The evolution of Spartina anglica C.E. Hubbard (Gramineae): origin and genetic variability

An extensive survey of isozyme phenotypes in British populations of the amphidiploid salt marsh grass Spartina anglica and its putative parents has confirmed that the species arose by chromosome doubling in S. × townsendii , a sterile hybrid between S. maritima and S. alterniflora. Isozyme phenotypes and seed protein profiles indicate that S. anglica is almost totally lacking in genetic variation. Isozyme evidence also indicates that the parental species are characterized by low levels of genetic variation. The lack of variation in S. anglica is proposed as being due to a narrow genetic base resulting from a single origin, or a multiple origin from uniform parents; the fact that many populations are derived from very small founder populations; and because preferential pairing between identical homologous chromosomes prevents recombination between the divergent component genomes of the species. The low levels of isozyme variation that occur appear to be due to chromosome loss.
The consequences for the future evolution of S. anglica , given its lack of genetic variation, are discussed.     

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

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. ×townsendii 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. ×townsendii were the aggregate of diagnostic DNA fragments from S. maritima and S. alterniflora, thus confirming its hybrid origin. The S. ×townsendii 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.     

Molecular investigations in populations of Spartina anglica C.E. Hubbard (Poaceae) invading coastal Brittany (France)

Spartina anglica is a classical example of recent alloploid speciation. It arose during the end of the nineteenth century in England by hybridization between the indigenous Spartina maritima and the introduced East-American Spartina alterniflora. Duplication of the hybrid genome (Spartina x townsendii) gave rise to a vigorous allopolyploid involved in natural and artificial invasions on different continents. Spartina anglica was first recorded in France in 1906, and since then, it has spread all along the western French coast. Earlier studies revealed that native British populations display consistent morphological plasticity and lack of isozyme variation. In this paper, we use different molecular markers (randomly amplified polymorphic DNA, intersimple sequence repeats and restriction patterns from nuclear and chloroplast DNA sequences) to analyse the genetic patterns of the French populations of S. anglica. Our results show that French populations are mainly composed of one "major" multilocus genotype. This genotype is identical to the first-generation hybrid S. x townsendii from England. Losses of few markers from this genotype are observed but are restricted to a few populations from Brittany; it is likely that they appeared independently, subsequent to their introduction. In southern Brittany, no hybrids between S. anglica and S. maritima have been found where the two species co-occur. All French populations of S. anglica display the same chloroplast DNA sequences as S. alterniflora, the maternal genome donor. These findings are consistent with a severe genetic bottleneck at the time of the species formation, as a consequence of a unique origin of the species. Both parental nuclear sequences are present in the allopolyploid populations, revealing that for the markers investigated, no extensive changes have occurred in this young species.     

Molecular evidence for the maternal parentage in the hybrid origin of Spartina anglica C.E. Hubbard

Spartina anglica is a textbook example of a natural amphiploid, which originated from hybridization between S. alterniflora and S. maritima . Which of these species was the maternal parent has remained a mystery. Inheritance of chloroplast DNA in most angiosperms is strictly maternal and can thus be used to test the parentage of hybrid taxa. The DNA sequence of the chloroplast leucine tRNA gene intron was used to show that the introduced North American S. alterniflora is the female parent of the F ₁ hybrid S. x townsendii and the amphiploid S. anglica . A possible scenario for their origin is given.     

Retrotransposons and genomic stability in populations of the young allopolyploid species Spartina anglica C.E. Hubbard (Poaceae)

Spartina x townsendii arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced Spartina alterniflora, native to the eastern seaboard of North America. Duplication of the hybrid genome gave rise to Spartina anglica, a vigorous allopolyploid involved in natural and artificial invasions on several continents. This system allows investigation of the early evolutionary changes that accompany stabilization of new allopolyploid species. Because allopolyploidy may be a genomic shock, eliciting retroelement insertional activity, we examined whether retrotransposons present in the parental species have been activated in the genome of S. anglica. For this purpose we used inter-retrotransposon amplified polymorphism (IRAP) and retrotransposons-microsatellite amplified polymorphism (REMAP) markers, which are multilocus PCR-based methods detecting retrotransposon integration events in the genome. IRAP and REMAP allowed the screening of insertional polymorphisms in populations of S. anglica. The populations are composed mainly of one major multilocus genotype, identical to the first-generation hybrid S. x townsendii. Few new integration sites were encountered in the young allopolyploid genome. We also found strict additivity of the parental subgenomes in the allopolyploid. Both these findings indicate that the genome of S. anglica has not undergone extensive changes since its formation. This contrasts with previous results from the literature, which report rapid structural changes in experimentally resynthesized allopolyploids.     

Spartina anglica C. E. Hubbard: a natural model system for analysing early evolutionary changes that affect allopolyploid genomes

Spartina anglica arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced East American Spartina alterniflora and following genome duplication of the hybrid ( S.  ×  townsendii ). This system allows investigations of the early evolutionary changes that accompany stabilization of a new allopolyploid species in natural populations. Various molecular data indicate that S. anglica has resulted from a unique parental genotype. This young species contains two distinctly divergent homoeologous genomes that have not undergone extensive change since their reunion. No burst of retroelements has been encountered in the F₁ hybrid or in the allopolyploid, suggesting a 'structural genomic stasis' rather than 'rapid genomic changes'. However, modifications of the methylation patterns in the genomes of S.  ×  townsendii and S. anglica indicate that in this system, epigenetic changes have followed both hybridization and polyploidization.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 475–484.     

Genetic and epigenetic consequences of recent hybridization and polyploidy in Spartina (Poaceae)

To study the consequences of hybridization and genome duplication on polyploid genome evolution and adaptation, we used independently formed hybrids (Spartina x townsendii and Spartina x neyrautii) that originated from natural crosses between Spartina alterniflora, an American introduced species, and the European native Spartina maritima. The hybrid from England, S. x townsendii, gave rise to the invasive allopolyploid, salt-marsh species, Spartina anglica. Recent studies indicated that allopolyploid speciation may be associated with rapid genetic and epigenetic changes. To assess this in Spartina, we performed AFLP (amplified fragment length polymorphism) and MSAP (methylation sensitive amplification polymorphism) on young hybrids and the allopolyploid. By comparing the subgenomes in the hybrids and the allopolyploid to the parental species, we inferred structural changes that arose repeatedly in the two independently formed hybrids. Surprisingly, 30% of the parental methylation patterns are altered in the hybrids and the allopolyploid. This high level of epigenetic regulation might explain the morphological plasticity of Spartina anglica and its larger ecological amplitude. Hybridization rather than genome doubling seems to have triggered most of the methylation changes observed in Spartina anglica.     

互花米草、狐米草和大绳草茎秆的比较解剖观察

互花米草、狐米草和大绳草的表皮均由长细胞、短细胞(栓质细胞和硅质细胞)、盐腺和气孔器组成。它们成纵行交互排列。盐腺的结构与大米草相似,但三个种的盐腺和气孔器的数目不同,尤其以大绳草最多。它们的内部结构是由气道、不同大小的维管束、基本组织以及厚壁组织组成。然而,维管束的数目及厚壁组织的发育各不相同。狐米草和大绳草有高度木质化的厚壁组织细胞,而互花米草的厚壁组织木质化较弱。大绳草的维管束多于其他两种。     

米草属引入中国海岸带的利弊分析

1963年以来,中国先后从国外引进3种米草。实践证明,大米草和互花米草是盐渍土壤植物群落形成的先锋植物,适应于中国海岸带自然环境,生长发育正常,具有明显的促淤造陆和保护海岸等生态功能,并具有显著的经济效益.大米草和互花米草在南方海岸繁殖过快是近岸水体富营养化的结果,但并未对环境、动植物种群和人类构成侵害,其在北方海岸未发生异常现象,故应客观地评估米草的功与过。本文从利弊2个方面对中国海岸滩涂引入米草进行了分析。     

Molecular phylogeny of hybridizing species from the genus Spartina Schreb. (Poaceae)

Interspecific hybridization events have been reported in the genus Spartina Schreb. (Poaceae), involving the east American species Spartina alterniflora, and including either introgression (e.g., with the western American Spartina foliosa) or allopolyploid speciation (e.g., with the Euro-African Spartina maritima). Molecular phylogenetic analysis of the genus has been undertaken in order to understand phylogenetic relationships and genetic divergence among these hybridizing species. Twelve Spartina species have been sequenced for two nuclear DNA regions (ITS of ribosomal DNA, and part of the Waxy gene) and one chloroplast DNA spacer (trnT-trnL). Separate and conditional combined phylogenetic analyses using Cynodon dactylon as the outgroup have been conducted. Spartina is composed of two lineages. The first clade includes all hexaploid species: the Euro-African S. maritima (2n = 60), the East-American S. alterniflora (2n = 62) and the West-American S. foliosa (2n = 60). Spartina alterniflora appears as a closely related sister species to S. foliosa. Although belonging to the same lineage, Spartina maritima appears consistently more genetically differentiated from S. alterniflora than S. foliosa. The tetraploid species S. argentinensis (2n = 40) is placed at the base of this first clade according to the Waxy data, but its position is not well resolved by the other sequences. The second well-supported main lineage within genus Spartina includes the other tetraploid American species. Significant incongruence has been encountered between the waxy based tree and both the ITS and trnT-trnL trees concerning the position of S. densiflora, suggesting a possible reticulate evolution for this species. The results agree with hybridization patterns occurring in Spartina: introgression involving closely related species (S. alterniflora and S. foliosa) on one hand, and alloploid speciation involving more differentiated species (S. alterniflora and S. maritima) on the other hand.     

外来种互花米草入侵模式与爆发机制

互花米草（Spartina alterniflora Loisel）因其促淤造陆和消浪护堤作用显著而被许多国家引种,如今却在侵入地快速蔓延并呈现爆发趋势,对生态系统造成了极大危害,被认为是研究生物入侵生态学和遗传学的模式植物。从种群的入侵力、生态系统可入侵性和入侵通道3个方面探讨互花米草的爆发机制,研究结果表明高遗传分化和基因渗入能力是互花米草爆发的遗传基础,对逆境的高抗性和强竞争力是其快速扩张的保障,而高繁殖系数是互花米草爆发的源泉。我国互花米草种群的早期扩散人为影响超过了自然过程,快速扩张呈现出点源扩散和多点爆发的特点,从而为其种群控制带来困难,同时种子的跳跃式和连续式扩散在互花米草种群维持、更新和爆发中有重要作用,强有力的克隆生长能力也为互花米草种群的连续扩张提供了保障。现阶段要完全控制和根除互花米草是不实际的,但在及时预测预警的基础上,应用成本一效益分析方法,采取有序控制和综合开发利用的策略,仍可望妥善解决互花米草入侵所带来的负面效应。     

Uniformity of the nuclear and chloroplast genomes of Spartina maritima (Poaceae), a salt-marsh species in decline along the Western European Coast

Spartina maritima is a salt-marsh species from European and African Atlantic coasts. In the northern range of the species (including north-west France), a rapid decline of the populations has been observed during the 20th century. In this paper, the molecular diversity of 10 populations of S. maritima from France has been investigated using nuclear and chloroplast DNA markers: inter-simple sequence polymorphism (ISSR), randomly amplified polymorphic DNA (RAPD), inter-retrotransposon amplified polymorphism (IRAP), and PCR-RFLP of a 5 kb long portion of chloroplast DNA. The results reveal an extremely low level of genetic variation in this species: only one nuclear marker (out of 98) was polymorphic, with the presence of two genotypes randomly distributed among the populations. The lack of genetic diversity is interpreted in light of the almost exclusive vegetative propagation of the species in its northern range, the colonization history of the populations, and the origin of S. maritima (2n = 60) in the hexaploid lineage of the genus and in the context of the management of S. maritima populations.     

AN ECOPHYSIOLOGICAL STUDY OF THE SALT SECRETION OF FOUR HALOPHYTES

Plants of Spartina anglica, Limonium vulgare, Armeria maritima and Glaux maritima were collected in the field and grown on different concentrations of NaCl, KCl and CaCl₂. Salt secretion, ion content, water content and transpiration rates were determined. The highest sodium secretion was found in Spartina anglica , a species from the most saline habitat; and a somewhat lower secretion rate in Limonium vulgare. The lowest rates were found in Glaux maritima and Armeria maritima. The sodium secretion efficiency, i.e. the ability to maintain an unchanged internal sodium content, was highest in Spartina anglica. Spartina anglica is the most successful in the removal of excessively absorbed salt, since it secretes 60% of the absorbed sodium. The values for Limonium vulgare, Glaux maritima and Armeria maritima were 33, 20 and 4% respectively. The species studied differ in the preferential sequence of ion secretion as well as in secretion rate and efficiency. This preferential sequence of ion secretion seems to be similar in members of the same taxonomic group (Plumbaginaceae). The comparability of the secretion parameters is discussed with regard to morphological differences between the species.     

Nitrogen level changes the interactions between a native (Scirpus triqueter) and an exotic species (Spartina anglica) in Coastal China

The exotic species Spartina anglica, introduced from Europe in 1963, has been experiencing a decline in the past decade in coastal China, but the reasons for the decline are still not clear. It is hypothesized that competition with the native species Scirpus triqueter may have played an important role in the decline due to niche overlap in the field. We measured biomass, leaf number and area, asexual reproduction and relative neighborhood effect (RNE) of the two species in both monoculture and mixture under three nitrogen levels (control, low and high). S. anglica showed significantly lower biomass accumulation, leaf number and asexual reproduction in mixture than in monoculture. The inter- and intra-specific RNE of S. anglica were all positive, and the inter-specific RNE was significantly higher than the intra-specific RNE in the control. For S. triqueter, inter- and intra-specific RNE were negative at the high nitrogen level but positive in the control and at the low nitrogen level. This indicates that S. triqueter exerted an asymmetric competitive advantage over S. anglica in the control and low nitrogen conditions; however, S. anglica facilitated growth of S. triqueter in high nitrogen conditions. Nitrogen level changed the interactions between the two species because S. triqueter better tolerated low nitrogen. Since S. anglica is increasingly confined to upper, more nitrogen-limited marsh areas in coastal China, increased competition from S. triqueter may help explain its decline.     

The enigmatic invasive Spartina densiflora: a history of hybridizations in a polyploidy context

The aim of this study was to explore the origin of the invasive Spartina densiflora by analysing samples from the native region (South America) and from a recently colonized area (California). A combination of various molecular data (chloroplast and nuclear sequences, molecular fingerprint) and ploidy level estimations was used to answer the question whether the reticulate phylogenetic pattern previously detected in S. densiflora was restricted to California, or alternatively, whether a more ancient hybrid origin preceded formation of this species in its native area. We found that this species is heptaploid in both its native and introduced range. Identification of nuclear homeologous sequences indicate that this species has a reticulate origin in its native range, involving a lineage related to the hexaploid clade formed by S. alterniflora, S.foliosa, and S. maritima, and another lineage related to the sub-Antarctic endemic S. arundinacea that provided the chloroplast genome. The samples from California displayed similar multilocus patterns to the samples from Chile, supporting the hypothesis that this species originated on the southeast American coast (Argentina), from where it eventually spread to the west coast of South America (Chile) first and to the Northern Hemisphere (California) later.     

Reciprocal hybrid formation of Spartina in San Francisco Bay

Diversity in the tRNALEU1 intron of the chloroplast genome of Spartina was used to study hybridization of native California cordgrass, Spartina foliosa, with S. alterniflora, introduced to San Francisco Bay approximately 25 years ago. We sequenced 544 bases of the tRNALEU1 intron and found three polymorphic sites, a pyrimidine transition at site 126 and transversions at sites 382 and 430. Spartina from outside of San Francisco Bay, where hybridization between these species is impossible, gave cpDNA genotypes of the parental species. S. foliosa had a single chloroplast haplotype, CCT, and this was unique to California cordgrass. S. alterniflora from the native range along the Atlantic coast of North America had three chloroplast haplotypes, CAT, TAA, and TAT. Hybrids were discriminated by random amplified polymorphic DNA (RAPD) phenotypes developed in a previous study. We found one hybrid that contained a cpDNA haplotype unknown in either parental species (TCT). The most significant finding was that hybridization proceeds in both directions, assuming maternal inheritance of cpDNA; 26 of the 36 hybrid Spartina plants from San Francisco Bay contained the S. foliosa haplotype, nine contained haplotypes of the invading S. alterniflora, and one had the cpDNA of unknown origin. Furthermore, cpDNA of both parental species was distributed throughout the broad range of RAPD phenotypes, suggesting ongoing contributions to the hybrid swarm from both. The preponderance of S. foliosa cpDNA has entered the hybrid swarm indirectly, we propose, from F1s that backcross to S. foliosa. Flowering of the native precedes by several weeks that of the invading species, with little overlap between the two. Thus, F1 hybrids would be rare and sired by the last S. foliosa pollen upon the first S. alterniflora stigmas. The native species produces little pollen and this has low viability. An intermediate flowering time of hybrids as well as pollen that is more vigourous and abundant than that of the native species would predispose F1s to high fitness in a vast sea of native ovules. Thus, spread of hybrids to other S. foliosa marshes could be an even greater threat to the native species than introductions of alien S. alterniflora.     

Evolution in Spartina (Gramineae) IV. The cytology of S. alterniflora Loisel. in North America

A cytological survey of Spartina alterniflora Loisel. populations in Canada and north-eastern U.S.A. establishes the widespread and seemingly exclusive occurrence of the 2 n = 62 chromosome number in that region. The data indicate that the introduced and relict populations of S. alterniflora in Southampton Water, Britain (whence arose S. × townsendii ) are not cyto-logically atypical of the main body of this species in North America, and there is therefore no reason to doubt, on the grounds of aberrant chromosome number, the role of S. alterniflora as a parent of S. × townsendii.
2 n = 62 is anomalous with respect to the basic number of x = 10 generally found in the genus. The present results indicate that 2 n = 62 is widespread in north-eastern North American S. alterniflora , and no other chromosome number has been found in a survey of the region. But the species is wide-ranging and the possibility of 2 n = 60 occurring in other areas, especially in the south of its range, cannot be ruled out. However, the only chromosome number so far confirmed in southern populations is a count of 2 n = 62 from Texas.     

Community modification by a grass invader has differing impacts for marine habitats

There is increasing recognition of invasive species impacts but less is known about how modifications may differ under variable environmental contexts. In particular, it is generally unknown whether impacts of single invasive species can vary among habitats and what the consequences of this variability may be. We used a multi-site comparative approach to examine the impacts of a single invader, the marine grass Spartina anglica , on estuarine habitats with different native species assemblages and physical conditions. We found that range (extent), abundance, and effects on sediment and native plant species vary depending on the habitat invaded. S. anglica has by far the greatest range and abundance in mudflats and low salinity marshes compared to high salinity marshes and cobble beaches. Changes in sediment characteristics also substantially differed among habitats, with invaded areas in some habitats experiencing greater sediment accretion, water content, and salinity than other habitats. In addition, in opposition to the theory that strong invaders decrease species diversity, we found that native plant diversity in our plots increased within invaded areas in some habitats while it declined in others. These variable modifications suggest that single invaders, even species that are considered strong interactors, do not produce the same effect in all habitats. We suggest that understanding impact variability can help predict how invasive species will respond to environmental changes, new habitats, and management strategies.     

New evidence for a postglacial homoploid hybrid origin of the widespread Central European Scabiosa columbaria L. s. str. (Dipsacaceae)

We have tested whether Scabiosa columbaria s. str. could have originated by recent homoploid hybrid speciation because it is of intermediate morphology and is distributed in areas under glacial influence. Existing morphological and new RAPD data for 11 ingroup populations were analysed using multivariate statistics. Most methods indicated that S. columbaria s. str. could be the first example for a homoploid hybrid origin from Central Europe although a rigorous proof was missing partly because of ongoing introgression. Sequence variation was absent for 5,000 bp from 11 regions supporting a postglacial origin of S. columbaria s. str. An analysis of realized macroclimatic niches using GIS data showed that the niches of S. columbaria s. str. were partly intermediate between its putative parental species rather than extreme as often found in other homoploid hybrids. This could be due to geographical preconditions: large new habitats were available in postglacial Europe while in the case of most other homoploid hybrids only ecologically challenging habitats were not occupied by parental taxa.     

大米草对双齿围沙蚕生境中土壤改良作用的研究

大米草对双齿围沙蚕生境中土壤改良作用的研究赵清良赵强（南京师范大学生物系,２１００９７）ＥｆｆｅｃｔｓｏｆＳｐａｒｔｉｎａａｎｇｌｉｃａｔｏｔｈｅＳｏｉｌＨａｂｉｔａｔｓｏｆＰｅｒｅｎｅｒｅｉｓａｉｂｕｈｉｔｅｎｓｉｓ．ＺｈａｏＱｉｎｇｌｉａｎｇ,Ｚ...