Effect of hybridization of the Quercus crassifolia×Quercus crassipes complex on the community structure of endophagous insects

In a previous study, we showed that the geographic proximity of hybrid plants to the allopatric areas of parental species increases their morphological and genetic similarity with them. In the present work, we explored whether the endophagous fauna of hybrid plants show the same pattern. We studied the canopy species richness, diversity and composition of leaf-mining moths (Lepidoptera: Tischeridae, Citheraniidae) and gall-forming wasps (Hymenoptera: Cynipidae) associated with two species of red oaks (Quercus crassifolia and Quercus crassipes) and their interspecific hybrid (Quercus×dysophylla Benth pro sp.) in seven hybrid zones in central Mexico, during four seasons in 2 years. The study was conducted on 194 oak trees with known genetic status [identified by leaf morphology and molecular markers (random amplified polymorphic DNAs)], and the results indicate a bidirectional pattern of gene flow. Hybrid plants supported intermediate levels of infestation of gall-forming and leaf-mining insects compared to their putative parental species. The infestation level of leaf-mining insects varied significantly following the pattern: Q. crassifolia>hybrids>Q. crassipes, whereas the gall-forming insects showed an inverse pattern. A negative and significant relationship was found between these two types of insect guilds in each host taxa, when the infestation percentage was evaluated. It was found that 31.5% (n=11) of the endophagous insects were specific to Q. crassipes, 22.9% (n=8) to Q. crassifolia, and 8.6% (n=3) to hybrid individuals. The hybrid bridge hypothesis was supported in the case of 25.7% (n=9) of insects, which suggests that the presence of a hybrid intermediary plant may favor a host herbivore shift from one plant species to another. Greater genetic diversity in a hybrid zone is associated with greater diversity in the endophagous community. The geographic proximity of hybrid plants to the allopatric site of a parental species increases their similarity in terms of endophagous insects and the Eje Neovolcánico acts as a corridor favoring this pattern. Electronic Supplementary Material Supplementary material is available for this article at     

Canopy arthropods community within and among oak species in central Mexico

Quercus rugosa and Q.laurina are species that presents a wide geographical distribution range in temperate forests of Mexico. Oak canopies contain a considerable portion of arthropod diversity and the arthropods fauna fulfill a wide variety of ecological roles. We examined the effect of oak species and seasonal changes on some community structure parameters (diversity, composition, similarity, biomass, rare species, and density of arthropod fauna) of canopy arthropods. In total, 40 oak canopies were fogged during rainy and dry season. A total of 614 identified arthropod morphospecies were recognized belonging to 22 orders associated with tree canopies. A separation of host tree species during both seasons, suggesting a different community structure on host plants species was demonstrated by the principal component analyses (PCA), therefore, differences between oak species results in phenotypes that structure the composition of the arthropod community. Q.laurina registered the highest densities, diversity index and number of rare species in comparison with Q.rugosa. While arthropod biomass showed an inverse pattern. Trees more close to one another (spatial distance) register a more similar canopy arthropod fauna. This study suggests that the trees of Q.laurina could act as a center of biodiversity by the accumulation of arthropod fauna with a considerable number of rare species, which presents wide ecological roles or is involved in critical processes that maintain forest ecosystems[Current Zoology 55(2):132-144,2009].     

Hybridization increases canopy arthropod diversity in the <Emphasis Type="Italic">Quercus affinis</Emphasis> × <Emphasis Type="Italic">Quercus laurina</Emphasis> complex

Understanding the factors that influence the diversity and composition of arthropod communities is a major topic in ecology. Canopy arthropod communities are a major constituent of biodiversity and show great variation in time and space according to different factors. Recently, genetic variation within tree species has attracted attention as a significant factor determining the diversity and composition of canopy arthropod communities. A major source of genetic and phenotypic novelty in plant species is interspecific hybridization, and therefore it is of interest to evaluate how this process affects the communities of associated organisms. In this study, we used microsatellite markers and geometric morphometry of leaf shape to analyze genetic and morphological variation in 45 individuals in a local hybrid zone between the oaks Quercus affinis and Q. laurina in Mexico. Individual trees were assigned to one of the parental species or to the hybrid category. The percentage of leaf area removed by herbivores was quantified in each individual and the canopies of five individuals of each categeory (two parental species and hybrids) was fogged with insecticide to assess the diversity and composition of arthropod communities. Results indicated that hybrid trees experience higher levels of herbivory than parental species and also sustain a higher abundance and richness of canopy arthropods. In general, our study supports the “hybrid susceptibility hypothesis” that predicts a higher incidence of associated arthropods on hybrid plants than in their parental species as result of the disruption of co-adapted gene complexes associated to resistance traits.     

Taxonomy of Quercus crassifolia (Fagaceae) and morphologically similar species in Mexico

In this work, ten species morphologically similar to Q. crassifolia are circumscribed based on a phylogenetic species concept. Examination of these species suggests that Q. dysophylla and Q. radiata are of hybrid origin; the putative parents of Q. dysophylla are Q. crassifolia and Q. crassipes, while Q. radiata is likely the result of hybridization between Q. coccolobifolia and Q. conzattii or Q. urbanii. A lectotype of Q. crassifolia is designated here. Dichotomous keys, as well as distribution ranges and photographs, are presented.     

Modeling of introgression between Elymus caninus and E. fibrosus (Poaceae) and their registration using one-dimensional SDS electrophoresis

Artificial sexual hybrids between Elymus caninus (L.) L. and E. fibrosus (Schrenk) Tzvel. were experimentally examined in generations F₁–F₅. The possibility of genetic introgression between these species was shown. Morphologically, the hybrid plants can be assigned to either of the parental species or to variety E. caninus var. muticus (Holmb.) Karlsson. Some traits (spike density, leaf blade width, leaf blade pubescence, awns of lemmas) exhibited considerable variation. Polypeptide spectra of endosperm proteins were characterized in the initial parental biotypes and the hybrid progeny, using a gel-buffer SDS electrophoresis system. It was suggested that successful interspecies hybridization requires backcrosses or normalizing crosses. The possibility of sexual genetic exchange enables to utilize the gene pools of the two species to transfer the required traits in selection forage forms.     

REPRODUCTIVE ISOLATION AND INTROGRESSION BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (FAMILY CYPRINIDAE): COMPARISON OF MORPHOLOGY,ALLOZYMES, AND MITOCHONDRIAL DNA

Hybrid zones in fluvial fishes may be heterogeneous from drainage to drainage. The comparison of data from morphology, allozymes, and mitochondrial DNA (mtDNA) indicates variability in the causes and degree of restriction of gene flow between Notropis cornutus and Notropis chrysocephalus. Allozyme marker loci show frequency-dependent introgression; i.e., the rarer species, whichever it is at a particular locality, tends to exhibit a higher proportion of introgressed alleles. Unlike allozymes, introgression of mtDNA haplotypes varies geographically. In westward-flowing Michigan drainages, N. cornutus mtDNA haplotypes are more common in F₁ hybrids and backcrosses, independent of parental frequencies. In eastward-flowing Michigan drainages, N. chrysocephalus mtDNA is more common in F₁ hybrids and backcrosses; this pattern may be due to local ecological effects or frequency-dependent introgression. Morphological data alone are not sufficient to distinguish all classes of hybrids. The lack of concordance of morphological, allozymic, and mtDNA introgression patterns implies operation of one or two factors: 1) geographically variable patterns of selection against different hybrid and backcross combinations or 2) genetic differences between Michigan populations inhabiting eastward- and westward-flowing drainage systems accumulated during historical isolation.     

Genetic structure of a hybrid zone between two violets,Viola rossii Hemsl. and V. bissetii Maxim.: dominance of F1 individuals in a narrow contact range

The genetic composition of a hybrid zone can provide insight into the evolution of diversification in plants. We carried out morphological and amplified fragment length polymorphism analyses to investigate the genetic composition of a hybrid zone between two violets, Viola bissetii Hemsl. and Viola rossii Maxim. Our aim was to clarify the formation and maintenance of hybrids between these Viola species. We found that most hybrid individuals (V. bissetii × V. rossii) were of the F₁ generation, with a few of the F₂ generation. We found no backcrosses. The scarcity of post‐F₁ hybrids indicates that a species barrier is established between the parental species. The F₁‐dominated hybrid zone occupied only a narrow, intermediate ecotone between the parental habitats, suggesting that selection by environmental factors against hybrids may help to maintain the current conditions in this hybrid zone.     

Arthropod communities on hybrid and parental cottonwoods are phylogenetically structured by tree type: Implications for conservation of biodiversity in plant hybrid zones

Although hybridization in plants has been recognized as an important pathway in plant speciation, it may also affect the ecology and evolution of associated communities. Cottonwood species (Populus angustifolia and P. fremontii) and their naturally occurring hybrids are known to support different plant, animal, and microbial communities, but no studies have examined community structure within the context of phylogenetic history. Using a community composed of 199 arthropod species, we tested for differences in arthropod phylogenetic patterns within and among hybrid and parental tree types in a common garden. Three major patterns emerged. (1) Phylogenetic diversity (PD) was significantly different between arthropod communities on hybrids and Fremont cottonwood when pooled by tree type. (2) Mean phylogenetic distance (MPD) and net relatedness index (NRI) indicated that communities on hybrid trees were significantly more phylogenetically overdispersed than communities on either parental tree type. (3) Community distance (D_pw) indicated that communities on hybrids were significantly different than parental species. Our results show that arthropod communities on parental and hybrid cottonwoods exhibit significantly different patterns of phylogenetic structure. This suggests that arthropod community assembly is driven, in part, by plant–arthropod interactions at the level of cottonwood tree type. We discuss potential hypotheses to explain the effect of plant genetic dissimilarity on arthropod phylogenetic community structure, including the role of competition and environmental filtering. Our findings suggest that cottonwood species and their hybrids function as evolutionarily significant units (ESUs) that affect the assembly and composition of associated arthropod communities and deserve high priority for conservation.     

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.     

Comparisons of arthropod assemblages on an invasive and native trees: abundance,diversity and damage

The success of exotic plants may be due to lower herbivore loads than those on native plants (Enemies Release Hypothesis). Predictions of this hypothesis include lower herbivore abundances, diversity, and damage on introduced plant species compared to native ones. Greater density or diversity of predators and parasitoids on exotic versus native plants may also reduce regulation of exotic plants by herbivores. To test these predictions, we measured arthropod abundance, arthropod diversity, and foliar damage on invasive Chinese tallow tree (Triadica sebifera) and three native tree species: silver maple (Acer saccharinum), sycamore (Platanus occidentalis), and sweetgum (Liquidambar styraciflua). Arthropod samples were collected with canopy sweep nets from six 20 year old monoculture plots of each species at a southeast Texas site. A total of 2,700 individuals and 285 species of arthropods were caught. Overall, the species richness and abundance of arthropods on tallow tree were similar to the natives. But, ordination (NMS) showed community composition differed on tallow tree compared to all three native trees. It supported an arthropod community that had relatively lower herbivore abundance but relatively more predator species compared to the native species examined. Leaves were collected to determine damage. Tallow tree experienced less mining damage than native trees. The results of this study supported the Enemies Release Hypothesis predictions that tallow tree would have low herbivore loads which may contribute to its invasive success. Moreover, a shift in the arthropod community to fewer herbivores without a reduction in predators may further limit regulation of this exotic species by herbivores in its introduced range.     

Structure of herbivore communities in two oak (Quercus spp.) hybrid zones

Summary We examined patterns of density and species diversity for leaf-mining Lepidopterans and gall-forming Hymenopterans in two oak (Quercus spp.) hybrid zones: Quercus depressipes x Q. rugosa and Q. emoryi x Q. coccolobifolia. In both species complexes, hybrid hosts typically supported significantly lower densities and species diversity of parasites than did parental types. This contradicts the findings of Whitham (1989) that suggested that hybrid hosts may act as parasite sinks both in ecological and evolutionary time. We discuss features of hybrid zones that are likely to influence patterns of herbivory.     

Diagnostic SNPs reveal widespread introgressive hybridization between introduced bighead and silver carp in the Mississippi River Basin

Hybridization among conspecifics in native and introduced habitats has important implications for biological invasions in new ecosystems. Bighead (Hypophthalmichthys nobilis) and silver carp (H. molitrix) are genetically isolated and occur in sympatry within their native range. Following their introduction to North America, however, introgressant hybrids have been reported throughout their expanded range within the Mississippi River Basin (MRB). The extent of introgression, both spatially and generationally, is largely unknown. Therefore, we examined mixed‐species populations from across the MRB to characterize the extent of interspecific gene flow. We assayed 2798 individuals from nine locations with a suite of species‐diagnostic SNPs (57 nuclear and one mitochondrial). Forty‐four per cent (n = 1244) of individuals displayed hybrid genotypes. Moreover, the composition of hybrid genotypes varied among locations and represented complex hybrid swarms with multiple generations of gene flow. Introgressive hybrids were identified from all locations, were bidirectional and followed a bimodal distribution consisting primarily of parental or parental‐like genotypes and phenotypes. All described hybrid categories were present among individuals from 1999 to 2008, with parents and later‐generation backcrosses representing the largest proportion of individuals among years. Our mitochondrial SNP (COII), tested on a subset of 730 individuals, revealed a silver carp maternal bias in 13 of 21 (62%) F₁ hybrids, in all silver carp backcrosses, and maintained throughout many of the bighead carp backcrosses. The application of this suite of diagnostic markers and the spatial coverage permits a deeper examination of the complexity in hybrid swarms between two invasive, introduced species.     

The influence of mistletoes on the litter-layer arthropod abundance and diversity in a semi-arid savanna,Southwest Zimbabwe

Aims

The below-canopy soil moisture content and litter-layer arthropod abundance and diversity of Acacia karroo trees parasitized by each of three mistletoe species (Erianthemum ngamicum, Plicosepalus kalachariensis, and Viscum verrucosum) and uninfected A. karroo trees were investigated in semi-arid savanna, southwest Zimbabwe.

Results

The soils below the canopies of mistletoe-infected trees were significantly low in moisture content compared to those beneath uninfected A. karroo trees. Nevertheless, arthropod species diversity was greater by up to 34 % below the canopies of mistletoe-infected trees than beneath uninfected A. karroo trees, with greater abundances beneath trees infected by E. ngamicum and P. kalachariensis. In addition, the majority of the arthropod species associated with mistletoe-infected trees had litter as their dominant foraging substrate.

Conclusions

Our findings show that mistletoes increase the abundance and diversity of litter-dwelling and –foraging arthropods due to increase in the quality and quantity of litterfall beneath mistletoe-infected trees. By altering the below-canopy arthropod communities and soil moisture content, mistletoes have potential to modify ecosystem processes such as decomposition, soil process rates, and nutrient cycling. Therefore, we suggest that the resulting increase in resource heterogeneity plays an important role in determining the structure and functioning of semi-arid savanna ecosystems.     

Maintenance of species boundaries in three sympatric Ligularia (Senecioneae,Asteraceae) species

The key process in speciation concerns the formation and maintenance of reproductive isolating barriers between diverging lineages. Although species boundaries are frequently investigated between two species across many taxa, reproductive isolating barriers among multiple species (>2) that would represent the most common phenomenon in nature, remain to be clarified. Here, we use double digest restriction‐site associated DNA (ddRAD) sequencing to examine patterns of hybridization at a sympatric site where three Ligularia species grow together and verify whether those patterns contribute to the maintenance of boundaries among species. The results based on the RAD SNP datasets indicated hybridization Ligularia cyathiceps × L. duciformis and L. duciformis × L. yunnanensis were both restricted to F₁s plus a few first‐generation backcrosses and no gene introgression were identified, giving rise to strong reproductive isolation among hybridizing species. Moreover, hybrid swarm simulation, using HYBRIDLAB, indicated the RAD SNP datasets had sufficient discriminatory power for accurate hybrid detection. We conclude that parental species show strong reproductive isolation and they still maintain species boundaries, which may be the key mechanism to maintain species diversity of Ligularia in the eastern Qinghai‐Tibetan Plateau and adjacent areas. Moreover, this study highlights the effectiveness of RAD sequencing in hybridization studies.     

Cryptic introgression into the kidney saxifrage (Saxifraga hirsuta) from its more abundant sympatric congener Saxifraga spathularis,and the potential risk of genetic assimilation

Background and Aims Although hybridization can play a positive role in plant evolution, it has been shown that excessive unidirectional hybridization can result in replacement of a species’ gene pool, and even the extinction of rare species via genetic assimilation. This study examines levels of introgression between the common Saxifraga spathularis and its rarer congener S. hirsuta, which have been observed to hybridize in the wild where they occur sympatrically.Methods Seven species-specific single nucleotide polymorphisms (SNPs) were analysed in 1025 plants representing both species and their hybrid, S. × polita, from 29 sites across their ranges in Ireland. In addition, species distribution modelling was carried out to determine whether the relative abundance of the two parental species is likely to change under future climate scenarios.Key Results Saxifraga spathularis individuals tended to be genetically pure, exhibiting little or no introgression from S. hirsuta, but significant levels of introgression of S. spathularis alleles into S. hirsuta were observed, indicating that populations exhibiting S. hirsuta morphology are more like a hybrid swarm, consisting of backcrosses and F₂s. Populations of the hybrid, S. × polita, were generally comprised of F₁s or F₂s, with some evidence of backcrossing. Species distribution modelling under projected future climate scenarios indicated an increase in suitable habitats for both parental species.Conclusions Levels of introgression observed in this study in both S. spathularis and S. hirsuta would appear to be correlated with the relative abundance of the species. Significant introgression of S. spathularis alleles was detected in the majority of the S. hirsuta populations analysed and, consequently, ongoing introgression would appear to represent a threat to the genetic integrity of S. hirsuta, particularly in areas where the species exists sympatrically with its congener and where it is greatly outnumbered.     

Further studies with interspecific hybridization among mediterranean/African lupin species

Summary Interspecific hybridization has a role to play in the domestication of wild species through the introgression of desirable genes from related domesticated species. Almost complete genetic isolation among the Mediterranean-African rough-seeded lupin species has hitherto ruled out introgression within that group. Recent work in Western Australia with L. atlanticus, L. cosentinii, L. pilosus and L. digitatus has nevertheless suggested that hybrid sterility may be overcome if specially selected lines of each are used in crossing. We have now selected F₂-F₅ plants and backcrosses (Bc₁; Bc₂) from L. atlanticus/L. cosentinii, some with improvement in seed fertility combined with domestication genes from either species. Relatively better F₁-F₂ plant fertility in subsequent crosses of L. atlanticus/L. digitatus than in L. atlanticus/L. cosentinii or L. digitatus/L. cosentinii indicate closer relationship between L. atlanticus and L. digitatus than in the other cross combinations. Use of embryo culture may help to overcome L. pilosus/L. atlanticus F₁ plant sterility. Some of the interspecific selections could act as genetic bridges between L. cosentinii and L. pilosus.     

Comparative genetic linkage maps of <Emphasis Type="Italic">Eucalyptus grandis</Emphasis>, <Emphasis Type="Italic">Eucalyptus globulus</Emphasis> and their F<Subscript>1</Subscript> hybrid based on a double pseudo-backcross mapping approach

Comparative genetic mapping in interspecific pedigrees presents a powerful approach to study genetic differentiation, genome evolution and reproductive isolation in diverging species. We used this approach for genetic analysis of an F₁ hybrid of two Eucalyptus tree species, Eucalyptus grandis (W. Hill ex Maiden.) and Eucalyptus globulus (Labill.). This wide interspecific cross is characterized by hybrid inviability and hybrid abnormality. Approximately 20% of loci in the genome of the F₁ hybrid are expected to be hemizygous due to a difference in genome size between E. grandis (640 Mbp) and E. globulus (530 Mbp). We investigated the extent of colinearity between the two genomes and the distribution of hemizygous loci in the F₁ hybrid using high-throughput, semi-automated AFLP marker analysis. Two pseudo-backcross families (backcrosses of an F₁ individual to non-parental individuals of the parental species) were each genotyped with more than 800 AFLP markers. This allowed construction of de novo comparative genetic linkage maps of the F₁ hybrid and the two backcross parents. All shared AFLP marker loci in the three single-tree parental maps were found to be colinear and little evidence was found for gross chromosomal rearrangements. Our results suggest that hemizygous AFLP loci are dispersed throughout the E. grandis chromosomes of the F₁ hybrid.Communicated by O. Savolainen     

Airborne‐pollen pool and mating pattern in a hybrid zone between Pinus pumila and P. parviflora var. pentaphylla

The reproductive isolation barriers and the mating patterns among Pinus pumila, P. parviflora var. pentaphylla and their hybrids were examined by flowering phenology and genetic assays of three life stages: airborne‐pollen grains, adults and seeds, in a hybrid zone on Mount Apoi, Hokkaido, Japan. Chloroplast DNA composition of the airborne‐pollen was determined by single‐pollen polymerase chain reaction. Mating patterns were analysed by estimating the molecular hybrid index of the seed parent, their seed embryos and pollen parents. The observation of flowering phenology showed that the flowering of P. pumila precedes that of P. parviflora var. pentaphylla by about 6 to 10 days within the same altitudinal ranges. Although this prezygotic isolation barrier is effective, the genetic assay of airborne‐pollen showed that the two pine species, particularly P. pumila, still have chances to form F₁ hybrid seeds. Both parental species showed a strong assortative mating pattern; F₁ seeds were found in only 1.4% of seeds from P. pumila mother trees and not at all in P. parviflora var. pentaphylla. The assortative mating was concluded as the combined result of flowering time differentiation and cross‐incompatibility. In contrast to the parental species, hybrids were fertilized evenly by the two parental species and themselves. The breakdown of prezygotic barriers (intermediate flowering phenology) and cross‐incompatibility may account for the unselective mating. It is suggested that introgression is ongoing on Mount Apoi through backcrossing between hybrids and parental species, despite strong isolation barriers between the parental species.