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(American Journal of Botany. 2001;88:1863-1867.)
© 2001 Botanical Society of America, Inc.

Systematics

Origin and genetic diversity of Spartina anglica (Poaceae) using nuclear DNA markers1

Debra R. Ayres2 and Donald R. Strong

Evolution and Ecology, One Shields Avenue, University of California, Davis, California 95616 USA

Received for publication November 2, 2000. Accepted for publication March 15, 2001.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Spartina alterniflora, introduced into the UK in the 1800s, was the seed parent in an interspecific hybridization with S. maritima. The sterile F1 hybrid S. xtownsendii gave rise to the fertile allopolyploid S. anglica by chromosomal doubling. Previous chromosome, isozyme, and cpDNA surveys did not reveal notable genetic variation within either the parental or the hybrid species. We used nuclear DNA markers (random amplified polymorphic DNA ([RAPD]) and inter-simple sequence repeats (ISSR) to further explore the origin, diversity, and parentage of S. anglica. We found DNA fragments in S. xtownsendii were the aggregate of diagnostic DNA fragments from S. maritima and S. alterniflora, thus confirming its hybrid origin. The S. xtownsendii genotype was identical to most of the S. anglica individuals analyzed, establishing the genetic concordance of these two taxa. We found widespread genetic variation within S. anglica. This could indicate that S. anglica arose several times, from different S. maritima sires. Alternatively, alleles could have been lost through recombination and/or through loss of entire chromosomes in S. anglica. Finally, all but one S. anglica individual had a S. alterniflora component that was indistinguishable from a S. alterniflora plant extant in Marchwood, UK, leaving open the possibility that this plant is the actual seed parent of S. anglica.

Key Words: allopolypoid • hybridization • ISSRs • Poaceae • RAPDs • Spartina alterniflora • Spartina maritima • Spartina xtownsendii


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
The origin of English cordgrass, Spartina anglica C. E. Hubbard, has been revealed by historical, morphological, and chromosomal research (Marchant, 1967, 1968 ), by isozyme studies (Raybould et al., 1991a ), and by chloroplast DNA analysis (Ferris, King, and Gray, 1997 ). Spartina alterniflora (2N = 62), native to the East and Gulf coasts of the United States, was introduced into Southampton, UK, waters prior to 1829 (Marchant, 1967 ). This exotic species was the seed parent (Ferris, King, and Gray, 1997 ) in an interspecific hybridization with the native S. maritima (2N = 60), producing a sterile F1 hybrid, S. xtownsendii (2N = 62) by 1870. The fertile allopolyploid, S. anglica (2N = 120, 122, 124), arose by the end of the 1880s in Southampton Water (Stapf, 1913 ).

Spartina xtownsendii and S. anglica from the UK have largely identical isoenzyme banding patterns (Raybould et al., 1991a ) and share a single cpDNA haplotype (Ferris, King, and Gray, 1997 ). This lack of variation could have arisen from either a single interspecific hybridization or from multiple crosses between genetically uniform parental plants. Neither the isozyme (Raybould et al., 1991b ) nor the cpDNA surveys of S. alterniflora and S. maritima showed adequate genetic variation to distinguish between these two hypotheses. DNA markers generated through random amplified polymorphic DNA (RAPD) and inter-simple sequence repeats (ISSR) analysis can reveal more genetic variation than either of these markers (Ayres and Ryan, 1999 ; Esselman et al., 1999 ). We have used RAPD and ISSR markers to distinguish Spartina hybrids in San Francisco Bay, California (Ayres et al., 1999 ; Ayres and Strong, unpublished data). Our goals in this study were to explore the hybrid origin of S. xtownsendii, assess the genetic relationship between S. xtownsendii and S. anglica, survey worldwide populations of S. anglica for genetic variation to evaluate the likelihood of multiple origins, and distinguish the lineage of the female parent using RAPD and ISSR nuclear DNA markers.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Leaves or seeds were collected from populations of S. alterniflora, S. maritima, S. xtownsendii, and S. anglica at sites around the world (Table 1). Leaves were collected from widely separated individuals to avoid multiple sampling of single clones. DNA was extracted from leaves using modifications of the Proteinase K-based methodology of Guidet (1994) developed for Spartina (Daehler et al., 1999 ). DNA was extracted from seed by pulverizing single seeds, stripped of lemma and palea, within a 1500 µL tube immersed in liquid nitrogen. Volumes of buffers throughout the seed DNA extraction were reduced to one-fifth the volume used for leaf material (i.e., 200 µL of buffer instead of 1000 µL).


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  		Table 1. Species, collection locations, and sample material of Spartina species included in this study

 
The polymerase chain reaction for RAPD reactions was performed with a Perkin Elmer 9600 thermocycler (Norwalk, Connecticut, USA) and an Eppendorf Mastercycler gradient (Westbury, New York, USA). Reproducibility of band arrays between machines was verified by running duplicate reactions. We used the following temperature regimen: 94°C (1.5 min); 40 cycles of 94°C (15 sec), 42°C (30 sec), 72°C (2 min); 1 cycle 72°C (4 min). Reaction conditions for ISSR PCR were optimized for MgCl2 concentration and annealing temperatures using the temperature gradient feature of the Eppendorf thermocycler. The temperature regimen for ISSR was the same as for RAPDs except the annealing temperature was 60°C (instead of 42°C). With modifications for RAPD and ISSR noted, we used reaction volumes of 15 µL containing10% by volume MgCl2-free 10x reaction buffer A (Promega, Madison, Wisconsin, USA), 0.6 units Taq polymerase (Promega, Madison, Wisconsin), 0.20 µmol/L primer (Operon Technologies, Alameda, California, USA ([RAPD]); University of British Columbia kit 800 ([ISSR]), 3 mmol/L MgCl2, 200 µmol/L each dATP, dCTP, dGTP,and dTTP (Promega, Madison, Wisconsin, USA), and 30 ng genomic DNA. Reaction products were stained with ethidium bromide and visualized under UV light following electrophoresis on 1.5% agarose gels.

Our goal was to identify RAPD and ISSR primers that strongly and reproducibly amplified species-specific diagnostic DNA fragments (= bands) in S. alterniflora and S. maritima. A band was considered diagnostic if it was present in only one species. From our previous work (Ayres et al., 1999 ), we had identified 84 Operon primers from kits A, B, C, D, F, G, and H that produced bright, repeatable bands in RAPD reactions with S. alterniflora DNA (data not presented). This subset of primers was used with S. maritima DNA in RAPD reactions to identify the primers that yielded bands specific to S. alterniflora or S. maritima (A7, B10, C1, C10, C12, F10, G2, H7). Two of the primers (C10 and C12) were used in previous studies (Ayres et al., 1999 ; Anttila et al., 2000 ) where they produced bands that were both specific to S. alterniflora and present in all 34 accessions from four populations that were examined. A set of 100 ISSR primers was screened using DNA from both species; we used primers 816, 830, 842, 850, and 888 in ISSR reactions. Most DNA-primer combinations were run at least twice to verify repeatability.

Bands were scored as present in or absent from all accessions. The NTSYS version 2.01 computer program (Exeter Software, Setauket, New York, USA) was used to calculate the genetic similarity between individuals using the simple matching coefficient (number of matches divided by the number of genetic characters), which gives equal weight to shared presence and shared absence of bands. Genetic similarity relationships were portrayed by unweighted pair group clustering (UPGMA). In addition to analyzing all bands, we also analyzed genetic similarity based only on the S. maritima-specific, or only on S. alterniflora-specific bands, to examine diversity patterns and identify the S. maritima and S. alterniflora individuals most similar to S. anglica.

Eight RAPD primers yielded 24 bands, and five ISSR primers yielded 14 bands. We combined both RAPD- and ISSR-generated bands in our analyses. Twenty-one bands were specific to S. alterniflora, and 17 bands were specific to S. maritima, for a total of 38 species-specific markers.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Spartina anglica contained species-specific DNA fragments from both S. alterniflora and S. maritima, and the majority of individuals we examined were indistinguishable from S. xtownsendii. Further, we found eight distinct genotypes among the 36 individuals of S. anglica analyzed (Fig. 1). Genetic variation was found in both seed and leaf material.



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  		Fig. 1. Dendrogram portraying genetic similarity among species, populations, and individuals of Spartina (ALT = S. alterniflora; GLA = S. alterniflora glabra; MAR = S. maritima; ANG = S. anglica; TOWN = S. xtownsendii). Populations: US = United States; FL = Florida; MD = Maryland; ME = Maine; WA = Washington; PS = Puget Sound, Washington; CA = California; UK = United Kingdom; AUS = Australia; TAS = Tasmania; SP = Spain

 
We did not find any DNA fragments in S. anglica that were not present in either of the parental species; different combinations of six species-specific bands (Table 2) resulted in eight genotypes. Considering only the S. alterniflora bands in S. anglica, all but one plant were identical to the S. alterniflora plant from the UK and were more similar to the S. alterniflora glabra (genetic similarity = 0.95) than they were to eastern United States/Washington (0.83) or the Gulf Coast Florida (0.71) plant. The single different individual was from Tasmania and lacked a single S. alterniflora-specific band; thus we found only two genotypes of S. anglica distinguishable using S. alterniflora-specific bands. All S. anglica individuals and S. alterniflora and S. alt. glabra plants from the UK lacked a diagnostic band at 200 base pairs (bp) from primer B10. In addition, only S. anglica and the single S. alterniflora collected from Marchwood, UK, lacked a diagnostic band at 475 bp from primer C10. The C10 band was found in all 34 S. alterniflora individuals tested in our previous study (Ayres et al., 1999 ), and a band of the same molecular weight was also found in a single individual of S. maritima collected in the UK for this study.


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  		Table 2. Parental species origin, identification, and number of Spartina anglica plants showing loss of DNA fragments

 
In S. alterniflora, the 21 species-specific bands separated the 11 individuals into six genotypes (Table 3). The two plants of UK S. alterniflora glabra had identical genotypes and were most similar to the single S. alterniflora collected from Marchwood, UK. These plants were contained within a larger grouping that included plants from the northeastern U.S. coast and the introduced populations from Washington State. The single plant from Gulf Coast Florida was well separated genetically from the main S. alterniflora cluster. These genetic relationships are in accord with the known history of the endemic source of the UK population of S. alterniflora, which is believed to be from somewhere between Boston, Massachusetts, USA, and Newfoundland, Canada (Hubbard, 1965 ).


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  		Table 3. Number of Spartina samples collected and genotypes found from each location

 
Based solely on S. maritima bands, eight genotypes of S. anglica were differentiated due to loss of S. maritima bands at five separate loci (Table 2) (5/17 bands for S. maritima, 1/21 bands for S. alterniflora; chi-square, NS). A large invariant assemblage contained S. xtownsendii, all accessions of S. maritima, and 21 S. anglica individuals including members of S. anglica from each worldwide population. Overall, two genotypes were found from the endemic UK population, while exotic Tasmanian populations had five genotypes.

We found no variation within the six S. maritima plants for the S. maritima-specific bands nor were any other polymorphic bands found, with one exception. A single individual from Strood estuary, Mersea Island was distinguishable on the basis of the (mostly) S. alterniflora-specific band from primer C10 discussed previously.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
DNA fragments in S. xtownsendii were the aggregate of diagnostic DNA fragments found in S. maritima and S. alterniflora, thus confirming the hybrid origin of S. xtownsendii at the nuclear DNA level. Furthermore, the S. xtownsendii genotype, in which 36 of the 38 species-specific bands were present, was identical to 20 of the 36 S. anglica individuals analyzed, establishing the genetic concordance of these two entities. The aggregate genotype common to S. xtownsendii and S. anglica has already been recognized at the level of protein variation (Raybould et al., 1991a ). However, unlike the previous isozyme and cpDNA studies (Ferris, King, and Gray, 1997 ), we found widespread genetic variation within S. anglica.

Genetic variation in 16 of the 36 individuals of S. anglica analyzed was due primarily to variable band loss at five S. maritima-specific loci, which resulted in eight distinct genotypes. Variation in the S. maritima component of S. anglica genomes is consistent with multiple interspecific hybridization events with genetically diverse S. maritima sires. However, we did not find a diversity of S. maritima genotypes; rather, there was no variation in the S. maritima-specific bands. Raybould et al. (1991b) found two rare isozyme variants in their collection of S. maritima from the UK, but all S. anglica isozyme patterns matched the "almost ubiquitous" S. maritima genotype. The patterns of genetic variation assayed by isozymes and cpDNA (Ferris, King, and Gray, 1997 ), coupled with the failure to produce artificial hybrids (Marchant, 1968 ; Raybould et al., 1991b ), support the notion of a single origin of S. anglica. However, the remaining extant populations of S. alterniflora and S. maritima have not yet revealed adequate genetic variation to refute rigorously this conclusion. Demonstration of multiple hybridizations requires matching variation between the allopolyploid and one of the parental species. More extensive DNA surveys of all three species, including herbarium specimens, may reveal genetic fragments that disprove the single origin hypothesis. In addition, DNA analyses of S. xneyrautii, an entity isozymically and chromosomally identical to S. xtownsendii, but evidently originating in France in 1892, may reveal a second naturally occurring F1 hybrid.

The loss of parental bands in S. anglica could stem from recombination of heterozygous species-specific bands. Recombination between chromosomes derived from the two component genomes probably does not occur (Raybould et al., 1991a ); however, recombination presumably can occur between homologous chromosomes from the same genome. Detecting heterozygosity with dominant markers such as RAPDs and ISSRs generally is not feasible; these difficulties will be exacerbated in hexaploid species such as S. alterniflora and S. maritima. Alternatively, allele loss could arise through mutation in S. anglica. Regardless of their origin, it is conceivable that null alleles could occasionally become homozygous through successive generations of recombination, even in an allododecaploid.

Loss of S. maritima chromosomes in S. anglica is another possible explanation for the loss of species-specific bands found in the present study. A single variant isozyme genotype was found in 12% of the S. anglica clones sampled throughout the UK in 1991 (Raybould et al., 1991a ). The variant genotype lacked two or more bands for a GOT locus contributed by S. maritima. As counts of fewer than 120 chromosomes were found in some plants with the variant genotype, they hypothesized that there had been a loss of S. maritima chromosomes from the S. anglica variants.

Coevolution of the nuclear and cytoplasmic genomes may result in greater nuclear DNA similarity between the allopolyploid and its seed parent than between the allopolyploid and its pollen parent (Soltis and Soltis, 1993 ). Song, Osborn, and Williams (1988) have suggested that the cytoplasmic genome of the seed parent may select against the nuclear genome of the pollen parent in Brassica. Significantly, S. alterniflora was the seed parent to the F1 hybrid (Ferris, King, and Gray, 1997 ), and S. anglica displays five alterations to the S. maritima component, but only a single alteration to the S. alterniflora component of its genome.

An intriguing question is whether the actual seed parent or its descendants are still present in the UK. Spartina alterniflora was first found along the Itchen River in 1829. From there it spread downstream into Southampton Water and along the English channel coast where it was still abundant as late as 1920. Loss of marshes to development, and possibly competition with S. anglica, have reduced the distribution of S. alterniflora to a single site at Marchwood, Southampton Water (Marchant, 1967 ). An invariant isozyme genotype and lack of fertile seed suggest that the lone extant population is a single clone (Raybould et al., 1991b ). Spartina maritima grew in Southampton Water in 1805 but disappeared after 1910 (Marchant, 1967 ). Spartina alterniflora glabra wasn't introduced into Southampton Water until 1924, and therefore could not have taken part in the interspecific hybridization. Spartina xtownsendii was first collected in 1870 at Hythe, Southampton Water (Stapf, 1913 ), where it grows today (Raybould et al., 1991b ; D. Strong, personal observation). Natural spread of this seed-sterile hybrid was negligible. Spartina anglica arose from S. xtownsendii through chromosome doubling and began spreading by seed in Southampton Water around 1890. It is likely that the S. alterniflora progenitor of S. anglica and the extant population/clone at Marchwood are both descended from the original S. alterniflora introduction in the Itchen River. The molecular evidence we have presented is consistent with this conclusion; there is no variation between the Marchwood S. alterniflora and the S. alterniflora component of the S. anglica genome. Actually, the data do not rule out the possibility that the Marchwood plant is the seed parent of S. anglica. Future research with DNA markers, especially codominant markers such as microsatellites, may characterize with greater precision the genetic relationship between the extant S. alterniflora in the UK and S. anglica. Surveying S. anglica with more markers would allow the creation of hypothetical genotypes of the S. alterniflora and S. maritima parents, which could be compared to genotypes of living plants and herbarium specimens.

We found that exotic populations of S. anglica from Tasmania and Australia were more genetically variable than the endemic population we sampled at Marchwood, UK. This could be due to differentiation owing to mutation, drift, and/or selection or to different founding populations. Poole Harbour was the source of at least 130 exotic plantations of S. anglica worldwide including Australia (Hubbard, 1965 ). Raybould et al. (1991a) found a single uncommon variant in their extensive isozyme survey of 261 S. anglica individuals in the UK; the majority of the variant clones (25 out of 31) were found in Poole Harbour. Comparisons of diversity of endemic and exotic populations of S. anglica may clarify worldwide genetic structure patterns. Finally, our assessment of genetic diversity in S. anglica was based on bands specific to the parental species, and thus represents a possibly conservative estimate of genetic diversity. Further research may reveal novel polymorphism in S. anglica not found in either parental species.


   FOOTNOTES
 
1 The authors would like to thank our laboratory assistant Jeannette Martinez and Alan Gray, David SanLeon, Paul Hedge, Eligio Bruzzeze, and Mei Wu for providing us with Spartina samples. Back

2 Author for correspondence (FAX: 530 752 1449; e-mail: drayres{at}ucdavis.edu ). Back


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Anttila C. K. A. R. King C. Ferris D. R. Ayres D. R. Strong 2000 Reciprocal hybrid formation of Spartina in San Francisco Bay. Molecular Ecology 9: 765-771[CrossRef][Medline]

Ayres D. R. D. Garcia-Rossi H. G. Davis D. R. Strong 1999 Extent and degree of hybridization between exotic (Spartina alterniflora) and native (S. foliosa) cordgrass (Poaceae) in California, USA determined by random amplified polymorphic DNA (RAPDs). Molecular Ecology 8: 1179-1186[CrossRef][Medline]

Ayres D. R. F. J. Ryan 1999 Genetic diversity and structure of the narrow endemic, Wyethia reticulata, and its congener W. bolanderi (Asteraceae) using RAPD and allozyme techniques. American Journal of Botany 86: 344-353[Abstract/Free Full Text]

Daehler C. C. C. K. Antilla D. R. Ayres D. R. Strong 1999 Evolution of a new ecotype of Spartina alterniflora (Poaceae) in San Francisco Bay, California, USA. American Journal of Botany 86: 543-546[Abstract/Free Full Text]

Esselman E. J. L. Jianqiang D. J. Crawford J. L. Winduss A. D. Wolfe 1999 Clonal diversity in the rare Calamagrostis porteri ssp. insperata (Poaceae): comparative results for allozymes and random amplified polymorphic DNA (RAPD) and intersimple sequence repeat (ISSR) markers. Molecular Ecology 8: 443-451[CrossRef][ISI]

Ferris C. R. A. King A. J. Gray 1997 Molecular evidence for the maternal parentage in the hybrid origin of Spartina anglica C. E. Hubbard. Molecular Ecology 6: 185-187[CrossRef]

Guidet F. 1994 A powerful new technique to quickly prepare hundreds of plant extracts for PCR and RAPD analysis. Nucleic Acids Research 22: 1772-1773[Free Full Text]

Hubbard J. C. E. 1965 Spartina marshes in southern England. VI. Pattern of invasion in Poole Harbour. Journal of Ecology 53: 799-813[CrossRef]

Marchant C. J. 1967 Evolution in Spartina (Gramineae). I. History and morphology of the genus in Britain. Botanical Journal of the Linnean Society 60: 1-24

Marchant C. J. 1968 Evolution in Spartina (Gramineae). II. Chromosomes, basic relationships and the problems of S. xtownsendii agg. Botanical Journal of the Linnean Society 60: 381-409

Raybould A. F. A. J. Gray M. J. Lawrence D. F. Marshall 1991a The evolution of Spartina anglica C. E. Hubbard (Gramineae): origin and genetic variability. Biological Journal of the Linnean Society 43: 111-126[CrossRef]

Raybould A. F. A. J. Gray M. J. Lawrence D. F. Marshall 1991b The evolution of Spartina anglica CE Hubbard (Gramineae): genetic variation and status of the parental species in Britain. Biological Journal of the Linnean Society 44: 369-380[CrossRef]

Stapf O. 1913 Townsend's grass or rice grass. Proceedings of the Bournemouth Natural Science Society 5: 76-82

Soltis D. E. P. S. Soltis 1993 Molecular data and the dynamic nature of polyploidy. Critical Reviews in Plant Science 12: 243-273

Song K. M. T. C. Osborn P. H. Williams 1988 Brassica taxonomy based on nuclear restriction fragments length polymorphism (RFLPs).I. Genome evolution of diploid and amphidiploid species. Theoretical and Applied Genetics 75: 784-794[ISI]




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