Fine-scale environmental variation contributes to introgression in a three-species spruce hybrid complex

Hybridization is common for many forest trees, where weak barriers to reproduction obscure species boundaries. We characterized the genomic structure of Picea populations comprising three species spanning two well-known contact zones, the Picea sitchensis?×?Picea glauca and the P. engelmannii?×?P. glauca hybrid zones, using a set of 71 candidate-gene single nucleotide polymorphisms. The genetic structure of populations suggests a complex genomic architecture shaped by interspecific gene flow and strong environmental selection, with increased genetic diversity in hybrids. The presence of admixture among all three species suggests that three-way hybrids with mixed ancestry occur where species ranges overlap in transitional environments. Significant clinal variation and associations with climatic variables (including continentality, temperature, and precipitation) differ between hybrid zones, indicating that individual species and their hybrids are adapted to distinct environmental niches. Allele–environmental association analysis revealed that most of the candidate genes with evidence of selection were unique to either the Sitka?×?white or the Engelmann?×?white hybrid zones, with few shared between these zones. Management of these widespread and diverse gene pools will be best served through development of climate-based seed transfer, with recommended seed sources informed by a combination of genetic and climatic information for future climates.     

Molecular evidence for hybridization between invasive <Emphasis Type="Italic">Solidago canadensis</Emphasis> and native <Emphasis Type="Italic">S</Emphasis>. <Emphasis Type="Italic">virgaurea</Emphasis>

Hybridization between alien and native species is biologically very important and could lead to genetic erosion of native taxa. Solidago × niederederi was discovered over a century ago in Austria and described by Khek as a natural hybrid between the alien (nowadays regarded also as invasive) S. canadensis and native S. virgaurea. Although interspecific hybridization in the genus Solidago is considered to be relatively common, hybrid nature of S. × niederederi has not been independently proven using molecular tools, to date. Because proper identification of the parentage for the hybrid Solidago individuals solely based on morphological features can be misleading, in this paper we report an additive polymorphism pattern expressed in the ITS sequences obtained from individuals representing S. × niederederi, and confirm the previous hypothesis that the parental species of this hybrid are S. canadensis and S. virgaurea. Additionally, based on variability at the cpDNA rpl32-trnL locus, we showed that in natural populations hybridization occurs in both directions.     

MicroRNA expression changes following synthesis of three full-sib <Emphasis Type="Italic">Populus</Emphasis> triploid populations with different heterozygosities

Key message

Through high-throughput sequencing, we compared the relative expression levels of miRNA in three full-sib Populus triploid populations with that in their parents and one diploid hybrid population. We found similar numbers of miRNAs differentially expressed between the parents and the four progeny hybrid populations. In addition, unbalanced parental expression level dominance of miRNAs were found in the three allotriploid and interspecific hybrid populations, which may reprogram gene expression networks and contribute to the growth of Populus hybrids. These results indicated that hybridization has a great impact on the miRNA expression variation in the newly synthesized Populus triploid and diploid hybrid populations. However, we also found no significant differences in miRNA expression among one diploid and three triploid hybrid populations, hinting that miRNA abundances do not increase with the genome content. No dosage effect of miRNA expression could lead to dosage-dependent negative effects on target genes and their downstream pathway in polyploids. We speculate that polyploids may gain advantages from the slight decrease in miRNA regulation, suggesting an important molecular mechanism of polyploid advantage.

Abstract

Hybridization with three types of induced 2n gametes transmitted different parental heterozygosities has been proven as an efficient method for Populus triploid production. Several researches have shown that miRNA could be non-additively expressed in allopolyploids. However, it is still unclear whether the non-additively expressed miRNAs result from the effect of hybridization or polyploidization, and whether a dose response to the additional genomic content exists for the expression of miRNA. Toward this end, through high-throughput sequencing, we compared the expression levels of miRNA in three full-sib Populus triploid populations with that in their parents and one interspecific hybrid population. We found similar numbers of miRNAs differentially expressed between the parents and the four progeny hybrid populations. Unbalanced parental expression level dominance of miRNAs were found in the three triploid and diploid hybrid populations, which may reprogram gene expression networks and affect the growth of Populus hybrids. These results indicated that hybridization has a great impact on the miRNA expression variation in the newly synthesized Populus triploid and diploid hybrid populations. However, we also found no significant differences in miRNA expression among the three triploid populations and the diploid hybrid population. No dosage effect of miRNA expression could lead to dosage-dependent negative effects on target genes and their downstream pathway in polyploids. We speculate that polyploids may gain advantages from the decrease in miRNA negative regulation, suggesting an important molecular mechanism of polyploid advantage.

     

Molecular signatures of divergence and selection in closely related pine taxa

Efforts to detect loci under selection in plants have mostly focussed on single species. However, assuming that intraspecific divergence may lead to speciation, comparisons of genetic variation within and among recently diverged taxa can help to locate such genes. In this study, coalescent and outlier detection methods were used to assess nucleotide polymorphism and divergence at 79 nuclear gene fragments (1212 SNPs) in 16 populations (153 individuals) of the closely related, but phenotypically and ecologically distinct, pine taxa Pinus mugo, P. uliginosa and P. uncinata across their European distributions. Simultaneously, mitochondrial DNA markers, which are maternally inherited in pines and distributed by seeds at short geographic distance, were used to assess genetic relationships of the focal populations and taxa. The majority of nuclear loci showed homogenous patterns of variation between the taxa due to a high number of shared SNPs and haplotypes, similar levels of polymorphism, and low net divergence. However, against this common genetic background and an overall low population structure within taxa at mitochondrial markers, we identified several genes showing signatures of selection, accompanied by significant intra- and interspecific divergence. Our results indicate that loci involved in species divergence may be involved in intraspecific local adaptation.     

Isolation mechanisms in closely related grasshopper species <Emphasis Type="Italic">Chorthippus albomarginatus</Emphasis> and <Emphasis Type="Italic">Ch. oschei</Emphasis> (Orthoptera: Acrididae)

Two closely related grasshopper species Chorthippus albomarginatus and Ch. oschei are known to hybridize in the narrow contact zone at the territory of Ukraine and Moldova. Different isolaton mechanisms providing reproductive isolation between the two species were studied. In choice mating experiments, females of the both species demonstrated a strong assortative mating (80–90% preference for the conspecific males). Comparison of the parental and hybrid viability revealed a reduced hatching and increased larval mortality in F₁ and F₂ hybrids. In choice mating experiments, the hybrid females mated less assortatively than the parental females. An assymmetry was found in mating preferences and in viability of hybrids. The results demonstrate the existence of pre-and post-mating isolation between Ch. albomarginatus and Ch. oschei. A possible fate of the hybrid zone is discussed.     

Introgression from non-native species unveils a hidden threat to the migratory Neotropical fish <Emphasis Type="Italic">Prochilodus hartii</Emphasis>

Invasive species are one of the greatest threats to biodiversity, due to competition, predation, pathogen spread, and hybridization. The latter may remain undetected and impair the survival of species, due to genetic admixture and hybrid swarming (i.e., interbreeding between hybrid individuals and backcrossing with parental species). The impact of invasive species remains poorly studied in the Neotropical ichthyofauna, particularly when considering the potential for hybridization between native and introduced species. Due to fisheries importance and its commercial value, species of the Prochilodus genus have been introduced to other catchments in Brazil. Here, we evaluate the introduction of non-native Prochilodus species and the potential effect of hybridization with the native migratory fish P. hartii. To evaluate possible introgression of Prochilodus spp. to P. hartii in the Jequitinhonha river basin (JRB), we employed a morphogenetic approach, analysing 219 specimens sampled from a broad extent of the river basin. Morphological analyses using meristic characters were incongruent with molecular identification by DNA barcoding (COI) in 22.83% of the analysed specimens. Haplotypes from three non-native species (P. argenteus, P. costatus, and P. lineatus) were recovered from specimens morphologically identified as P. hartii. Hybridization between P. hartii and introduced species was confirmed using co-dominant nuclear microsatellite markers. We observed a pronounced introgression pattern in this Neotropical basin, and paradoxically, despite being one of the most abundant migratory species native to the JRB, due to ongoing levels of introgression, P. hartii’s genetic integrity and conservation might be affected.     

Field-based artificial crossings indicate partial compatibility of reciprocal crosses between <Emphasis Type="Italic">Pinus sylvestris</Emphasis> and <Emphasis Type="Italic">Pinus mugo</Emphasis> and unexpected chloroplast DNA inheritance

Crossability relationships between Scots pine (Pinus sylvestris L.) and mountain dwarf pine (Pinus mugo Turra) was studied, using artificial pollination approach. Partial compatibility of the reciprocal crossings of these species was proved experimentally, validating the idea of a spontaneous formation of their hybrid swarms under natural conditions. The hybrids were validated using organellar DNA markers and nuclear DNA microsatellites. Based on the percentage of filled seeds, the interspecific crossings were less efficient than the intraspecific cross-pollinations of P. sylvestris and P. mugo individuals. Both species were found to intercross readily with individuals of their putative hybrid swarm, P. mugo exhibiting a higher hybridological affinity towards putatively hybrid individuals than P. sylvestris. Validation of the hybrids confirmed the paternal inheritance of chloroplast DNA (cpDNA) in the combination P. sylvestris × P. mugo only. Surprisingly, in the reciprocal crossing P. mugo × P. sylvestris, maternal inheritance of cpDNA was revealed. Obtained results offer a new insight into the direction and intensity of gene flow within the hybrid swarms of Scots pine and mountain dwarf pine.     

An overlooked hybrid between the two diploid <Emphasis Type="Italic">Chenopodium</Emphasis> species in Central Europe determined by microsatellite and morphological analysis

The presence and extent of hybridization within the Chenopodium album aggregate (Amaranthaceae) is still unclear. Although many hybrid combinations have been described, their existence in the field has never been systematically studied and verified. The main aim of this study was to ascertain the extent of interspecific hybridization between the diploid species C. ficifolium and C. suecicum using highly variable nuclear microsatellite markers. Due to the absence of such kind of molecular markers for the whole C. album group, we divided the analysis into two steps: (1) Eleven microsatellite loci designed for the closely related species C. quinoa were cross-amplified in five Eurasian species of the C. album diploid–polyploid complex, i.e. C. album s.s. (6x), C. striatiforme (4x), C. strictum (4x), C. ficifolium (2x) and C. suecicum (2x); (2) For the detection of interspecific hybridization between C. ficifolium and C. suecicum, we sampled 480 individuals from five localities in Central Europe. We also investigated morphological differences between the parental taxa and their hybrid and devised a key for their determination. Analysis of variation in microsatellite loci using Bayesian methods, PCoA and Neighbour-joining tree identified 32 F1 hybrids. These F1 hybrids, described here as C. paradoxum Mandák, formed a cluster between well-differentiated parental species, combining the morphological characters of both their parents. Moreover, genetic analyses also recognized several F2 or backcross hybrids, whose delimitation, mainly from C. suecicum and F1 hybrids, based on morphological characters, is problematic.     

Independent hybrid populations of <Emphasis Type="Italic">Formica polyctena X</Emphasis><Emphasis Type="Italic">rufa</Emphasis> wood ants (Hymenoptera: Formicidae) abound under conditions of forest fragmentation

Combined genetic and morphological data indicate frequent hybridisation between the wood ants Formica polyctena Förster 1850 and F. rufa Linnaeus 1761 in Central Europe. The genetic and morphological traits give a concordant picture of hybridisation with a strong correlation between the genotypic admixture proportions at 19 microsatellite loci and the first vectors of a principal component analysis (P < 0.001) and of a 3-class discriminant analysis (P < 0.001) of 15 quantitative morphological characters. This integrative approach enabled a grouping into F. polyctena, the hybrid and rufa. Genetic differentiation between the hybrid and F. rufa is significantly larger than between the hybrid and polyctena, indicating gene flow mainly between the latter entities. A suggested gene flow bias towards F. polyctena agrees with differential queen acceptance and mating behaviour. Both genetic and phenotypic colony parameters indicate predominance of monogyny in F. rufa but of polygyny in polyctena and the hybrid. Hybrids are intermediate between the parental species in body size, diagnostic morphological characters, monogyny frequency, size of nest population, nest diameter and infestation rate with epizootic fungi. The three entities respond differently to woodland fragmentation. Hybrids are significantly more abundant in forests with a coherent area <300 ha than in woodland above this size. Regions with high hybrid frequency in Germany—the Eastern Oberlausitz (23%) and the Baltic Sea islands Darss, Hiddensee and Rügen (28%)—are characterised by a fragmented woodland structure whereas regions with low hybrid frequency—Brandenburg and the lower Erzgebirge (3.4%)—have clearly larger and more coherent forest systems. Data from other European countries indicate habitat fragmentation to be a facilitating factor but no essential precondition for interspecific hybridisation in these ants. Hybrids are hypothesised to have selective advantage in fragmented systems because of combining the main reproductive and dispersal strategies of the parental species.     

Genomic clues to the parental origin of the wild flowering cherry <Emphasis Type="Italic">Prunus yedoensis</Emphasis> var. <Emphasis Type="Italic">nudiflora</Emphasis> (Rosaceae)

Prunus yedoensis Matsumura is one of the popular ornamental flowering cherry trees native to northeastern Asia, and its wild populations have only been found on Jeju Island, Korea. Previous studies suggested that wild P. yedoensis (P. yedoensis var. nudiflora) is a hybrid species; however, there is no solid evidence on its exact parental origin and genomic organization. In this study, we developed a total of 38 nuclear gene-based DNA markers that can be universally amplifiable in the Prunus species using 586 Prunus Conserved Orthologous Gene Set (Prunus COS). Using the Prunus COS markers, we investigated the genetic structure of wild P. yedoensis populations and evaluated the putative parental species of wild P. yedoensis. Population structure and phylogenetic analysis of 73 wild P. yedoensis accessions and 54 accessions of other Prunus species revealed that the wild P. yedoensis on Jeju Island is a natural homoploid hybrid. Sequence-level comparison of Prunus COS markers between species suggested that wild P. yedoensis might originate from a cross between maternal P. pendula f. ascendens and paternal P. jamasakura. Moreover, approximately 81% of the wild P. yedoensis accessions examined were likely F1 hybrids, whereas the remaining 19% were backcross hybrids resulting from additional asymmetric introgression of parental genotypes. These findings suggest that complex hybridization of the Prunus species on Jeju Island can produce a range of variable hybrid offspring. Overall, this study makes a significant contribution to address issues of the origin, nomenclature, and genetic relationship of ornamental P. yedoensis.     

Hybridization between Arctostaphylos viscida and A. canescens in Oregon

Evidence of natural hybridization betweenArctostaphylos viscida andA. canescens in southwestern Oregon was obtained from morphological studies of field populations. Hybridization occurs where populations ofA. viscida andA. canescens meet in areas where serpentine and nonserpentine soils abut. At these contacts,A. viscida forms large populations only on serpentine andA. canescens only on nonserpentine. Most of the indentifiable hybrids survive on either transitional or nonserpentine soils. The small proportion that are found on serpentine are considered to be backcrosses toA. viscida. The hybrid plants are similar to several putative species maintained in the standard floras, and it is suggested that these no longer be given taxonomic recognition.     

A diversity study of <Emphasis Type="Italic">Saccharomycopsis fibuligera</Emphasis> in rice wine starter <Emphasis Type="Italic">nuruk</Emphasis>, reveals the evolutionary process associated with its interspecies hybrid

The amylolytic yeast Saccharomycopsis fibuligera is the predominant yeast in the starter product, nuruk, which is utilized for rice wine production in South Korea. Latest molecular studies explore a recently developed interspecific hybridization among stains of S. fibuligera with a unique genetic feature. However, the origin of the natural hybridization occurrence is still unclear. Thus, to respectively distinguish parental and hybrid strains, specific primer sets were applied on 141 yeast strains isolated from different nuruk samples fermented in different provinces. Sixty-seven strains were defined accordingly as parental species with genome A while 8 strains were defined as hybrid strains. Unexpectedly, another parental species with genome B could not be found among the strain pools yet. Furthermore, it was observed that hybrid strains are phenotypically different from A genome strains; asci containing tetrad ascospores were observed in A genome strains more frequent than in hybrid strains. Nevertheless, hybrid strains were slightly more thermotolerant than A genome strains. Interestingly, all hybrid strains were located only in Jeju province. Based on these sets of data, we speculated that the unique climate of Jeju province might play an evolutionary role in the interspecific hybridization between A genome strains, as well as the unculturable allopatric B genome strains.     

Phylogenetic relationships and intraspecific diversity of a North Patagonian Fescue: evidence of differentiation and interspecific introgression at peripheral populations

Peripheral sites usually offer suboptimal conditions for species with wide distributions, where expression of phenotypic variability and potential interspecific hybridization might be enhanced. The Patagonian steppe, the largest and southernmost dryland ecosystem in South America, is characterized by natural rangelands dominated by grasses. Festuca pallescens is a keystone species with a wide distribution in Patagonia over diverse environments reaching the extreme arid zones in the Somuncura plateau, a biogeographical island. Our aim is to study the phylogenetic relationships among Festuca pallescens populations as well as between this species and the sympatric F. argentina in North Patagonia. We analysed fourteen populations along a west-east transect of about 500 km in North Patagonia with three types of molecular markers: ITS, chloroplast trnL-F and eight nuclear microsatellites. Bayesian inferences, maximum parsimony and maximum likelihood analyses with trnL-F and ITS showed that F. pallescens is related to the Patagonian clade within the Festuca phylogeny. However, the easternmost populations of F. pallescens at Somuncura plateau were highly differentiated from the other populations and clustered with F. argentina (a sympatric species of the Asian-American clade). Principal coordinates analyses and Bayesian clustering performed with nuclear microsatellites as well as morphoanatomical traits, showed an intermediate position of one of these easternmost populations with respect to the two species, suggesting admixture. The high genetic variability observed in these peripheral populations highlight their relevance for conservation and might be indicating the existence of evolutionary processes triggering events of speciation in the Patagonian fescues.     

Concordant morphological and molecular clines in a contact zone of the Common and Spined toad (<Emphasis Type="Italic">Bufo bufo</Emphasis> and <Emphasis Type="Italic">B. spinosus</Emphasis>) in the northwest of France

Background

Hybrid zones are regions where individuals of two species meet and produce hybrid progeny, and are often regarded as natural laboratories to understand the process of species formation. Two microevolutionary processes can take place in hybrid zones, with opposing effects on population differentiation. Hybridization tends to produce genetic homogenization, reducing species differences, whereas the presence of mechanisms of reproductive isolation result in barriers to gene flow, maintaining or increasing differences between taxa.

Results

Here we study a contact zone between two hybridizing toad species, Bufo bufo and B. spinosus, through a combination of molecular (12 polymorphic microsatellites, four nuclear and two mitochondrial SNP markers) and morphological data in a transect in the northwest of France. The results show largely concordant clines across markers, defining a narrow hybrid zone of ca. 30 km wide. Most hybrids in the centre of the contact zone are classified as F₂ or backcrossed individuals, with no individuals assigned to the F₁ hybrid class.

Conclusions

We discuss the implications of these results for our understanding of the evolutionary history of these species. We anticipate that the toad contact zone here described will become an important asset in the study of hybrid zone dynamics and evolutionary biology because of its easy access and the abundance of the species involved.

     

RAPD-PCR Analysis of Ground Squirrels from the Tobol-Ishim Interfluve: Evidence for Interspecific Hybridization between Ground Squirrel Species <Emphasis Type="Italic">Spermophilus major</Emphasis> and <Emphasis Type="Italic">S. erythrogenys</Emphasis>

Populations of two ground squirrel species, Spermophilus major and S. erythrogenys, from the interfluvial area of the Tobol and Ishim rivers, where their ranges overlap, have been examined using RAPD-PCR. We have identified 253 loci, which included taxon-specific markers for S. major and S. erythrogenys as well as markers for geographic populations. Estimation of genetic diversity and construction of phylogenetic relationships were performed using software programs POPGENE, TEPGA, and TREECON. In all, based on morphological traits, animals from the Tobol-Ishim interfluve were assigned to the two parental morphotypes and showed similar levels of genetic variability (H, n_a, n_e). However, the total polymorphism level proved to be higher in ground squirrels with the major morphotype (P = 40.32%,P₉₅ = 27.27%) than in animals with the erythrogenys morphotype (P = 32%,P₉₅ = 22.13%). Nevertheless, the number of rare alleles was high in both cases, constituting about 70% of the total number. Interpopulation differentiation was considerably higher in S. major δ = 0.50) than in S. erythrogenys δ = 0.41). The genetic differentiation between local samples from the Tobol-Ishim interfluvial area was lower than that between the parental species. A significant part of the genetic diversity of the species examined and animals from the zone of overlapping ranges was accounted for by intrapopulation variability. Animals from the northern and southern parts of the Tobol-Ishim interfluve were charac-terized by the core traits of S. major and S. erythrogenys, respectively, falling into two distinct clusters in the UPGMA and NJ reconstructions. In addition to three hybrid individuals, identified by the bioacoustic method, three hybrid animals were distinguished using RAPD analysis. These animals earlier were thought to be “pure” species and formed their own clusters in phylogenetic reconstructions. Thus, the RAPD-PCR results directly showed the existence of stable hybridization (20% genetic hybrids) between S. major and S. erythrogenys in the Tobol-Ishim interfluvial area, which is more extensive than inferred previously from morphological and bioacoustic data.     

Nucleolar organizing regions (NORs) of common vole chromosomes as nuclear markers of genome differentiation in data from a hybrid zone of two karyoforms, <Emphasis Type="Italic">arvalis</Emphasis> and <Emphasis Type="Italic">obscurus</Emphasis>

Contrasting chromosome location of nucleolar organizing regions (NORs), a poorly studied differentiating feature of two karyoforms of the 46-chromosome common voles, arvalis and obscurus, is demonstrated in the heterozygous karyotype of a natural hybrid from the arvalis/obscurus contact zone in European Russia. It was deduced from the karyotype analysis that the obscurus form was parental in this case and the arvalis form was defined as maternal via the markers of the mitochondrial genome. The similarities in the levels of chromosome polytypy and the lineage differentiation of the arvalis/obscurus and obscurus mitochondrial genomes according to the new and earlier published data is noticeable.     

<Emphasis Type="Italic">Bruguiera hainesii</Emphasis>, a critically endangered mangrove species,is a hybrid between <Emphasis Type="Italic">B. cylindrica</Emphasis> and <Emphasis Type="Italic">B. gymnorhiza</Emphasis> (Rhizophoraceae)

Bruguiera hainesii (Rhizophoraceae) is one of the two Critically Endangered mangrove species listed in the IUCN Red List of Threatened Species. Although the species is vulnerable to extinction, its genetic diversity and the evolutionary relationships with other Bruguiera species are not well understood. Also, intermediate morphological characters imply that the species might be of hybrid origin. To clarify the genetic relationship between B. hainesii and other Bruguiera species, we conducted molecular analyses including all six Bruguiera species using DNA sequences of two nuclear genes (CesA and UNK) and three chloroplast regions (intergenic spacer regions of trnL-trnF, trnS-trnG and atpB-rbcL). For nuclear DNA markers, all nine B. hainesii samples from five populations were heterozygous at both loci, with one allele was shared with B. cylindrica, and the other with B. gymnorhiza. For chloroplast DNA markers, the two haplotypes found in B. hainesii were shared only by B. cylindrica. These results suggested that B. hainesii is a hybrid between B. cylindrica as the maternal parent and B. gymnorhiza as the paternal one. Furthermore, chloroplast DNA haplotypes found in B. hainesii suggest that hybridization has occurred independently in regions where the distribution ranges of the parental species meet. As the IUCN Red List of Threatened Species currently excludes hybrids (except for apomictic plant hybrids), the conservation status of B. hainesii should be reconsidered.     

Effects of Host Interspecific Interaction in the <Emphasis Type="Italic">Maculinea</Emphasis>–<Emphasis Type="Italic">Myrmica</Emphasis> Parasite–Host System

A model of interspecific host competition in a system with one parasite (butterfly—Maculinea) and multiple potential hosts (ants—Myrmica) is presented. Results indicate that host interspecific competition increases the occurrence of multiple host behaviour in Maculinea natural populations but decreases the ability of the parasite populations to adapt to the most abundant host species. These qualitative predictions were compared with data on host specificity, with good agreement. Analysis of the data also indicates that Maculinea teleius and Maculinea arion respond differently to changes in relative host abundances. Maculinea teleius shows a larger fraction of sites where it displays multiple host behaviour and a larger fraction of sites where the niches of the hosts overlap. In some instances, Maculinea teleius is adapted to Myrmica hosts that are present in lower frequencies. Maculinea arion is locally more host-specific and occurs at sites where host interspecific competition is unlikely and is more frequently adapted to the most abundant host species.     

<Emphasis Type="Italic">Viola woosanensis</Emphasis>, a recurrent spontaneous hybrid between <Emphasis Type="Italic">V. ulleungdoensis</Emphasis> and <Emphasis Type="Italic">V. chaerophylloides</Emphasis> (Violaceae) endemic to Ulleung Island,Korea

Ulleung Island is an oceanic volcanic island in Korea, which has never been connected to the adjacent continent. Previous studies highlighted Ulleung Island as an excellent system to study the pattern and process of early stages of flowering plant evolutions on oceanic island. The predominant mode of speciation in flowering plants on Ulleung Island appears to be anagenesis. However, the potentially important role of hybrid speciation among incompletely reproductively isolated lineages cannot be ruled out. Viola woosanensis (Violaceae) is of purportedly hybrid origin between V. ulleungdoensis (i.e., formerly recognized as V. selkirkii in Ulleung Island) and V. chaerophylloides, based on morphology. To examine the origin of V. woosanensis, we sampled a total of 80 accessions, including V. woosanensis and its putative parental species and sequenced nrDNA ITS, and four highly variable chloroplast noncoding regions (trnL-trnF, rpl16 intron, atpF-atpH, and psbA-trnH). Representative species of Viola from Korea were also included in the phylogenetic analyses (maximum parsimony, maximum likelihood, and Bayesian inference). Additive polymorphic sites in the nrDNA ITS regions were confirmed by cloning amplicons from representative species. The molecular data strongly supported the hybrid origin of V. woosanensis, and the maternal and paternal parent were determined to be V. ulleungdoensis and V. chaerophylloides, respectively. The presence of two parental ribotypes in V. woosanensis (with the exception in one population) was confirmed by cloning, suggesting V. woosanensis is primarily the F₁ generation. No trace of backcrossing and introgression with its parents was detected due to low fertility of hybrid species. We found a multiple and unidirectional hybrid origin of V. woosanensis. Additional studies are required to determine which factors contribute to asymmetric gene flow of Viola species in Ulleung Island.     

Persistence,dispersal and genetic evolution of recently formed <Emphasis Type="Italic">Spartina</Emphasis> homoploid hybrids and allopolyploids in Southern England

In Southampton Water, UK, the recent (c. 150 years ago) interspecific hybridisation between Spartina alterniflora (2n = 6x = 62; A-genome) and S. maritima (2n = 6x = 60; M-genome) gave rise to the homoploid hybrid (S. × townsendii, 2n = 6x = 62), and subsequently to the invasive allododecaploid species S. anglica (2n = 12x = 120–124) that has since spread worldwide. To address the question of dynamics of mixed ploidy populations involving these plants, we analysed several Spartina populations (fifty one individuals) in Southern England, UK, one of which was the presumed place of origin of the homoploid hybrid (Hythe). Using a combination of flow cytometry and ribosomal DNA (rDNA) genotyping we were able to identify the genomic composition and ploidy level of each individual analysed. The data show that the homoploid hybrid still dominates the population at Hythe (82 % of individuals collected in that locality) since its origin in the nineteenth century. We also identified S. × townsendii for the first time on Hayling Island (66 % individuals), indicating dispersal beyond its likely origin. The fertile allododecaploid S. anglica was mainly found in populations outside the initial hybridisation site, on Hayling Island and at Eling Marchwood. Quantification of the rDNA contributions from each parental genome showed that the ratios were mostly balanced in S. × townsendii. However, two (3 %) S. anglica individuals analysed have lost nearly all M-genome homeologs, indicating extensive repeat loss. Such variation indicates that despite the presumed single allopolyploid origin of S. anglica and genetic uniformity at other loci, it has undergone substantial changes at the rDNA loci following genome duplication.