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.     

Hybridization between introduced smooth cordgrass (Spartina alterniflora; Poaceae) and native California cordgrass (S. foliosa) in San Francisco Bay, California, USA

Introduced Spartina alterniflora (smooth cordgrass) is rapidly invading intertidal mudflats in San Francisco Bay, California. At several sites, S. alterniflora co-occurs with native S. foliosa (California cordgrass), a species endemic to California salt marshes. In this study, random amplified polymorphic DNA markers (RAPDs) specific to each Spartina species were identified and used to test for hybridization between the native and introduced Spartina species in the greenhouse and in the field. Greenhouse crosses were made using S. alterniflora as the pollen donor and S. foliosa as the maternal plant, and these crosses produced viable seeds. The hybrid status of the crossed offspring was confirmed with the RAPD markers. Hybrids had low self-fertility but high fertility when back-crossed with S. foliosa pollen. Hybrids were also found established at two field sites in San Francisco Bay; these hybrids appeared vigorous and morphologically intermediate between the parental species. Field observations suggested that hybrids were recruiting more rapidly than the native S. foliosa. Previous work identified competition from introduced S. alterniflora as a threat to native S. foliosa. In this study, we identify introgression and the spread of hybrids as an additional, perhaps even more serious threat to conservation of S. foliosa in San Francisco Bay.     

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.     

Sexual reproduction of cordgrass hybrids (Spartina foliosa x alterniflora) invading tidal marshes in San Francisco Bay

We genetically analysed cordgrass plants and seedlings throughout the San Francisco, California, USA, estuary and found that hybrids between exotic Spartina alterniflora and native Spartina foliosa are the principal cordgrass invaders and colonizers. We hypothesized that this was due to higher seed set and siring ability by hybrids relative to the native species; too few alien parents remained in San Francisco Bay for our comparative studies. Hybrid seed comprised 91% to 98% of that set in the marsh study plants over the 2 years of the study. Total viable pollen production by hybrid plants was 400 times that of the native plants. Seed and pollen production were highly skewed towards a few hybrid genotypes. In addition to seed produced by hybrid plants, hybrid seed was produced by S. foliosa due to hybrid backcrossing. While the greatest advantage for hybrids was in pollen and seed production, hybrid seeds germinated, and seedlings survived and grew as well or better than the native species. As native S. foliosa becomes increasingly rare, hybrid seed floating on the tides will predominate, overwhelming recruitment sites and resulting in further colonization by hybrids. In an evolutionary context, hybrids with exceptional pollen and seed production will be initially favoured by natural selection, leading to the evolution of even more fertile hybrid genotypes.     

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.     

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.     

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.     

Greater male fitness of a rare invader (Spartina alterniflora, Poaceae) threatens a common native (Spartina foliosa) with hybridization

Hybridization with abundant invaders is a well-known threat to rare native species. Our study addresses mechanisms of hybridization between a rare invader, smooth cordgrass (Spartina alterniflora) and the common native California cordgrass (S. foliosa) in the salt marshes of San Francisco Bay. These species are wind-pollinated and flower in summer. The invader produced 21-fold the viable pollen of the native, and 28% of invader pollen germinated on native stigmas (1.5-fold the rate of the native's own pollen). Invader pollen increased the seed set of native plants almost eightfold over that produced with native pollen, while native pollen failed to increase seed set of the invader. This pollen swamping and superior siring ability by the invader could lead to serial genetic assimilation of a very large native population. Unlike California cordgrass, smooth cordgrass can grow into low intertidal habitats and cover open mud necessary to foraging shorebirds, marine life, navigation, and flood control in channels. To the extent that intertidal range of the hybrids is more similar to the invader than to the native parent, introgression will lead to habitat loss for shore birds and marine life as well to genetic pollution of native California cordgrass.     

Morphological, cytogenetic, and molecular evidence for introgressive hybridization in birch.

Extensive morphological variation of tetraploid birch (Betula pubescens) in Iceland is believed to be due to gene flow from diploid dwarf birch (B. nana) by means of introgressive hybridization. A combined morphological and cytogenetic approach was used to investigate this phenomenon in two geographically separated populations of natural birch woodland in Iceland. The results not only confirmed introgressive hybridization in birch, but also revealed bidirectional gene flow between the two species via triploid interspecific hybrids. The populations showed continuous morphological variation connecting the species, but karyotypically they consisted of only three types of plants: diploids, triploids, and tetraploids. No aneuploids were found. Some of the tetraploid plants had B. pubescens morphology as expected, but most of them had intermediate characters. Most of the diploid plants were B. nana, but some were intermediates and a few had B. pubescens morphology. The triploid plants were either intermediates or they resembled one of the two species. Similar introgressive variation was observed among the diploid and triploid progeny of open-pollinated B. nana in a garden. Birch samples including field plants and artificial hybrids were further examined using a molecular method based on genomic Southern hybridization. The experiments verified introgression at the DNA level.     

Variation in nuclear DNA content in Malus species and cultivated apples.

The nuclear DNA content for a group of 40 Malus species and hybrids has been estimated using flow cytometry. Estimates of nuclear DNA content for this germplasm collection range from 1.45 pg for Malus fusca (diploid) to 2.57 pg for Malus ioensis (triploid). Among diploids, the nuclear (2C) DNA ranges from 1.45 pg for M. fusca to 1.68 pg for Malus transitoria. Among triploids, the nuclear (3C) DNA content ranges from 2.37 pg / 3C for Malus sikkimensis to 2.57 pg / 3C for M. ioensis. Given the complexity of the apple genome and its suggested allopolyploid origin, the results obtained in this study confirm earlier reports that polyploids can easily withstand the loss of a certain amount of DNA, and that there is a slight tendency towards diminished haploid nuclear DNA content with increased polyploidy.     

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)

Spartina alterniflora, smooth cordgrass, native to the eastern USA, was introduced into south San Francisco Bay ≈ 25 years ago. It has spread by purposeful introduction of rooted plants and dispersal of seeds on the tides. Previous work suggested that S. alterniflora was competitively superior to the native California cordgrass, S. foliosa, and that the two species hybridized. The present study determined the spread of S. alterniflora and S. foliosa × alterniflora hybrids in California and examined the degree of hybridization. We used nuclear DNA markers diagnostic for each species to detect the parental species and nine categories of hybrids. The California coast outside San Francisco Bay contained only the native species. All hybrid categories exist in the Bay, implying that several generations of crossing have occurred and that hybridization is bidirectional. Hybrids were found principally near sites of deliberate introduction of the exotic species. Where S. alterniflora was deliberately planted, we found approximately equal numbers of S. alterniflora and hybrid individuals; S. foliosa was virtually absent. Marshes colonized by water-dispersed seed contained the full gamut of phenotypes with intermediate-type hybrids predominating. The proliferation of hybrids could result in local extinction of S. foliosa. What is more, S. alterniflora has the ability to greatly modify the estuary ecosystem to the detriment of other native species and human uses of the Bay. To the extent that they share these engineering abilities, the proliferation of cordgrass hybrids could grossly alter the character of the San Francisco Bay.     

Molecular cytogenetic study on the ploidy status in Acipenser mikadoi

The ploidy status of Acipenser mikadoi was examined using nuclear DNA contents, karyotypes and fluorescence in situ hybridization (FISH) with 5.8S + 28S rDNA as a probe. In flow‐cytometrically sorted specimens with 8.2–9.1 pg DNA content per somatic cell, i.e. genetic diploid, the best informative metaphase with 268 chromosomes had 80 biarmed meta‐ or submetacentric (M or SM) chromosomes, 48 monoarmed telocentric (T) chromosomes and 140 microchromosomes. In genetic triploid specimens with 12.6–13.0 pg DNA content, the best informative metaphase with 402 chromosomes showed 120 biarmed M or SM, 72 monoarmed T chromosomes and 210 microchromosomes. The rDNA FISH detected a maximum 18 and 27 signals in the diploid and triploid A. miakdoi, respectively. The obtained findings thus corroborated a clear parallel between nuclear DNA contents and karyological or FISH profiles in the genetic diploid and triploid specimens, suggesting 1.5 times chromosome complements of diploid counterparts or three sets of homologues in the triploid sturgeons. Moreover, the estimated genome size and the observed molecular cytogenetic features in the diploid A. mikadoi strongly suggest that this species is a member of a functional tetraploid group recently proposed in the literature.     

Chromosome set manipulations and hybridization experiments in gilthead seabream (Sparus aurata). II. Assessment of diploid and triploid hybrids between gilthead seabream and red seabream (Pagrus major)

The experiments were designed to evaluate the suitability for mariculture of the diploid and triploid hybrids of gilthead seabream Sparus aurata and red seabream Pagrus major , both of which are members of the Sparidae family. Performance testing of the hybrids was carried out in comparison with the parental species under the same controlled environment. The reciprocal diploid hybrids as well as the triploid hybrid did not exhibit any significant growth advantage over diploid parental species either before or after sexual maturity. The adults, both reciprocal diploid hybrids and triploid hybrid (female S. aurata × male P. major ), were clearly immature and had only vestigial gonads; neither ovaries nor released milts were observed. Histological examination of the gonads, carried out during associated periods of the first and second maturation cycles of the parental species, showed that all the hybrids were completely sterile. Despite their sterility, the growth of the hybrids did not display any positive effect, and for this reason their commercial culture appears to be questionable. On the other hand, the use of sterilization through hybridization and chromosome set manipulation may be important when there is a need to restrict the ecological impact on a wild population.     

Distant hybridization leads to different ploidy fishes

Distant hybridization makes it possible to transfer the genome of one species to another, which results in changes in phenotypes and genotypes of the progenies. This study shows that distant hybridization or the combination of this method with gynogenesis or androgenesis lead to different ploidy fishes with genetic variation, including fertile tetraploid hybrids, sterile triploid hybrids, fertile diploid hybrids, fertile diploid gynogenetic fish, and their derived progenies. The formations of the different ploidy fishes depend on the genetic relationship between the parents. In this study, several types of distant hybridization, including red crucian carp (Carassius auratus red var.) (2n=100, abbreviated as RCC) (♀)×common carp (Cyprinus carpio L.) (2n=100, abbreviated as CC) (♂), and RCC (2n=100) (♀)×blunt snout bream (Megalobrama amblycephala) (2n=48, abbreviated as BSB) (♂) are described. In the distant hybridization of RCC (♀)×CC (♂), bisexual fertile F3–F18 allotetraploid hybrids (4n=200, abbreviated as 4nAT) were formed. The diploid hybrid eggs and diploid sperm generated by the females and males of 4nAT developed into diploid gynogenetic hybrids and diploid androgenetic hybrids, respectively, by gynogenesis and androgenesis, without treatment for doubling the chromosome. Improved tetraploid hybrids and improved diploid fishes with genetic variation were derived from the gynogenetic hybrid line. The improved diploid fishes included the high-body RCC and high-body goldfish. The formation of the tetraploid hybrids was related to the occurrence of unreduced gametes generated from the diploid hybrids, which involved in premeiotic endoreduplication, endomitosis, or fusion of germ cells. The sterile triploid hybrids (3n=150) were produced on a large scale by crossing the males of tetraploid hybrids with females of diploid fish (2n=100). In another distant hybridization of RCC (♀)×BSB (♂), different ploidy fishes were obtained, including diploid bisexual fertile natural gynogenetic fish (2n=100), sterile triploid hybrids (3n=124), and bisexual fertile tetraploid hybrids (4n=148). Furthermore, two kinds of pentaploid hybrids (5n=172 and 5n=198) were formed. The biological characteristics and the mechanisms of formation of the different ploidy fish were compared and discussed at the cellular and molecular level. The results indicated distant hybridization or the combination of this method with gynogenesis or androgenesis affects the formation of different ploidy fish with genetic variation.     

Interploidal hybridization and mating patterns in the Sphagnum subsecundum complex

Polyploidization is thought to result in instant sympatric speciation, but several cases of hybrid zones between one of the parental species and its polyploid derivative have been documented. Previous work showed that diploid Sphagnum lescurii is an allopolyploid derived from the haploids S. lescurii (maternal progenitor) and S. subsecundum (paternal progenitor). Here, we report the results from analyses of a population where allodiploid and haploid S. lescurii co-occur and produce sporophytes. We tested (i) whether haploids and diploids form hybrid triploid sporophytes; (ii) how hybrid and nonhybrid sporophytes compare in fitness; (iii) whether hybrid sporophytes form viable spores; (iv) the ploidy of any viable gametophyte offspring from hybrid sporophytes; (v) the relative viability of sporelings derived from hybrid and nonhybrid sporophytes; and (vi) if interploidal hybridization results in introgression between the allopolyploid and its haploid progenitor. We found that triploid hybrid sporophytes do occur and are larger than nonhybrid sporophytes, but exhibit very low germination percentages and produce sporelings that develop more slowly than those from nonhybrid sporophytes. All sporophytes attached to haploid gametophytes were triploid and were sired by diploid males, but all sporophytes attached to diploid gametophytes were tetraploid. This asymmetric pattern of interploidal hybridization is related to an absence of haploid male gametophytes in the population. Surprisingly, all sporelings from triploid sporophytes were triploid, yet were genetically variable, suggesting some form of aberrant meiosis that warrants further study. There was limited (but some) evidence of introgression between allodiploid and haploid S. lescurii.     

Bypassing reproductive barriers in hybrid seeds using chemically induced epimutagenesis

The triploid block, which prevents interploidy hybridizations in flowering plants, is characterized by a failure in endosperm development, arrest in embryogenesis, and seed collapse. Many genetic components of triploid seed lethality have been successfully identified in the model plant Arabidopsis thaliana, most notably the paternally expressed genes (PEGs), which are upregulated in tetraploid endosperm with paternal excess. Previous studies have shown that the paternal epigenome is a key determinant of the triploid block response, as the loss of DNA methylation in diploid pollen suppresses the triploid block almost completely. Here, we demonstrate that triploid seed collapse is bypassed in Arabidopsis plants treated with the DNA methyltransferase inhibitor 5-Azacytidine during seed germination and early growth. We identified strong suppressor lines showing stable transgenerational inheritance of hypomethylation in the CG context, as well as normalized expression of PEGs in triploid seeds. Importantly, differentially methylated loci segregate in the progeny of “epimutagenized” plants, which may allow epialleles involved in the triploid block response to be identified in future studies. Finally, we demonstrate that chemically induced epimutagenesis facilitates hybridization between different Capsella species, thus potentially emerging as a strategy for producing triploids and interspecific hybrids with high agronomic interest.

Genome-wide loss of DNA methylation induced by 5-Azacytidine allows interploidy and interspecific hybridization barriers to be bypassed in Arabidopsis and Capsella.     

Nonadditive changes in genome size during allopolyploidization in the wheat (aegilops-triticum) group

Interspecific or intergeneric hybridization, followed by chromosome doubling, can lead to the formation of new allopolyploid species. Recent studies indicate that allopolyploid formation is associated with genetic and epigenetic changes. Despite these studies, it is not yet clear whether the C value of an allopolyploid is the sum of its diploid parents. To address this question, six newly synthesized wheat allopolyploids and their parental plants were investigated. It was found that allopolyploids have a genome size significantly smaller than the expected value. The reduction of the nuclear genome size in the synthetic allotetraploids and allohexaploids was 2 pg DNA at 2C. It was also found that changes in the genome size already existed in the first generation amphiploids, indicating that the change was a rapid event. There was no difference in the reduction of nuclear genome size between the allotetraploid and the allohexaploid. These data clearly show that genome differentiation in allopolyploids was not related to the ploidy level. The data obtained clearly suggested that the nonadditive change in genome size that occurred during allopolyploidization may represent a preprogrammed adaptive response to genomic stress caused by hybridization and allopolyploidy, which serves to stabilize polyploid genomes.     

Genetic relationships among Hylocereus and Selenicereus vine cacti (Cactaceae): evidence from hybridization and cytological studies

BACKGROUND AND AIMS: Hylocereus and Selenicereus are native to tropical and sub-tropical America. Based on its taxonomic status and crossability relations it was postulated that H. megalanthus (syn. S. megalanthus) is an allotetraploid (2n = 4x = 44) derived from natural hybridization between two closely related diploid taxa. The present work aimed at elucidating the genetic relationships between species of the two genera. METHODS: Crosses were performed and the putative hybrids were analysed by chromosome counts and morphological traits. The ploidy level of hybrids was confirmed by fluorescent in situ hybridization (FISH) of rDNA sites. Genomic in situ hybridization (GISH) was used in an attempt to identify the putative diploid genome donors of H. megalanthus and an artificial interploid hybrid. KEY RESULTS: Reciprocal crosses among four diploid Hylocereus species (H. costaricensis, H. monacanthus (syn. H. polyrhizus), H. undatus and Hylocereus sp.) yielded viable diploid hybrids, with regular chromosome pairing. Reciprocal crosses between these Hylocereus spp. and H. megalanthus yielded viable triploid, pentaploid, hexaploid and aneuploid hybrids. Morphological and phenological traits confirm the hybrid origin. In situ detection of rDNA sites was in accord with the ploidy status of the species and hybrid studied. GISH results indicated that overall sequence composition of H. megalanthus is similar to that of H. ocamponis and S. grandiflorus. High sequence similarity was also found between the parental genomes of H. monacanthus and H. megalanthus in one triploid hybrid. CONCLUSIONS: The ease of obtaining partially fertile F1 hybrids and the relative sequence similarity (in GISH study) suggest close genetic relationships among the taxa analysed.     

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.