Conservation Implications of Invasion by Plant Hybridization

The increasing number of invasive exotic plant species in many regions and the continuing alteration of natural ecosystems by humans promote hybridization between previously allopatric species; among both native as well as between native and introduced species. We review the ecological factors and mechanisms that promote such hybridization events and their negative consequences on biological diversity. Plant invasions through hybridization may occur in four different ways: hybridization between native species, hybridization between an exotic species and a native congener, hybridization between two exotics and by the introduction and subsequent spread of hybrids. The main harmful genetic effect of such hybrids on native species is the loss of both genetic diversity and of locally adapted populations, such as rare and threatened species. The spread of aggressive hybrid taxa can reduce the growth of, or replace, native species. The main factor promoting the formation of hybrids is species dispersal promoted by humans. However, the success and spread of hybrids is increased by disturbance and fragmentation of habitats, thus overcoming natural crossing barriers, and range expansions due to human activity. There are differences in flowering, pollination and seed dispersal patterns between parental species and hybrids. Hybrid resistance to pathogens and herbivores may also enhance the success of hybrids. To predict the mechanisms and consequences of invasions mediated by hybridization, extensive data on hybrid ecology and biology are needed, as well as carefully designed field experiments focused on the comparative ecology of parental populations and hybrids.     

Hybrid zones between invasive Rorippa austriaca and native R. sylvestris (Brassicaceae) in Germany: ploidy levels and patterns of fitness in the field

Hybrid zones may serve as natural laboratories for evolutionary studies. One common viewpoint is that hybrids may always be less fit than their parents due to genetic discontinuities. An alternative idea is that genotype-environment interactions influence the outcome of natural hybridization. Our comparative study of two different natural hybrid zones between the invasive diploid Rorippa austriaca and the native polyploid R. sylvestris in Germany identified the ploidy level as a major determinant of hybrid fitness. Different ploidy levels and patterns of fitness were detected in different hybrid zones. In one hybrid zone (Mülheim, Ruhr valley) hybrids were pentaploid and showed a relatively high seed set, whereas in the second hybrid zone (Randersacker, Main valley) hybrids were triploid and displayed extremely low fitness values. Analyses of fitness values in different natural hybrid zones between the same two species may lead to very different conclusions about the evolutionary significance of natural hybridization.     

Natural hybridization and hybrid zones between Quercus crassifolia and Quercus crassipes (Fagaceae) in Mexico: morphological and molecular evidence

Hybrid zones provide interesting systems to study genetic and ecological interaction between different species. The correct identification of hybrids is necessary to understand the evolutionary process involved in hybridization. An oak species complex occurring in Mexico formed by two parental species, Quercus crassifolia H. & B. and Q. crassipes H. & B., and their putative hybrid species, Q. dysophylla, was analyzed with molecular markers (random amplified polymorphic DNA [RAPDs]) and morphological tools in seven hybrid zones (10 trees per taxa in each hybrid zone) and two pure sites for each parental species (20 trees per site). We tested whether geographic proximity of hybrid plants to the allopatric site of a parental species increases its morphological and genetic similarity with its parent. Seventeen morphological traits were measured in 8700 leaves from 290 trees. Total DNA of 250 individuals was analyzed with six diagnostic RAPD primers. Quercus crassifolia differed significantly from Q. crassipes in all the examined characters. Molecular markers and morphological characters were highly coincident and support the hypothesis of hybridization in this complex, although both species remain distinct in mixed stands. Clusters and a hybrid index (for molecular and morphological data) showed that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Evidence does not indicate a unidirectional pattern of gene flow.     

Morphological variation of genetically confirmed Alouatta Pigra × A. palliata hybrids from a natural hybrid zone in Tabasco,Mexico

While hybridization has been reported for a large number of primate taxa, there is a general lack of data on hybrid morphology for wild individuals with known genetic ancestry. A confirmed hybrid zone for the closely related Neotropical primates Alouatta palliata and A. pigra has provided a unique opportunity to study primate hybrid morphological variation. Here we used molecular evidence based on mitochondrial, Y‐chromosome, and autosomal data to assess hybrid ancestry. We conducted univariate and multivariate statistical comparisons of morphometric data collected from individuals both outside and within the hybrid zone in Tabasco, Mexico. Our results show that of all the hybrids detected (N = 128), only 12% of them were approximately genetically intermediate, and none of them were first generation hybrids. Univariate pairwise comparisons among parental individuals, multigenerational backcrossed hybrids, and intermediate hybrids showed that overall, multigenerational backcrossed hybrids resemble the parental species with which they share most of their alleles. Conversely, intermediates were highly variable. Similarly, principal component analysis depicts an overlap between the parental species and their backcrosses when considering overall morphological differences. Finally, discriminant function analysis of the morphological variables was overall unreliable for classifying individuals into their assigned genotypic classes. Taken together, our results suggest that primate natural hybridization studies should incorporate molecular methods for determining ancestry, because morphology may not always be a reliable indicator of hybrid status. Hybrid zones could comprise a large number of multigenerational backcrossed hybrids that are indistinguishable from the parental species. The implications for studying hybridization in the primate fossil record are discussed. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Molecular and morphological evidence reveals introgression in swarms of the invasive taxa Fallopia japonica, F. sachalinensis, and F. xbohemica (Polygonaceae) in the United States

Fallopia japonica (Japanese knotweed, Polygonaceae) is a well-known East Asian perennial that is established throughout the U.S. and Europe. Another congener, F. sachalinensis, and their hybrid, F. ×bohemica, also persist on both continents. Their invasive success is primarily attributed to their ability to spread via clonal growth. However, mounting evidence suggests invasion history and dynamics differ between continents and that sexual reproduction is more common than previously assumed. We used published morphological traits designed to distinguish the three taxa to characterize their distribution in 24 New England towns. We found continuous variation of all five traits, with 84% of our 81 individuals having at least one trait outside parental limits. Hierarchical cluster analysis, along with two chloroplast and one nuclear species-specific markers, suggests the presence of intercrossing, segregating hybrids, and likely introgression between F1 hybrids and F. japonica. Our markers also show the first evidence of bidirectional hybridization between parental taxa in the U.S., emphasizing the complex structure of populations in our region. This study is a first step toward unraveling the evolutionary forces that have made these taxa such aggressive invaders in the U.S. The data may also affect management strategies originally designed for largely monomorphic, clonal populations.     

Incomplete reproductive isolation between Rhododendron taxa enables hybrid formation and persistence

The evolutionary consequences of hybridization ultimately depend on the magnitude of reproductive isolation between hybrids and their parents. We evaluated the relative contributions of pre-and post-zygotic barriers to reproduction for hybrid formation, hybrid persistence and potential for reproductive isolation of hybrids formed between two Rhododendron species,R. spiciferum and R. spinuliferum. Our study established that incomplete reproductive isolation promotes hybrid formation and persistence and delays hybrid speciation.All pre-zygotic barriers to reproduction leading to hybrid formation are incomplete: parental species have overlapping flowering; they share the same pollinators;reciprocal assessments of pollen tube germination and growth do not differ among parents. The absence of post-zygotic barriers between parental taxa indicates that the persistence of hybrids is likely. Reproductive isolation was incomplete between hybrids and parents in all cases studied, although asymmetric differences in reproductive fitness were prevalent and possibly explain the genetic structure of natural hybrid swarms where hybridization is known to be bidirectional but asymmetric. Introgression, rather than speciation, is a probable evolutionary outcome of hybridization between the two Rhododendron taxa. Our study provides insights into understanding the evolutionary implications of natural hybridization in woody plants.     

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.     

The effects of hybridization in plants on secondary chemistry: implications for the ecology and evolution of plant-herbivore interactions

Natural hybridization is a frequent phenomenon in plants. It can lead to the formation of new species, facilitate introgression of plant traits, and affect the interactions between plants and their biotic and abiotic environments. An important consequence of hybridization is the generation of qualitative and quantitative variation in secondary chemistry. Using the literature and my own results, I review the effects of hybridization on plant secondary chemistry, the mechanisms that generate patterns of chemical variation, and the possible consequences of this variation for plants and herbivores. Hybrids are immensely variable. Qualitatively, hybrids may express all of the secondary chemicals of the parental taxa, may fail to express certain parental chemicals, or may express novel chemicals that are absent in each parent. Quantitatively, concentrations of parental chemicals may vary markedly among hybrids. There are five primary factors that contribute to variation: parental taxa, hybrid class (F(1), F(2), etc.), ploidy level, chemical class, and the genetics of expression (dominance, recessive vs. additive inheritance). This variation is likely to affect the process of chemical diversification, the potential for introgression, the likelihood that hybrids will facilitate host shifts by herbivores, and the conditions that might lead to enhanced hybrid susceptibility and lower fitness.     

Taxa distribution and RAPD markers indicate different origin and regional differentiation of hybrids in the invasive Fallopia complex in central-western Europe

Interspecific hybridization can be a driving force for evolutionary processes during plant invasions, by increasing genetic variation and creating novel gene combinations, thereby promoting genetic differentiation among populations of invasive species in the introduced range. We examined regional genetic structure in the invasive Fallopia complex, consisting of F. japonica var. japonica , F.   sachalinensis and their hybrid F.  ×  bohemica , in seven regions in Germany and Switzerland using RAPD analysis and flow cytometry. All individuals identified as F. japonica var . japonica had the same RAPD phenotype, while F. sachalinensis (11 RAPD phenotypes for 11 sampled individuals) and F.  ×  bohemica (24 RAPD phenotypes for 32 sampled individuals) showed high genotypic diversity. Bayesian cluster analysis revealed three distinct genetic clusters. The majority of F . ×  bohemica individuals were assigned to a unique genetic cluster that differed from those of the parental species, while the other F . ×  bohemica individuals had different degrees of admixture to the three genetic clusters. At the regional scale, the occurrence of male-fertile F. sachalinensis coincided with the distribution of F . ×  bohemica plants showing a high percentage of assignment to both parental species, suggesting that they originated from hybridization between the parental species. In contrast, in regions where male-fertile F. sachalinensis were absent, F . ×  bohemica belonged to the non-admixed genetic group, indicating multiple introductions of hybrids or sexual reproduction among hybrids. We also found regional differentiation in the gene pool of F.  ×  bohemica , with individuals within the same region more similar to each other than to individuals from different regions.     

Morphological and chromosomal variation in Fallopia section Reynoutria (Polygonaceae) in Korea

We have examined morphological and chromosomal variation inFallopia sect.Reynoutria in Korea to clarify their taxonomic identities and to determine whether their morphological variability is associated with ploidy levels. Principal components analysis (PCA) of individuals from 21 populations, using major distinguishing characters, revealed the presence of four major entiries of sect.Reynoutria in Korea; these includeF. sachalinensis, F. japonica var.japonica, F. forbesii, and the Nonsan population consisting of presumed hybrids. Based on morphology, it is hypothesized that the Nonsan population was probably derived from multiple hybridization events involving the three named taxa. The results also indicate thatF. forbesii is distinct fromF. japonica var.japonica. Polyploidy is more prevalent in sect.Reynoutria than has been previously recognized.Fallopia sachalinensis in Korea occurs as dodecaploids with 2n=132; our count is the first dodecaploid count for the species, and represents the highest chromosome number known in the genus.Fallopia japonica var.japonica occurs as tetraploids (2n=44), hexaploids (2n=66), and octoploids (2n=88), whileF. forbesii occurs as hexaploids (2n=66) and octoploids (2n=88); our counts appear to be the first reported chromosome numbers forF. forbesii. Morphological analysis indicates that there is no apparent correlation between the ploidy levels in these taxa and the morphological characters that we have considered in this study except that the tetraploids ofF. japonica var.japonica tend to have somewhat thicker leaves.     

Morphological variation among Betula nana (diploid), B. pubescens (tetraploid) and their triploid hybrids in Iceland

BACKGROUND AND AIMS: Introgressive hybridization between two co-existing Betula species in Iceland, diploid dwarf birch B. nana and tetraploid downy birch B. pubescens, has been well documented. The two species are highly variable morphologically, making taxonomic delineation difficult despite stable ploidy levels. Here an analysis is made of morphological variation within each ploidy group with an aim to establishing a reliable means to distinguish the species. METHODS: Plant materials were collected from 14 woodlands in Iceland. The plants were identified based on 2n chromosome numbers. Morphological variation in species-specific characters within each ploidy group was analysed qualitatively and quantitatively. The morphological index was based on eight discrete characters, whereas the multivariate analysis was based on nine leaf variables. KEY RESULTS: Of the 461 plants examined, 9.5 % were found to be triploid hybrids. The three ploidy groups were morphologically distinguishable but their variation overlapped. The diploid, triploid and tetraploid groups had average scores of 1.3, 4.1 and 8.3, respectively, in the morphology index scale from 0 (B. nana) to 13 (B. pubescens). A linear discriminant analysis also revealed significant separation among the three ploidy groups and the model assigned 96 % and 97 % of the B. nana and B. pubescens individuals correctly. The triploid hybrids were difficult to predict since only half of them could be assigned correctly. Leaf length was the most useful variable identifying triploid hybrids. Geographical patterns within the ploidy groups could partly be explained by differences in mean July temperature. CONCLUSIONS: Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.     

Genetic divergence and hybrid speciation

Although the evolutionary importance of natural hybridization has been debated for decades, it has become increasingly clear that hybridization plays a fundamental role in the evolution of many plant and animal taxa, sometimes resulting in the formation of entirely new species. Although some hybrid species retain the base chromosome number of their parents, others combine the full chromosomal complements of their progenitors. Hybrid speciation can thus produce two fundamentally different types of evolutionary lineages, yet relatively little is known about the factors influencing ploidy level in hybrid neospecies. We estimated genetic divergence between species pairs that have given rise to homoploid and polyploid hybrid species and found that divergence is significantly greater for the parents of polyploids, even after controlling for potentially confounding factors. Our data thus provide the first direct evidence in support of the notion that the extent of genomic divergence between hybridizing species influences the likelihood of diploid versus polyploid hybrid speciation.     

Morphological Variation in Wild Marmosets (Callithrix penicillata and C. geoffroyi) and Their Hybrids

Evolutionary theory and observation predict wider phenotypic variation in hybrids than parental species. Emergent phenotypic novelty in hybrids may in turn drive new adaptations or speciation by breaking parental phenotypic constraints. Primate hybridization is often documented through genetic evidence, but knowledge about the primate hybrid phenotype remains limited due to a small number of available studies on hybrid primate morphology. Here, we examine pelage and morphometric variation in two Brazilian marmoset species (Callithrix penicillata and C. geoffroyi) and their hybrids. Hybrids were sampled in an anthropogenic hybrid zone in the municipality of Viçosa, Minas Gerais state, Brazil. We analyzed hybrid facial and body pelage color variation, and compared 13 morphometric measures between hybrids and parental species. Five different hybrid facial morphotypes were observed, varying from intermediate to parental-like. Hybrid facial morphotypes were biased towards C. penicillata, suggesting that the pelage of this species may be dominant to that of C. geoffroyi in this context, and indicating that mate preference, and therefore gene flow/introgression, may be biased towards C. penicillata within the hybrid zone. Hybrid morphometric features were on average intermediate to parental species traits, but transgressive hybrids were also observed, suggesting that morphometric variation for the studied traits is consistent with Rieseberg’s complementary allele model. Finally, we observed a decoupling of facial patterning and size/shape in hybrids, relative to parent phenotypes, suggesting that an important factor driving phenotypic novelty within the Viçosa marmoset hybrid zone might be the loosening of evolutionary constraints on phenotypic trait integration.     

Interspecific hybridization of plants and resistance to herbivores: hypotheses,genetics, and variable responses in a diverse herbivore community

We studied the morphology, molecular genetics, and hebivory of two species of willows (Salix sericea and S. eriocephala) and their interspecific hybrids to test four alternative hypotheses concerning the effects of hybridization on plant resistance. Individually marked plants were identified using morphological traits in the field and measurements of stipule and leaf pubescence were made and compared using Canonical Discriminant Function Analysis. DNA was extracted from the leaves of a sample of the marked plants and RAPD-PCR analysis was performed to establish the genetic status of parental and hybrid plants. RAPD band analysis generally verified the genetic status of parental plants. Hybrid plants were usually correctly identified in the field with a few exceptions. However, the hybrid plants were a heterogeneous group of plants made up of most plants that appear to be F₁s and a few plants that appear to be backcrosses to S. sericea. Morphological variables were useful for distinguishing S. sericea from S. eriocephala and hybrids, but were not as dependable in distinguishing between S. eriocephala and hybrids. We compared the densities of 11 herbivore species and the infection by a leaf rust pathogen (Melampsora sp.) on the leaves and stems of two parents and the hybrids in the field. We found support for the Additive hypothesis (3 species), the Dominance hypothesis (2 species) and the Hybrid Susceptibility hypothesis (7 species, 6 herbivores and the Melampsora rust). We found no evidence for the Hybrid Resistance hypothesis. Guild membership was not a good predictor of similar responses of species to hybrid versus parental plants. A Canonical Discriminant Function Analysis showed discrete separation of the taxa based on herbivore densities, illustrating different community structures on hybrid and parental plants. This study demonstrates the diversity of responses of phytophages in response to interspecific hybridization.     

Genome size variation and species relationships in Hieracium sub-genus Pilosella (Asteraceae) as inferred by flow cytometry

BACKGROUND AND AIMS: Hieracium sub-genus Pilosella (hawkweeds) is a taxonomically complicated group of vascular plants, the structure of which is substantially influenced by frequent interspecific hybridization and polyploidization. Two kinds of species, 'basic' and 'intermediate' (i.e. hybridogenous), are usually recognized. In this study, genome size variation was investigated in a representative set of Central European hawkweeds in order to assess the value of such a data set for species delineation and inference of evolutionary relationships. METHODS: Holoploid and monoploid genome sizes (C- and Cx-values) were determined using propidium iodide flow cytometry for 376 homogeneously cultivated individuals of Hieracium sub-genus Pilosella, including 24 species (271 individuals), five recent natural hybrids (seven individuals) and experimental F(1) hybrids from four parental combinations (98 individuals). Chromosome counts were available for more than half of the plant accessions. Base composition (proportion of AT/GC bases) was cytometrically estimated in 73 individuals. KEY RESULTS: Seven different ploidy levels (2x-8x) were detected, with intraspecific ploidy polymorphism (up to four different cytotypes) occurring in 11 wild species. Mean 2C-values varied approx. 4.3-fold from 3.53 pg in diploid H. hoppeanum to 15.30 pg in octoploid H. brachiatum. 1Cx-values ranged from 1.72 pg in H. pilosella to 2.16 pg in H. echioides (1.26-fold). The DNA content of (high) polyploids was usually proportional to the DNA values of their diploid/low polyploid counterparts, indicating lack of processes altering genome size (i.e. genome down-sizing). Most species showed constant nuclear DNA amounts, exceptions being three hybridogenous taxa, in which introgressive hybridization was suggested as a presumable trigger for genome size variation. Monoploid genome sizes of hybridogenous species were always between the corresponding values of their putative parents. In addition, there was a good congruency between actual DNA estimates and theoretical values inferred from putative parental combinations and between DNA values of experimental F(1) hybrids and corresponding established hybridogenous taxa. CONCLUSIONS: Significant differences in genome size between hawkweed species from hybridogenous lineages involving the small-genome H. pilosella document the usefulness of nuclear DNA content as a supportive marker for reliable delineation of several of the most problematic taxa in Hieracium sub-genus Pilosella (including classification of borderline morphotypes). In addition, genome size data were shown to have a good predictive value for inferring evolutionary relationships and genome constitution (i.e. putative parental combinations) in hybridogenous species.     

ITS Polymorphisms Shed Light on Hybrid Evolution in Apomictic Plants: A Case Study on the Ranunculus auricomus Complex

The reconstruction of reticulate evolutionary histories in plants is still a major methodological challenge. Sequences of the ITS nrDNA are a popular marker to analyze hybrid relationships, but variation of this multicopy spacer region is affected by concerted evolution, high intraindividual polymorphism, and shifts in mode of reproduction. The relevance of changes in secondary structure is still under dispute. We aim to shed light on the extent of polymorphism within and between sexual species and their putative natural as well as synthetic hybrid derivatives in the Ranunculus auricomus complex to test morphology-based hypotheses of hybrid origin and parentage of taxa. We employed direct sequencing of ITS nrDNA from 68 individuals representing three sexuals, their synthetic hybrids and one sympatric natural apomict, as well as cloning of ITS copies in four representative individuals, RNA secondary structure analysis, and landmark geometric morphometric analysis on leaves. Phylogenetic network analyses indicate additivity of parental ITS variants in both synthetic and natural hybrids. The triploid synthetic hybrids are genetically much closer to their maternal progenitors, probably due to ploidy dosage effects, although exhibiting a paternal-like leaf morphology. The natural hybrids are genetically and morphologically closer to the putative paternal progenitor species. Secondary structures of ITS1-5.8S-ITS2 were rather conserved in all taxa. The observed similarities in ITS polymorphisms suggest that the natural apomict R. variabilis is an ancient hybrid of the diploid sexual species R. notabilis and the sexual species R. cassubicifolius. The additivity pattern shared by R. variabilis and the synthetic hybrids supports an evolutionary and biogeographical scenario that R. variabilis originated from ancient hybridization. Concerted evolution of ITS copies in R. variabilis is incomplete, probably due to a shift to asexual reproduction. Under the condition of comprehensive inter- and intraspecific sampling, ITS polymorphisms are powerful for elucidating reticulate evolutionary histories.     

The production of hybrids with high ecological amplitude between exotic Spartina densiflora and native S. maritima in the Iberian Peninsula

Aim Hybridization is a common and potent mechanism of plant evolution that has the potential to be evolutionary significant in its own right, and hybrids are common between invasive and native congeneric species. Our aims were to document the existence and nature of new Spartina hybrids arising between introduced Spartina densiflora and native S. maritima in tidal marshes of the Iberian Peninsula, to examine the actual and potential range of hybrids in the intertidal zone, and to analyse the seed set of hybrids. Location South‐West Iberian Peninsula. Methods Hybrids were characterized using chloroplast and nuclear DNA, and ploidy assessments. The ecological tolerance of the hybrids was studied using vegetation surveys and transplant experiments. Results We found that both parental species have been seed parents to hybrids and that all hybrids had an additive pattern of species‐specific nuclear markers consistent with F₁ hybrids. Hybrid chromosome numbers varied between 2n = ca. 65 and 2n = ca. 94–97, while S. maritima had 2n = ca. 60 and S. densiflora had 2n = ca. 70. Hybrids grew in three discrete locations along the intertidal zone but were capable of growing throughout the ranges of both parental species in transplanted gardens, and in most cases, grew better than the parental species. While the potential exists for the origination of another invasive Spartina hybrid species, thus far hybrid plants are not fertile, limiting their ability to invade and spread. Main conclusions We recommend the eradication of all of the currently quite limited hybrid cordgrass and to fight the S. densiflora invasion in the Iberian Peninsula to prevent the origination of a new invasive allopolyploid Spartina species.     

Genetic and genomic aspects of hybridization in ferns

The morphological and ecological intermediacy of hybrid taxa has long interested and challenged fern biologists, resulting in numerous systematic contributions focused on disentangling relationships within reticulate species complexes. From a genetic perspective, hybrid ferns are especially interesting because they represent the union of divergent parental genomes in unique evolutionary entities. This review summarizes advances in our knowledge of the genetic and genomic aspects of hybridization in ferns from the mid-20th century to the present. The different organismal products of hybridization, evolutionary aspects of additive and non-additive gene expression in allopolyploids, genetic and genomic mechanisms leading to gene silencing and loss, the roles of multiple origins and introgression for imparting genetic variation to hybrid fern taxa and their progenitors, and the utility of allopolyploid ferns to investigate mechanisms of genome evolution in the homosporous ferns are discussed. Comparisons are made to other plant lineages and important future research directions are highlighted, with the goal of stimulating additional research on hybrid ferns.     

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.     

Hybrid genome evolution by transposition

Species hybridization is reviewed focusing on its role as a source of evolutionary novelties. Contrary to the view that hybrids are lineages devoid of evolutionary value, a number of case studies are given that show how hybrids are responsible for reticulate evolution that may lead to the origin of new species. Hybrid evolution is mediated by extensive genome repatterning followed by rapid stabilization and fixation of highly adapted genotypes. Some well-documented cases demonstrate that bursts of transposition follow hybridization and may contribute to the genetic instability observed after hybridization. The mechanism that triggers transposition in hybrids is largely unknown, but coupling of hybrid transposition and demethylation has been observed in mammals and plants. A natural scenario is proposed in which marginal small hybrid populations undergo transposition mediated genome reorganizations accompanied by exogenous and endogenous selection that, in concert with drift, lead to rapid fixation of high fitness hybrid genotypes. These genotypes may represent parental introgressed species or be entirely new species.