
Hedrick PW. (1987) Genetic load and mating system in homosporous ferns. Evolution 41:1282–1289.[CrossRef][Web of Science]
Hultén E. (1956) The Cerastium alpinum complex. Sven Bot Tidskr 50:411–495.
Jonsell B. (2001) Cerastium alpinum. In Jonsell B (Ed.). Flora Nordica, vol 2 (Bergianska stiftelsen, Stockholm, Sweden) pp. 140–143.
Kashkush K, Feldman M, Levy AA. (2002) Gene loss, silencing, and activation in a newly synthesized wheat allotetraploid. Genetics 160:1651–1659.
Lack AJ and Kay QON. (1986) Phosphoglucose isomerase (EC 5.3.1.9) isozymes in diploid and tetraploid Polygala species: evidence for gene duplication and diversification. Heredity 55:111–118.
Löve A and Löve D. (1975) Cytotaxonomical atlas of the arctic flora (Cramer, Vaduz, Liechtenstein).
Masterson J. (1994) Stomatal size in fossil plants—evidence for polyploidy in majority of angiosperms. Science 264:421–424.
Müntzing A. (1938) Sterility and chromosome pairing in intraspecific Galeopsis hybrids. Hereditas 24:117–188.[Web of Science]
Nei M. (1987) Molecular evolutionary genetics. (Columbia University Press, New York).
Nyberg Berglund A-B, Saura A, Westerbergh A. (2001) Genetic differentiation of a polyploid plant on ultramafic soils in Fennoscandia. S Afr J Sci 97:533–535.
Nyberg Berglund A-B and Westerbergh A. (2001) Two postglacial immigration lineages of the polyploid Cerastium alpinum (Caryophyllaceae). Hereditas 134:171–183.[CrossRef][Web of Science][Medline]
Otto SP and Whitton J. (2000) Polyploid incidence and evolution. Ann Rev Genet 34:401–437.[CrossRef][Web of Science][Medline]
Raymond M and Rousset F. (1995) GENEPOP (version 1.2): population genetics software for exact tests and ecumenicism. J Hered 86:248–249.
Rune O. (1953) Plant life on serpentines and related rocks in the north of Sweden. Acta Phytogeogr Suec 31:1–139.
Rune O. (1957) De serpentinicola elementen i Fennoscandiens flora. Sven Bot Tidskr 51:43–105.
Soltis DE, Haufler CH, Darrow DC, Gastony GJ. (1983) Starch gel electrophoresis of ferns: a compilation of grinding buffers, gel and electrode buffers, and staining schedules. Am Fern J 73:9–26.[CrossRef][Web of Science]
Soltis DE and Rieseberg LH. (1986) Autopolyploidy in Tolmiea menziesii (Saxifragaceae): genetic insights from enzyme electrophoresis. Am J Bot 73:310–318.[CrossRef][Web of Science]
Soltis PS and Soltis DE. (2000) The role of genetic and genomic attributes in the success of polyploids. Proc Natl Acad Sci USA 97:7051–7057.
Stebbins GL. (1950) Variation and evolution in plants. (Columbia University Press, New York).
Stebbins GL. (1984) Polyploidy and the distribution of the arctic-alpine flora: new evidence and a new approach. Bot Helv 94:1–13.
Sun M. (1996) The allopolyploid origin of Spiranthes hongkongensis (Orchidaceae). Am J Bot 83:252–260.[CrossRef][Web of Science]
Totland Ø and Schulte-Herbrüggen B. (2003) Breeding system, insect flower visitation, and floral traits of two alpine Cerastium species in Norway. Arct Antarct Alp Res 35:242–247.[CrossRef]
Warnke SE, Douches DS, Branham BE. (1998) Isozyme analysis supports allotetraploid inheritance in tetraploid creeping bentgrass (Agrostis palustris Huds.). Crop Sci 38:801–805.
Weir BS and Cockerham CC. (1984) Estimating F-statistics for the analysis of population structure. Evolution 38:1358–1370.[CrossRef][Web of Science]
Werth CR, Hilu K, Langner CA, Baird WV. (1993) Duplicate gene-expression for isocitrate dehydrogenase and 6-phosphogluconate dehydrogenase in diploid species of Eleusine (Gramineae). Am J Bot 80:705–710.[CrossRef][Web of Science]
Westerbergh A and Saura A. (1992) The effect of serpentine on the population structure of Silene dioica (Caryophyllaceae). Evolution 46:1537–1548.[CrossRef]
Westerbergh A and Saura A. (1994) Gene flow and pollinator behaviour in Silene dioica populations. Oikos 71:215–224.
Wu R, Gallo-Meagher M, Littell RC, Zeng ZB. (2001) A general polyploid model for analyzing gene segregation in outcrossing tetraploid species. Genetics 159:869–882.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||