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.     

Inbreeding depression in smooth cordgrass (Spartina alterniflora, Poaceae) invading San Francisco Bay

The magnitude of inbreeding depression in invading plant populations is often presumed to be small and of little consequence. The purpose of this study was to assess the magnitude of inbreeding depression in a pollen-limited, partially self-incompatible, invading plant population. The magnitude and timing of inbreeding depression were compared among ten maternal plants sampled from a population of smooth cordgrass (Spartina alterniflora) invading San Francisco Bay. Selfed and outcrossed progeny were compared for embryo abortion, survival of seedlings, and growth/survival at the end of the first growing season in three greenhouse environments. Estimates of inbreeding depression varied among environments, with competitive environment > high-nutrient environment > low-nutrient environment. Population-level estimates of inbreeding depression ranged from 0.61 to 0.81; however, maternal plants varied significantly in their magnitude of inbreeding depression, ranging from 0.1 to 0.97. The 95% confidence interval for inbreeding depression for some maternal plants included zero. There was a significant negative correlation between the overall magnitude of inbreeding depression and self-fertility rate among maternal plants. The few maternal plants with high self-fertility carried relatively little genetic load, and their selfed progeny are likely to survive on open mudflats. The noncompetitive, pollen-limited growing conditions associated with invasion may allow self-fertility to spread in this population.     

Hybridization between invasive Spartina densiflora (Poaceae) and native S. foliosa in San Francisco Bay, California, USA

Rapid evolution in contemporary time can result when related species, brought together through human-aided introduction, hybridize. The significant evolutionary consequences of post-introduction hybridization range from allopolyploid speciation to extinction of species through genetic amalgamation. Both processes are known to occur in the perennial cordgrass genus, Spartina. Here we report the existence of a third recent Spartina hybridization, discovered in 2002, between introduced S. densiflora and native S. foliosa in San Francisco Bay, California, USA. We used nuclear and chloroplast DNA analysis and nuclear DNA content with chromosome counts to examine plants of morphology intermediate between S. densiflora and S. foliosa in a restored marsh in Marin County, California. We found 32 F(1) diploid hybrids and two triploid plants, all having S. densiflora and S. foliosa as parents; there is also evidence of a genetic contribution of S. alterniflora in some hybrids. None of these hybrids set germinable seed. In 2007 we found a hybrid over 30 miles away in a marsh where both parental species occurred, suggesting hybridization may not be a localized phenomenon. The presence of diploid and triploid hybrids is important because they indicate that several avenues existed that may have given rise to a new allopolyploid species. However, such an event is now unlikely because all hybrids are targets of eradication efforts.     

Evolution of a new ecotype of Spartina alterniflora (Poaceae) in San Francisco Bay, California, USA

We report the discovery and spread of a dwarf ecotype of Spartina alterniflora in San Francisco Bay. Relative to typical S. alterniflora, this dwarf ecotype has one-fifth the tiller height (～21 cm), tenfold the tiller density (～4000 tillers/m(2)), and is restricted to growth in the upper intertidal zone. Chromosome counts of the dwarfs are identical to typical smooth cordgrass (2n = 62), and smooth cordgrass-specific random amplified DNA markers confirm the species identity of the dwarf. Field-collected clonal fragments of the dwarf grown for 2 yr under high-nutrient conditions maintained the dwarf syndrome, as did plants grown from the seed of a dwarf. The dwarf condition is not caused by endophytic fungi. The first dwarf smooth cordgrass patch was discovered in 1991, and by 1996 five separate dwarf patches had appeared within 200 m of the original. Since 1991, total area covered by the dwarf ecotype has increased sixfold to 140 m(2). The ecological range of the dwarf smooth cordgrass ecotype is similar to that of S. patens, a competitor on the Atlantic coast. We suggest that the absence of S. patens from most of San Francisco Bay has allowed the dwarf ecotype of smooth cordgrass to survive and spread.     

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.     

互花米草黄酮含量分析及其生态学意义

互花米草原产于北美洲大西洋沿岸和墨西哥海湾,经引种和其他原因,已扩散到包括中国在内的多个国家和地区,并对被入侵地生态系统的结构和功能均造成了显著影响。有关该物种的入侵机制和控制策略是当前入侵生态学研究领域的热点。互花米草生物质材料中富含黄酮类物质,与该物种向海扩张的能力可能存在一定联系。鉴于黄酮广泛的应用价值,能否借助黄酮成分的开发带动生物质的收割,以达到有效控制该物种的目的,也是当前研究者所关注的问题。以芦丁为标准参照物,NaNO2-Al（ NO3）3法比较温室互花米草不同器官中总黄酮含量的差异,分析盐城滩涂互花米草越冬芽黄酮含量与所在地理位置之间的关系,采用L9（34）正交试验对干储互花米草叶总黄酮的提取工艺进行优化。互花米草叶片中黄酮类物质远高于根、根状茎、茎、叶鞘、颖稃及种子。盐城滩涂上互花米草越冬芽的黄酮含量随着由陆向海方向基本呈升高趋势。互花米草总黄酮最佳提取工艺为料液比1∶30,乙醇浓度70％,水浴温度80℃,水浴时间2h。互花米草植物材料中,尤其在叶部,黄酮含量十分丰富。该黄酮物质与互花米草在海岸带盐沼生态系统中的适应和向海扩张也存在正相关关系,可作为探索互花米草入侵机制的一个方向。利用本研究中所得优化提取工艺可从互花米草干储生物质中获得稳定的黄酮来源。配合黄酮经济成分的开发而推动生物质材料的收割利用,将成为互花米草生态控制的一项潜在策略。     

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.     

Spread of Exotic Cordgrasses and Hybrids (Spartina sp.) in the Tidal Marshes of San Francisco Bay, California, USA

Four species of exotic cordgrass (Spartina sp.) occur in the San Francisco estuary in addition to the California native Spartina foliosa. Our goal was to map the location and extent of all non-native Spartina in the estuary. Hybrids of S. alterniflora and S. foliosa are by far the most numerous exotic and are spreading rapidly. Radiating from sites of deliberate introduction, S. alterniflora and hybrids now cover ca. 190 ha, mainly in the South and Central Bay. Estimates of rate of aerial increase range from a constant value to an accelerating rate of increase. This could be due to the proliferation of hybrid clones capable of rapid expansion and having superior seed set and siring abilities. The total coverage of 195 ha by hybrids and other exotic cordgrass species is slightly less than 1% of the Bay's tidal mudflats and marshes. Spartina anglica has not spread beyond its original 1970s introduction site. Spartina densiflora has spread to cover over 5 ha at 3 sites in the Central Bay. Spartina patens has expanded from 2 plants in 1970 to 42 plants at one site in Suisun Bay. Spartina seed floats on the tide, giving it the potential to export this invasion throughout the San Francisco estuary, and to estuaries outside of the Golden Gate. We found isolated plants of S. alterniflora and S. densiflora in outer coast estuaries north of the Bay suggesting the likelihood for the San Francisco Bay populations to found others on the Pacific coast.     

Effects of drainage and soil organic content on growth of Spartina alterniflora (Poaceae) in an artificial salt marsh mesocosm

An artificial salt marsh mesocosm was constructed using 680-L polypropylene tanks to determine the effect of soil drainage depth and organic content on growth and rhizome proliferation of the salt marsh smooth cordgrass Spartina alterniflora. Soil drainage depth had no effect on accumulation of aerial or subsurface plant tissue, but tanks that had 2.5% soil organic content supported enhanced aerial tissue and rhizome growth compared to tanks that contained sand alone. We propose a mathematical model for predicting the mass of photosynthetically significant leaf tissue without cutting and drying leaves. Implications of these findings for salt marsh creation projects are discussed.     

Genetic evidence for hybridization between the native Spartina maritima and the introduced Spartina alterniflora (Poaceae) in South-West France: Spartina × neyrautii re-examined

 Spartina alterniflora, a perennial grass native to the North American Atlantic coast, was introduced during the 19th century in western Europe (Southern England and western France) where it hybridized with the native Spartina maritima. In England, the sterile hybrid S. × townsendii gave rise by chromosome doubling to the highly fertile allopolyploid Spartina anglica, which has now invaded many salt marshes and estuaries in western Europe, and has been introduced in several continents. In South-West France, another sterile hybrid was discovered in 1892 in the Bidassoa Estuary, and named Spartina × neyrautii. According to their morphology, some authors suggested that S. × neyrautii and S. × townsendii result from reciprocal crosses. During the 20th century, the hybridization site was severely disturbed, and surviving of S. × neyrautii was questioned. In this paper, various Spartina populations are investigated in the Basque region (France and Spain), and compared to the hybrid taxa formed in England (S. × townsendii and S. anglica). The samples were analyzed using molecular fingerprinting (RAPD and ISSR) and Chloroplast DNA sequence (trnL-trnT spacer, trnL intron and trnL-trnF spacer). In the Bidassoa estuary, a hybrid isolated clone has been found, that displays additive species-specific nuclear markers of S. maritima and S. alterniflora, and that is subsequently considered as a surviving clone of S. × neyrautii. The molecular analyses indicate that S. × neyrautii and S. × townsendii share the same maternal (S. alterniflora), and paternal (S. maritima) parental species, but also that the two independent hybridization events have involved different parental (nuclear) genotypes in England and in South-West France. Received July 12, 2002; accepted October 4, 2002 Published online: March 20, 2003     

One hundred years of Spartina altemiflora (Poaceae) in Willapa Bay, Washington: random amplified polymorphic DNA analysis of an invasive population

Spartina altemiflora (smooth cordgrass) has invaded Willapa Bay, Washington, covering bare mud flats and seagrass beds. At current expansion rates it threatens to occupy most of the intertidal habitat in the bay within 40 years. Although a major mitigation effort is underway, nothing is known about the genetic structure of the Willapa Bay S. altemiflora population or the underlying causes for the dramatic increase in seed set that has occurred in the last 15 years. Random amplified polymorphic DNA (RAPD) analysis was assessed as a method of DNA fingerprinting to analyse the genetic history and current structure of the Willapa population. The results suggest that all S. alterniflora clones in Willapa Bay are descended from a single genet. Given the limited genetic variability expected in a clonally founded population, we discuss the utility of RAPDs as genetic markers to explore the large morphological and reproductive differences reported among Willapa clones. Cluster analysis of RAPD fragments was used to show relatedness among S. alteniflora genets and may be helpful in locating the most prolific clones for mitigation efforts. Overall, RAPDs show great potential in genetic analysis of S. altemiflora, particularly in invasive populations where isozyme variability is low or nonexistent.     

Effect of oxygen availability and salinity on EARLY LIFE HISTORY STAGES OF SALT MARSH PLANTS. I. Different germination strategies of Spartina alterniflora and Phragmites australis (Poaceae)

Gradients in oxygen availability and salinity are among the most important environmental parameters influencing zonation in salt marsh communities. The combined effects of oxygen and salinity on the germination of two salt marsh grasses, Spartina alterniflora and Phragmites australis, were studied in growth chamber experiments. Germination of both species was initiated by emergence of the shoot and completed by root emergence. Percentage S. alterniflora germination was reduced at high salinity (40 g NaCl/L) and in decreased oxygen (5 and 2.5%). In 0% oxygen shoots emerged, but roots did not. P. australis germination was reduced at a lower salinity (25 g NaCl/L) than S. alterniflora, and inhibited at 40 g NaCl/L and in anoxia. However, a combination of hypoxia (10 and 5% O2) and moderate salinity (5 and 10 g NaCl/L) increased P. australis germination. When bare areas in the salt marsh are colonized, the different germination responses of these two species to combinations of oxygen and salt concentrations are important in establishing their initial zonation. In high salinity wetlands S. alterniflora populates the lower marsh and P. australis occupies the high marsh at the upland boundary.     

Variable reproductive output among clones of Spartina alterniflora (Poaceae) invading San Francisco Bay,California: the influence of herbivory,pollination, and establishment site

Spartina alterniflora has recently been introduced to San Francisco Bay, California, and is rapidly invading open mud flats, growing in circular patches that we found to be individual genetic clones. We collected spikelet samples from more than 200 clones and observed germination rates ranging from 0% to 59%, indicating substantial variation in reproductive output among clones. Several experiments were performed to explore the cause of variation. Pollination manipulations showed that S. alterniflora is outcrossing, but pollen supplements did not increase spikelet germination rates. Exclusion of the only insect herbivore (a phloem feeder, Prokelisia marginata) from developing inflorescences increased the proportion of spikelets containing seed, but failed to increase germinations per spikelet. Spikelets from Willapa Bay, Washington, grown free of insect herbivores, had germination rates similar to San Francisco Bay. These results suggest herbivory is not limiting reproductive output of S. alterniflora. Spikelet viability was not related to clone size; however, clones located lower in the intertidal or far up a drainage slough averaged fewer germinations per spikelet, suggesting clones in areas with lower genet density may have lower spikelet viabilities. Spikelet samples from different sections of clones growing across wide environmental ranges had similar rates of germination, suggesting some genetic influence on spikelet viability. Differential reproductive output among clones and the novel selective environment of San Francisco Bay are expected to cause gene frequency changes in this rapidly expanding population.     

Introgression between invasive and native blue mussels (genus Mytilus) in the central California hybrid zone

The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. Here, we investigate the relative importance of isolating barriers and the demographic expansion of invasive Mytilus galloprovincialis on the magnitude and the direction of introgression with the native Mytilus trossulus in a hybrid zone in central California. We use double‐digest restriction‐site‐associated DNA sequencing (ddRADseq) to genotype 1337 randomly selected single nucleotide polymorphisms and accurately distinguish early and advanced generation hybrids for the first time in the central California Mytilus spp. hybrid zone. Weak levels of introgression were observed in both directions but were slightly more prevalent from the native M. trossulus into the invasive M. galloprovincialis. Few early and advanced backcrossed individuals were observed across the hybrid zone confirming the presence of strong barriers to interbreeding. Heterogeneous patterns of admixture across the zone of contact were consistent with the colonization history of M. galloprovincialis with more extensive introgression in northern localities furthest away from the putative site of introduction in southern California. These observations reinforce the importance of dynamic spatial and demographic expansions in determining patterns of introgression between close congeners, even in those with high dispersal potential and well‐developed reproductive barriers. Our results suggest that the threat posed by invasive M. galloprovincialis is more ecological than genetic as it has displaced, and continues to displace the native M. trossulus from much of central and southern California.     

Hybridization and introgression in Carpobrotus spp. (Aizoaceae) in California. II. Allozyme evidence

Morphological evidence has indicated that hybridization and introgression are occurring between Carpobrotus edulis L., a nonindigenous, invasive species in California (Bolus), and its putative native congener, C. chilensis. The identification of allozyme markers has enabled us to quantify hybridization and the extent of introgression between these species. Samples from 20 individuals of each of five morphological types (both parent species and three hybrid phenotypes) were collected from 39 populations along the coast of California from the Mexico to Oregon borders. Ten enzyme systems revealed a total of 17 resolvable loci, eight of which were polymorphic for the genus. Five of the polymorphic loci easily differentiate C. edulis and C. chilensis. Allele frequencies among the morphologically defined types are consistent with estimations of allele frequency changes due to first- and second-generation backcrossing. In comparison to long-lived, herbaceous perennials and widespread species, C. edulis and intermediate types have more variation in their populations (P = 41.73, Ap = 2.11, Ho = 0.246, and P = 38.13, Ap = 2.06, Ho = 0.216, respectively) than C. chilensis (P = 11.76, Ap = 2.00, Ho = 0.082). Indirect estimates of gene flow indicate four of the five morphological types are outcrossing. Gene flow between previously allopatric species may have broad implications if it results in an increase in fitness; further experimentation is needed to determine the ultimate ecological consequences of this phenomenon and its possible threat to limited, remaining natural habitat in California.     

Variable extent of hybridization between invasive sika (Cervus nippon) and native red deer (C. elaphus) in a small geographical area

Instances of hybridization between endemic and alien species pose a threat to species integrity but also provide us with an opportunity to study the dynamics of gene flow between two species as they first meet. Here, we used variation at 22 highly differentiated microsatellite loci and one mitochondrial DNA (mtDNA) marker in a sample of 735 individuals, to investigate the genetic consequences of an introduction of Japanese sika deer ( Cervus nippon ) for native red deer ( C. elaphus ) on the Kintyre Peninsula in Scotland. We investigated population structure, estimated null-allele frequency and assigned individual hybrid scores using a Bayesian clustering algorithm implemented in structure 2.2. The dataset clearly divided into two clusters and generally, introgression into red and sika was low. However at one site, West Loch Awe, 43% of individuals were hybrids. MtDNA introgression indicated that hybridization was occurring between red-deer hinds and sika-deer stags. We argue that the pattern of differential introgression across the study area is primarily due to the rarity of hybridization events between the two species and the limited time the two species have been in contact (< 120 years). This contrasts with the causes of classic mosaic hybrid zones (selection induced by habitat variability). Currently, it seems possible that, in time, the level of hybridization found at West Loch Awe could also be found across the whole of the peninsula.     

Hybridization between native barbless carp (Cyprinus pellegrini) and introduced common carp (C. carpio) in Xingyun Lake, China

Hybridization with introduced fish species is an important threat to native fish species. Here we investigated hybridization between native barbless carp (Cyprinus pellegrini) and introduced common carp (C. carpio) in Xingyun Lake in the Yunnan-Guizhou plateau of China. A total of 203 individuals of Cyprinus from Xingyun Lake were studied by combination of morphological and genetic analyses. Most individuals were strictly intermediate between the two parental species in morphology, strongly suggesting that extensive hybridization has occurred. Bayesian model-based clustering of the genetic data suggests that there are two distinct genetic groups corresponding to barbless and common carp respectively. Many individuals in the two genetic groups showed intermediate morphology, suggesting that both groups actually contain massively introgressed genes. Only two individuals were identified as barbless carp both morphologically and genetically, hinting that this native species is at risk of genetic extinction in Xingyun Lake.