Interspecific and temporal variation in herbivore responses to hybrid willows

We studied herbivory of two species of willows (Salix sericea and S. eriocephala) and their interspecific hybrids to test alternative hypotheses concerning the effects of hybridization on plant resistance. Individually marked plants were identified using morphological traits in the field and random amplified polymorphic DNA (RAPD) band analysis was used to verify the genetic status of many parental and hybrid plants. The desities of 12 herbivore species on plants in the field were compared between two parents and their F₂-type hybrids. We found about equal support for the additive, dominance, and hybrid susceptibility hypotheses over 4 years. In one year, one species supported the hybrid resistance hypothesis. Guild membership was not a good predictor of similar responses of species to hybrid versus parental plants. There were marked differences in support for particular hypotheses among years for four herbivore species. This study demonstrates the diversity of responses of phytophages in response to interspecific hybridization, and indicates that year-to-year variation in relative resistance of hybrid plants can be important.     

Plant genetic differences influence herbivore community structure: evidence from a hybrid willow system

To determine the influence of plant genetic variation on community structure of insect herbivores, we examined the abundances of 14 herbivore species among six genetic classes of willow: Salix eriocephala, S. sericea, their F₁ and F₂ interspecific hybrids, and backcross hybrids to each parental species. We placed 1-year-old plants, grown from seeds generated from controlled crosses, in a common garden. During the growing season, we censused gall-inducing flies and sawflies, leaf-mining insects, and leaf-folding Lepidoptera to determine the community structure of herbivorous insects on the six genetic classes. Our results provided convincing evidence that the community structure of insect herbivores in this hybrid willow system was shaped by genetic differences among the parental species and the hybrid genetic classes. Using MANOVA, we detected significant differences among genetic classes for both absolute and relative abundance of herbivores. Using canonical discriminant analysis, we found that centroid locations describing community structure of the insect herbivores differed for each genetic class. Moreover, the centroids for the four hybrid classes were located well outside of the range between the centroids for the parental species, suggesting that more than additive genetic effects of the two parental species influenced community formation on hybrid classes. Line-cross analysis suggested that plant genetic factors responsible for structuring the herbivore community involved epistatic effects, as well as additive and dominance effects. We discuss the ramifications of these results in regard to the structure of insect herbivore communities on plants and the implications of our findings for the evolution of interspecific interactions.     

An experimental test of hybrid resistance to insects and pathogens using Salix caprea, S. repens and their F1 hybrids

The aim of this study was to assess the responses of herbivores and pathogens to hybrid plants under controlled conditions. F1 hybrids and parental species, produced by hand-pollinating willows in the field, were potted and kept in an experimental field under controlled conditions. In 1997, plant growth and survival were measured along with densities of insects and the degree of pathogen infection on the willows. The survival rate was higher for S. repens than for the hybrids and lowest for S. caprea. Densities of the sawflies Pontania pedunculi and P. brigmanii and the leaf-galling midge Iteomyia capreae were higher on hybrids and on S. caprea than on S. repens. The densities of Crepidodera fulvicornis (Chrysomelidae), chrysomelid larvae and the bud-galling midge Dasineura rosaria did not differ between any of the plant categories. Hybrids were more severely infected by rust (Melampsora sp.) than S. caprea and the totally resistant S. repens. Densities of herbivores on hybrid willows were consistent with the dominance hypothesis (i.e. herbivore densities were similar to densities on one of the parental species) or supported the no-difference hypothesis. Furthermore, herbivore densities on hybrid plants were most similar to densities on the more susceptible parent. The breakdown in rust resistance in hybrid plants suggests that resistance traits are severely disrupted by the genetic re-arrangement in hybrids and that this increased susceptibility could select against hybridisation. Received: 17 February 1998 / Accepted: 15 June 1998     

Seedling herbivory by slugs in a willow hybrid system: developmental changes in damage, chemical defense, and plant performance

We evaluated feeding preference and damage by the slug, Arion subfuscus, on seedlings of two willow species, Salix sericea and S. eriocephala, and their F₁ interspecific hybrids. Trays of seedlings were placed in the field and excised leaves were presented to slugs in choice tests. Slugs preferred feeding on and caused the most damage to S. eriocephala seedlings. S. sericea seedlings were least preferred and least damaged. F₁ hybrid seedlings were intermediate in preference and damage. Slug preference of and damage to these seedlings decreased over time, suggesting developmental changes in resistance. Seedlings were sampled for phenolic glycoside and tannin chemistry weekly to coincide with the field and laboratory experiments. Concentrations of phenolic glycosides and tannins increased linearly with seedling age, coincident with changes in slug preference and damage, indicating a developmental change in defense. Slug deterrence was not detected at low concentrations of salicortin when painted on leaves or discs, but both salicortin and condensed tannins deterred slug feeding at concentrations between 50 and 100 mg/g, levels found in adult willows. Seedling performance was related to damage inflicted by slugs. Due to lower levels of damage when exposed to slugs in the field, S. sericea plants had significantly greater biomass than S. eriocephala plants. Biomass of F₁ hybrids was equal to S. sericea when damaged. However, undamaged S. eriocephala and F₁ hybrid plants had the greatest biomass. Because F₁ hybrid seedlings performed as well as the most fit parent in all cases, slugs could be an important selective factor favoring introgression of defensive traits between these willow species.     

Willow hybrids and herbivory: a test of hypotheses of phytophage response to hybrid plants using the generalist leaf-feeder Lochmaea caprea (Chrysomelidae)

To test hypotheses of herbivore response to hybrid plants, I determined the preference of the generalist beetle Lochmeae capreae for individuals of Salix caprea, Salix phylicifolia and F1 hybrids between these species. F1 hybrids and pure individuals were created by hand-pollination of willow individuals in the field. The preference tests were performed in 1994 and 1995 and in both years there was a tendency for higher leaf area consumption from hybrids than from the parental species. However, statistically significant differences in consumption were only found in 1995 between hybrids and S. phylicifolia. The result from this study therefore lends most support to the dominance hypothesis: hybrid susceptibility was similar to that of one of the parental species. This is one of the first strict preference tests showing that a generalist herbivore can detect differences in palatability between hybrids and parental species. Furthermore, the experimental design used in the study minimized environmental variation. The results should therefore primarily reflect genetically based quality differences between hybrids and parental species. Received: 4 March 1996 / Accepted: 12 August 1996     

The susceptibility of parental and hybrid willows to plant enemies under contrasting soil nutrient conditions

 We conducted an experimental study of the effects of nutrient addition on the susceptibility of two species of willows (Salix eriocephala and S. sericea) and their hybrid to a pathogen and several herbivores. We hypothesized that the relative susceptibility of parental and hybrid willows would depend upon soil nutrient availability and vary among plant enemies. Using potted plants in a common garden, we found that S. eriocephala was significantly more susceptible to attack by a fungal rust (Melampsora sp.), a leaf-chewing beetle (Popillia japonica), and a leaf-folding sawfly (Phyllocolpa nigrita) than was S. sericea. Conversely, S. sericea was significantly more susceptible to attack by a spider mite (Tetranychus sp.) and a leaf-mining caterpillar (Phyllocnistis sp.) than was S. eriocephala. Hybrid susceptibility to Melampsora sp. and to Phyllocnistis sp. resembled S. eriocephala’s, while hybrid susceptibility to Phyllocolpa nigrita, Tetranychus sp., and Popillia japonica resembled S. sericea’s. Susceptibility to a sixth enemy, another leaf-mining caterpillar, Phyllonorycter salicifoliella, did not differ among the parents and hybrid. Susceptibility to herbivores and pathogens increased along a gradient of increasing fertilizer application, and this effect was independent of plant taxon or enemy. The results of our study point to the contrasting influences of a taxonomically and functionally diverse enemy community, reinforce the hybrid dominance model of enemy susceptibility, and demonstrate that physiological stress and enemy susceptibility can be inversely related. Received: 25 March 1996 / Accepted: 6 August 1996     

Differential effects of host plant hybridization on herbivore community structure and grazing pressure on forest canopies

Interspecific hybridization in plants is known to have ecological effects on associated organisms. We examined the differences in insect herbivore community structure and grazing pressure on tree canopy leaves among natural hybrids and their parental oak species. We measured leaf traits, herbivore community structure, and grazing pressure on leaves of two oak species, Quercus crispula and Q. dentata, and their hybrids. The concentration of nitrogen in canopy leaves was greater in hybrids and in Q. dentata than in Q. crispula. The concentration of total phenolics was lower in hybrids than in Q. crispula. The concentration of condensed tannin was greater in hybrids than in Q. crispula. Relative herbivore abundance and species richness were greater on oak hybrids than on either parental species; herbivore species diversity and composition on hybrids were close to those on Q. crispula. Herbivore grazing pressure was lower on hybrids and Q. dentata than on Q. crispula. There was a negative correlation between herbivore grazing pressure and leaf nitrogen, suggesting that interspecific variation among oak taxa in herbivore pressure may be explained by leaf nitrogen; variation in herbivore community structure among oak taxa is likely to be controlled by polygenic leaf traits. Differing responses of (1) herbivore community structure and (2) herbivore grazing pressure to host plant hybridization may play important roles in regulating herbivore biodiversity in cool‐temperate forest canopies.     

Narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata: Asteraceae). II. Selection gradients and hybrid fitness

The dynamic equilibrium hypothesis proposes that hybrid zones are stabilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower fitness than either parental taxon, regardless of habitat. Hybrid big sagebrush (Artemisia tridentata ssp. tridentata × ssp. vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore loads, however, are not greater on the hybrids than on the parental subspecies. Browsing by mule deer is greatest on ssp. vaseyana in both hybrid zones. Incidence of galls is also greatest on ssp. vaseyana in one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on ssp. tridentata in one hybrid zone. Hybrids produce more flowers than either ssp. tridentata or ssp. vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybrid seeds germinate as well as those of ssp. tridentata and better than those of ssp. vaseyana. Thus, our data do not support the dynamic equilibrium hypothesis.     

Herbivore resistance of invasive <Emphasis Type="Italic">Fallopia</Emphasis> species and their hybrids

Hybridization has been proposed as a mechanism by which exotic plants can increase their invasiveness. By generating novel recombinants, hybridization may result in phenotypes that are better adapted to the new environment than their parental species. We experimentally assessed the resistance of five exotic Fallopia taxa, F. japonica var. japonica, F. sachalinensis and F. baldschuanica, the two hybrids F. × bohemica and F. × conollyana, and the common European plants Rumex obtusifolius and Taraxacum officinale to four native European herbivores, the slug Arion lusitanicus, the moth Noctua pronuba, the grasshopper Metrioptera roeselii and the beetle Gastrophysa viridula. Leaf area consumed and relative growth rate of the herbivores differed significantly between the Fallopia taxa and the native species, as well as among the Fallopia taxa, and was partly influenced by interspecific variation in leaf morphology and physiology. Fallopia japonica, the most abundant Fallopia taxon in Europe, showed the highest level of resistance against all herbivores tested. The level of resistance of the hybrids compared to that of their parental species varied depending on hybrid taxon and herbivore species. Genotypes of the hybrid F. × bohemica varied significantly in herbivore resistance, but no evidence was found that hybridization has generated novel recombinants that are inherently better defended against resident herbivores than their parental species, thereby increasing the hybrid’s invasion success. In general, exotic Fallopia taxa showed higher levels of herbivore resistance than the two native plant species, suggesting that both parental and hybrid Fallopia taxa largely escape from herbivory in Europe.     

CROSSING RELATIONSHIPS AMONG DIPLOID SPECIES OF THE SOLANUM NIGRUM COMPLEX IN NORTH AMERICA

Crosses were made in all combinations of the six diploid species of the Solarium nigrum complex that occur in North America. Some interspecific pollinations failed to yield viable seed; successful crosses gave rise to moderately to highly sterile F₁ hybrids. Results of interspecific crosses suggested phylogenetic relationships that were not completely in accord with those suggested by morphology. Interspecific crosses also gave varied results. All interpopulational crosses within S. interius and S. sarachoides produced fully fertile hybrids. In contrast, hybrids within S. americanum and S. douglasii varied from fully fertile to almost completely sterile. Populations of S. pseudogracile could be divided into two groups which are geographically separated but not morphologically differentiated. Fully fertile hybrids resulted from crosses within a group, whereas crosses between groups gave hybrids with reduced fertility. Four crossing groups were observed within 5. nodiflorum; three of the parental groups are sympatric and are morphologically differentiated. Although hybrid sterility in interspecific crosses is sometimes used to support delimitation of species, the presence of sterility in intraspecific crosses suggests that such an interpretation is unwarranted for the S. nigrum complex. Hybrid sterility, therefore, is not considered to have special taxonomic significance in this complex.     

Morphological and molecular evidence of natural hybridization in Shorea (Dipterocarpaceae)

Shorea (Dipterocarpaceae) is a large genus in which many closely related species often grow together in Southeast Asian lowland tropical rain forests. Many Shorea species share common pollinators, and earlier studies suggested occurrence of interspecific hybridization and introgression. Here, we show morphological and molecular evidence of hybridization between Shorea species. In the census of all the trees of Shorea curtisii, Shorea leprosula, and Shorea parvifolia (>30 cm dbh) within the 164-ha area of Bukit Timah Nature Reserve in Singapore, we found 21 morphologically recognizable hybrid individuals. All of the putative hybrids could be distinguished obviously from the parental species on the basis of vegetative characters. Population genetic analysis of DNA sequences of two nuclear (GapC and PgiC) and chloroplast (trnL-trnF) regions demonstrated that each of the three species had several species-specific mutations. The nuclear sequences of the putative hybrids were heterozygote at all the species-specific sites between two parental species. Hybrid between S. curtisii and S. leprosula was found most, while S. curtisii × S. parvifolia and S. leprosula × S. parvifolia hybrids were also found. Almost no shared polymorphism between populations of the parental species suggests rarity of introgression. The study indicated that natural hybridization between sympatric Shorea species should not be uncommon, but all of the hybrid individuals were F₁, and the post-F₁ hybrids were considerably rare.     

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     

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.     

Production and characterization of fertile somatic hybrids of eggplant (Solanum melongena L.) with Solanum aethiopicum L.

Summary In order to produce fertile somatic hybrids, mesophyll protoplasts from eggplant were electrofused with those from one of its close related species, Solanum aethiopicum L. Aculeatum group. On the basis of differences in the cultural behavior of the parental and hybrid protoplasts, 35 somatic hybrid plants were recovered from 85 selected calli. When taken to maturity either in the greenhouse or in the field, the hybrid plants were vigorous, all rapidly overtopping parental individuals. The putative hybrids were intermediate with respect to morphological traits, and all of their organs were larger, particularly the leaves and stems. DNA analysis of the hybrids using flow cytometry in combination with cytological analysis showed that 32 were tetraploids, 1 hexaploid and 2 mixoploids. The hybrid nature of the 35 selected plants was confirmed by a comparison of the isoenzyme patterns of isocitrate dehydrogenase (Idh), 6-phosphogluconate dehydrogenase (6-Pgd) and phosphoglucomutase (Pgm). Chloroplast DNA (ctDNA) restriction analysis using Bam HI revealed that among the 27 hybrid plants analyzed, 10 had S. aethiopicum patterns and the 17 remaining hybrids exhibited bands identical with those of eggplant without any changes. All of the somatic hybrid plants flowered. Both parental plants had 94% stainable pollen, while the hybrids varied widely in pollen viability ranging from 30% to 85%. The somatic hybrids showed high significant variation in fruit production. Nevertheless, there was a tendency for low fertility to be associated often with S. aethiopicum chloroplast type and/or with an abnormal ploidy level, while good fertility was mostly associated with the tetraploid level and eggplant chloroplasts. Interestingly, 2 tetraploid somatic hybrid clones were among the most productive, yielding up to 9 kg/plant. As far as the fertility of the F₁ sexual counterpart was concerned, only 2 fruits of 50 g were obtained. Hybrid fertility in relation to phylogenetic affinities of the fusion partners is discussed.     

Evidence for strong assortative mating,limited gene flow,and strong differentiation across the blue‐footed/Peruvian booby hybrid zone in northern Peru

Hybrid zones represent natural laboratories in which the processes of divergence and genetic isolation can be examined. The generation and maintenance of a hybrid zone requires mispairing and successful reproduction between organisms that differ in one or more heritable traits. Understanding the dynamics of hybridization between two species requires an understanding of the extent to which they have diverged genetically, the frequency of misparing and hybrid production, and the extent of introgression. Three hundred and twenty one blue‐footed Sula nebouxii and Peruvian S. variegata boobies from the eastern tropical Pacific Ocean were analyzed using 19 putatively neutral genetic markers to evaluate interspecific differentiation, to classify morphological hybrids using Bayesian assignments, and to characterize hybridization using cline theory and Bayesian assignments. The species were well differentiated at mitochondrial and nuclear microsatellites, the hybrid zone was bimodal (contained a high frequency of each parental species but a low frequency of hybrids), and morphologically intermediate individuals were most likely F1 hybrids resulting from mating between female Peruvian boobies and male blue‐footed boobies. Clines in allele frequency could be constrained to share a common geographic centre but could not be constrained to share a common width. Peruvian and blue‐footed boobies hybridize infrequently, potentially due to strong premating reproductive isolation; however, backcrossing appears to facilitate introgression from blue‐footed to Peruvian boobies in this hybrid system.     

Variable responses of insects to hybrid versus parental sagebrush in common gardens

Both ecological and genetic mechanisms have been proposed to explain patterns of herbivore attack on interspecific plant hybrids, but distinguishing among them can be difficult in natural hybrid zones. We performed a common-garden experiment to evaluate four genetic hypotheses: dominance, additivity, elevated hybrid susceptibility, and elevated hybrid resistance. Censuses and cage experiments were used to compare insect responses to basin big sagebrush (Artemisia tridentata spp. tridentata), mountain big sagebrush (A. t. vaseyana), and their F₂ progeny. After two growing seasons, hybrid shrubs resembled mountain big sagenbrush in size, but were more similar to basin big sagebrush in flower production. Censuses of naturally colonizing insects (the gall midge Rhopalomyia obovata, the bagworm moth Apterona helix, and the aphid Obtusicauda coweni) tended to support the dominance hypothesis: if the insect clearly discriminated between the two parents, its frequency on hybrids closely resembled that on one of the parents. Moreover, colonization of hybrids in all three cases suggested a dominance deviation toward the susceptible parent rather than toward the resistant parent. In contrast to the censuses, cage experiments involving two insects supported the hybrid-susceptibility hypothesis; both survival and growth of the grasshopper Melanoplus sanguinipes and growth of the leaf beetle Trirhabda pilosa were higher on hybrid shrubs than on either parent. Because many secondary compounds have been determined to occur at intermediate concentrations in F₂ shrubs, dominance for susceptibility may indicate that insects respond to plant traits (e.g., oviposition stimulants and deterrents) in a threshold manner. Mechanisms underlying increased hybrid susceptibility are less clear, but our experimental design makes environmental explanations (e.g., the plant-stress hypothesis) unlikely. Although we eliminated several confounding factors, our results agree with the conclusion from natural hybrid zones that insect responses to hybrid plants are likely to be idiosyncratic; even congeneric species did not respond similarly to hybrid and parental plants.     

Community structure of canopy arthropods associated to Quercus crassifolia×Quercus crassipes complex

Quercus crassifolia and Q. crassipes are dominant species in temperate forests of central Mexico and hybridize between each other when they occur in sympatry. Oak canopies contain a considerable portion of arthropod diversity and the hybrid zones can provide new habitats to epiphyte fauna. We tested if the establishment of hybrids in contact zones with their parental hosts increases the species diversity of canopy arthropods assuming that hybrid trees constitute new genotypes of potential new habitats to small organisms. We examined the effect of hybridization on some community structure parameters (diversity, composition, similarity and density of arthropod fauna) of canopy arthropods compared to their parental species in a hybrid zone located in central Mexico. We employed 17 leaf morphological traits and six diagnostic RAPD primers to identify parental and hybrid plants. The RAPDs marker showed unidirectional introgression towards Q. crassifolia, and were detected hybrid (F₁), backcrosses and introgression individual trees. In total, 30 oak canopies were fogged during rainy and dry season. We recognized 532 taxa of arthropods belonging to 22 orders associated with tree canopies. The taxonomic status of host‐trees may be an important factor in the arthropod community structure and that seasonality (dry and rainy) is not a factor that could modify their organization. Trees of Q. crassipes registered the highest densities of arthropod fauna followed by hybrid hosts (F₁); trees originated by backcrosses towards Q. crassifolia registered a significant less arthropod density than F₁ hybrids; and trees of Q. crassifolia had the lowest density. Hybrid plants and Q. crassipes individuals had higher diversity (H′) of arthropods than Q. crassifolia plants. Hybrid plants had also more rare species in both seasons in comparison with parental species. This study suggests that hybrid oaks act as a center of biodiversity by accumulating arthropods of both parental and different species including a considerable number of rare species.     

Consequences of interspecific hybridization and virus infection on the growth and fecundity of three threatened coastal Lepidium (Brassicaceae) species from New Zealand

Lepidium castellanum, L. juvencum and L. oleraceum are threatened coastal cresses endemic to New Zealand. These three species were selfed and interspecific hybrids generated for examination of hybrid fitness and inbreeding depression. In controlled glasshouse experiments, the interspecific hybrids and selfed progeny were inoculated with a strain of the introduced Turnip mosaic virus (TuMV) previously isolated from wild populations of L. aegrum. Experiments tested the hypothesis that heterosis in the interspecific hybrids provides a gain in TuMV resistance in comparison to selfed plants. We show that interspecific hybrids of three genetically distinct species of Lepidium increased plant performance and reduced susceptibility to the effects of the TuMV. We suggest that interspecific hybridization could be implemented as a conservation management strategy and that a broader outlook may be required to mitigate the negative impacts of introduced pathogens on threatened species.     

Physiological adaptation along environmental gradients and replicated hybrid zone structure in swordtails (Teleostei: Xiphophorus)

Local adaptation is often invoked to explain hybrid zone structure, but empirical evidence of this is generally rare. Hybrid zones between two poeciliid fishes, Xiphophorus birchmanni and X. malinche, occur in multiple tributaries with independent replication of upstream‐to‐downstream gradients in morphology and allele frequencies. Ecological niche modelling revealed that temperature is a central predictive factor in the spatial distribution of pure parental species and their hybrids and explains spatial and temporal variation in the frequency of neutral genetic markers in hybrid populations. Among populations of parentals and hybrids, both thermal tolerance and heat‐shock protein expression vary strongly, indicating that spatial and temporal structure is likely driven by adaptation to local thermal environments. Therefore, hybrid zone structure is strongly influenced by interspecific differences in physiological mechanisms for coping with the thermal environment.