Plant hybrid zones as centers of biodiversity: the herbivore community of two endemic Tasmanian eucalypts

We found the hybrid zone between Eucalyptus amygdalina and Eucalyptus risdonii to be a center of insect and fungal species richness and abundance. Of 40 taxa examined, 73% were significantly more abundant in the hybrid zone than in pure zones, 25% showed on significant differences, and 2% were most abundant on a pure host species. The average hybrid tree supported 53% more insect and fungal species, and relative abundances were, on average, 4 times greater on hybrids than on either eucalypt species growing in pure stands. Hybrids may act as refugia for rare species: 5 of 40 species were largely restricted to the hybrid zone. Also, 50% of the species coexisted only in the hybrid zone, making for mique species assemblages. Although hybrids support more species and greater abundances, all hybrids are not equal: 68% of the 40 taxa examined were significantly more abundant on one hybrid phenotype than another. While herbivore concentrations on F1 type intermediates were rare, concentrations were common on phenotypes resembling backcrosses either to E. amygdalina or E. risdonii. For specialist herbivores, the hybrid phenotype most heavily utilized appears to be determined by its phenotypic affinity to its host species. Generalists exhibit an overall greater abundance on hybrids, but are less likely to utilize one hybrid phenotype over another. Mechanistic explanations for these distributions are numerous and probably species specific, but are likely to include: increased genetic susceptibility of hybrids due to hybrid breakdown; increased stress in the hybrid zone resulting in greater plant susceptibility; and a greater diversity of resources in the hybrid zone which could support more species. Seed capsule production by hybrids and their parental species is negatively correlated with herbivory. However, it is difficult to determine whether herbivores cause this pattern as hybrids may have inherently lower sexual reproduction. Laws enacted to protect rare and endangered species do not include hybrids. We argue that a re-examination of our current hybrid policy is warranted. Plant hybrid zones are centers of plant evolution and speciation, sources of economically important plants and potential biocontrol agents, and, as our study suggests, also provide essential habitats for phytophagous communities.     

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.     

Induced resistance in mountain birch: defence against leaf-chewing insect guild and herbivore competition

Summary The effect of leaf damage simulating the feeding of early season insect herbivore species, e.g. Epirrita autumnata, to mountain birch, Betula pubescens ssp. tortuosa, on the performance of insect larvae was studied with eleven leaf-chewing sawfly species. I found variation in the results that was due to short- and long-term inducible responses and to the phenology of herbivore species. In general, early and mid-season species were more strongly affected by induced reactions than late-season species. This finding is in accordance with earlier results but I could show that the persistance of induced reactions rather than the influence of timing of damage is responsible for the result. The growth of the larvae of mid-season sawfly species was affected by both short- and long-term induced reactions. This result shows that early season species may escape short-term induced reactions of mountain birch in current year but may not avoid long-term effects. It is supposed that seasonal deterioration of leaf quality either masks the effects of induced defences or late-season species are better adapted to low-quality leaves. Some species show variation in their response to induced defence in different years. This may be due to yearly differences in induced reactions as well as to species-specific responses. Induced defence reactions may play a role in competitive interactions between herbivore species in leaf-chewing guild of mountain birch.     

Genetic and soil-nutrient effects on the abundance of herbivores on willow

The effects of soil-nutrient environment, plant genotype, and the interaction between the two on the resistance of the willow, Salix sericea, to insect species in a diverse herbivore community was measured. We found that soil-nutrient environment influenced plant growth and the abundance of most herbivores of S. sericea. However, environmental effects on herbivore abundance were often modified by plant genetics; the abundance of four of seven herbivores exhibited significant genotypeby-environment interaction effects. Pure genotype effects were mostly small and non-significant. The effects of fertilization differed among herbivores. Several herbivores were more abundant on fertilized plants, one was less abundant, and the abundance of others did not change. We found that feeding guild was a poor predictor of herbivore response. Finally we found significant phenotypic and genetic correlations among growth rate, internode length, and the abundances of several herbivores.     

Fertility and sexual structure in a polygamous willow population

This note reports on an extraordinary polygamous population of Salix acmophylla from Nahal Dishon, Israel. Remarkably, all individuals in this population are bisexuals, that is, they all contain typically female catkins (with some or without any male florets), typically male catkins (with some or without any female florets) and mixed catkins. The proportions of these three catkin types in populations are 36.3%, 46.8% and 16.9% respectively. The fertility of these willows was found to be poor: no mature seeds were found at all and mean pollen stainability was low as 15.6%. The possible reasons for this semi-sterility are discussed.     

Molecular evidence indicates that subarctic willow communities in Scotland support a diversity of host-associated Melampsora rust taxa

Rare and threatened subarctic willow scrub communities in the UK are the subject of ongoing conservation programmes, yet little is known about the diversity of fungal taxa that they support. Isolates of the rust genus Melampsora were sampled from 112 leaves of eight subarctic willow (Salix) taxa and their hybrids from twelve sites in the UK. In order to determine the number of Melampsora taxa present in the samples, isolates were sequenced for the Internal Transcribed Spacer (ITS) region of rDNA and data were subject to phylogenetic analysis. Maximum likelihood and Bayesian analysis indicated that the isolates fell into three strongly supported host-associated clades. Clade I contained only isolates from Salix herbacea and was distinguished morphologically by dense urediniospore echinulation and thin cell walls. Clade II contained isolates from Salix arbuscula and Salix reticulata only. These could not be distinguished morphologically from isolates in Clade III which were found on Salix lapponum, Salix myrsinites, Salix myrsinifolia, Salix aurita, Salix lanata, and their hybrids. Clade II was most distinct in ITS sequence, differing by 50 bases from Clades I and III, while the latter clades differed in sequence by only 24 bases on average. Clades I and III are likely to represent the previously recognised taxa Melampsora alpina Juel 1894 and Melampsora epitea Thüm. 1879 respectively, but Clade II has not apparently been described before. Significant differences in the intensity of infection by isolates of Clade III were found among different Salix species at a single site, suggesting either differences in resistance among Salix taxa, or the presence of further cryptic taxa within Clade III. The study illustrates the power of molecular phylogenetic analysis to reveal cryptic biodiversity within Melampsora, and suggests that conserving Salix host diversity within subarctic willow communities will ensure that a diversity of associated Melampsora taxa is maintained.     

Fungal colonization of shrub willow roots at the forefront of a receding glacier

Shrub willows (Salix spp.) form associations with arbuscular mycorrhizal (AM), ectomycorrhizal (EM) and dark septate endophytic (DSE) fungi. Willow root colonization by these three types of fungi was studied on a deglaciated forefront of Lyman Glacier, Washington, USA. Root colonization was low; less than 1% of the root length was colonized by AM and 25.6% by DSE. EM colonized 25% of the root tips and 19.4% of the root length. AM and DSE colonization were not related to distance from the present glacier terminus or to canopy cover. EM colonization increased with distance from the glacier terminus based on gridline intercept data but not on root tip frequency data. Availability of propagules in the substrate was low, but numbers of propagules increased with distance from the glacier terminus. The EM communities were dominated by three ascomycetes showing affinity to Sordariaceae in BLAST analyses. Other frequent taxa on the glacier forefront included species of Cortinariaceae, Pezizaceae, Russulaceae, Thelephoraceae and Tricholomataceae. When occurrence of individual taxa was used as a response variable to canopy cover, distance from the glacier terminus, and their interaction, four different fungal guilds were identified: 1) fungi that did not respond to these environmental variables; 2) fungi that occurred mainly in intercanopy areas and decreased with distance from the glacier terminus; 3) fungi that were insensitive to canopy cover but increased with distance from the glacier terminus; 4) fungi that occurred mainly under willow canopies and increased with distance from the glacier terminus. We suggest that fungal colonization is mainly limited by fungal propagule availability. Environmental conditions may also limit successful establishment of plant-fungus associations. We propose that the four EM guilds partly explain successional dynamics. The initial EM community comprises fungi that tolerate low organic matter and nitrogen environment (first and second guilds above). During later community development, these fungi are replaced by those that benefit from an increased organic matter and nitrogen environment (third and fourth guilds above).     

Host plants affect predator fitness via the nutritional value of herbivore prey: Investigation of a plant-aphid-ladybeetle system

The interactions among host plants(Medicago sativa L., cv. `OKO8' and Vicia faba L., cv. `Windsor'), aphid prey(Acyrthosiphon pisum Harris, Homoptera:Aphididae), and Coccinella septempunctata L. (Coleoptera: Coccinellidae) preimaginal biology were evaluated. Interactions were measured over a range of limiting daily prey levels (1.2 mg–16.4 mg)from each host plant colony. Compared withA. pisum reared on V. faba, A. pisum reared on M. sativa storedsignificantly more fatty acids which resultedin a 1.17-fold increase in available caloriesfor developing C. septempunctata. Theincreased survival, decreased developmentaltimes, and larger size of C.septempunctata supplied with A. pisumreared on M. sativa clearly demonstratehost plant effects at the third trophic level. At low very limiting daily prey levels, A. pisum reared on M. sativa were moresuitable prey for C. septempunctatasurvival, development, and adult size thanA. pisum reared on V. faba. Coccinella septempunctata survival ratios(larval), developmental times, and adult sizeconverged (were not statistically different)between host plants at higher daily A.pisum levels. These convergence's supportthe hypothesis that there were quantitativedifferences in the nutritional value ofaphids, as influenced by differences in fattyacids and subsequent nutritional levels(calories), between aphids reared on separateplant hosts. The observed tritrophicinteractions appear to be modulated by thebiochemical response of A. pisum to hostplants.     

Population structure and genetic diversity in two species of Hawaiian picture-winged Drosophila

Over the last several decades many picture-winged Drosophila have become less common in both geographical distribution and local population size (pers. obs., Foote pers. comm., Montgomerey pers. comm.). Here we report on a study of two Hawaiian Drosophila species, D. engyochracea, and D. hawaiiensis, to determine the impact that changes in population sizes over the past thirty years have had on the genetic diversity of these species. D. engyochracea is known from only two locations on the Island of Hawai'i (Kipuka Ki and Kipuka Pua'ulu), while D. hawaiiensis is currently more wide spread across Hawai'i Island. We collected 65 D. hawaiiensis and 66 D. engyochracea from two forest patches (kipuka) isolated by a 400 year old volcanic ash deposit. DNA sequence data for 515 bases of the mitochondrial gene COII was analyzed for both species to estimate relative total genetic diversity as well as inter-kipuka gene flow. The more wide spread species, D. hawaiiensis, has more genetic diversity (23 vs. 11 unique haplotypes) than the rarer species, D. engyochracea. The distribution of haplotypes in the kipuka is consistent with more gene flow in D. engyochracea than in D. hawaiiensis. Phylogenetic analysis indicates a small number of individuals morphologically identified as one species but have DNA sequence diagnostic for the other species. These results are consistent with these individuals being descendant from hybrids between species.     

Secretory avocado idioblast oil cells: evidence of their defensive role against a non-adapted insect herbivore

We tested the hypothesis that avocado idioblast oil cells play a defensive role against herbivorous insects. Toxicities of the intact avocado idioblast oil cells and the extracted idioblast oil were compared for three insect herbivores. Spodoptera exigua (Hübner) larvae are generalists that do not feed on avocados. By contrast, Sabulodes aegrotata (Guenée) and Pseudoplusia includens (Walker) larvae are generalist herbivores that readily feed on avocados. All bioassays were performed at a naturally occurring concentration of idioblast oil cells (2% w/w). Choice experiments showed that S. exigua larvae avoided diet treated with avocado idioblast oil cells and consume more control than treated diet. In contrast, idioblast oil cells had no significant antifeedant effects on the adapted S. aegrotata and P. includens larvae. Subsequent experiments designed to assess resistance mechanisms separated pre-ingestive (behavioral) and post-ingestive (physiological) effects of the avocado idioblast oil cells, and the extracted idioblast oil, on the two adapted herbivores. Post-ingestive adaptation was the mechanism that allows feeding. Because the impact of the avocado idioblast oil cells was greatest on the performance of non-adapted S. exigua, additional experiments determined that larvae fed diet containing the oil cells had higher mortality and reduced larval growth compared to controls. Developmental times were significantly prolonged for the survivors. Thus, increased mortality, reduced developmental rates, and antifeedant activity in the non-adapted insect indicate that defense against non-adapted herbivores may be an important function of idioblast cells in avocados.     

The influence of phenolic compounds on the suitability of three Salix species as hosts for the willow beetle Phratora vulgatissima

The phenolic glycosides salicin and salicortin were found to influence larval growth and development rates and adult feeding preference of Phratora vulgatissima in laboratory feeding studies. Salicortin was more toxic to larvae than salicin, and none of the third instar larvae fed on Salix viminalis leaves amended with 1.52% (fresh mass) salicortin pupated. Condensed tannins (proanthocyanadins) did not affect larval performance. It was concluded that Salix burjatica resistance to willow beetle is due to the high levels of salicortin which occur in leaves of this species.     

Upward cascading effects of nutrients: shifts in a benthic microalgal community and a negative herbivore response

We evaluated the effects of nutrient addition on interactions between the benthic microalgal community and a dominant herbivorous gastropod, Cerithidea californica (California horn snail), on tidal flats in Mugu Lagoon, southern California, USA. We crossed snail and nutrient (N and P) addition treatments in enclosures on two tidal flats varying from 71 to 92% sand content in a temporally replicated experiment (summer 2000, fall 2000, spring 2001). Diatom biomass increased slightly (~30%) in response to nutrient treatments but was not affected by snails. Blooms of cyanobacteria (up to 200%) and purple sulfur bacteria (up to 400%) occurred in response to nutrient enrichment, particularly in the sandier site, but only cyanobacterial biomass decreased in response to snail grazing. Snail mortality was 2–5 times higher in response to nutrient addition, especially in the sandier site, corresponding to a relative increase in cyanobacterial biomass. Nutrient-related snail mortality occurred only in the spring and summer, when the snails were most actively feeding on the microalgal community. Inactive snails in the fall showed no response to nutrient-induced cyanobacterial growths. This study demonstrated strongly negative upward cascading effects of nutrient enrichment through the food chain. The strength of this upward cascade was closely linked to sediment type and microalgal community composition.     

Parasitism rates and sex ratios of a parasitoid wasp: effects of herbivore and plant quality

Summary We studied interactions among collards, Brassica oleracea var. acephala, the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Yponomeutidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae) by manipulating plant nitrogen (N) concentrations in field and laboratory experiments. Parasitoid abundance strongly reflected DBM abundance and was related to total leaf N. Parasitism rates were high (70.7%) and density-independent. Wasp sex ratios varied markedly (3–93% female) in response to the herbivores, the plants, or both. Higher proportions of female wasps emerged from DBM larvae on plants with high leaf N than on unfertilized plants. More female wasps also emerged from larvae parasitized as larger instars. We suggest that wasps have the potential to control DBM populations through long-term numerical responses mediated by variable sex ratios.     

Impacts of savanna trees on forage quality for a large African herbivore

Recently, cover of large trees in African savannas has rapidly declined due to elephant pressure, frequent fires and charcoal production. The reduction in large trees could have consequences for large herbivores through a change in forage quality. In Tarangire National Park, in Northern Tanzania, we studied the impact of large savanna trees on forage quality for wildebeest by collecting samples of dominant grass species in open grassland and under and around large Acacia tortilis trees. Grasses growing under trees had a much higher forage quality than grasses from the open field indicated by a more favourable leaf/stem ratio and higher protein and lower fibre concentrations. Analysing the grass leaf data with a linear programming model indicated that large savanna trees could be essential for the survival of wildebeest, the dominant herbivore in Tarangire. Due to the high fibre content and low nutrient and protein concentrations of grasses from the open field, maximum fibre intake is reached before nutrient requirements are satisfied. All requirements can only be satisfied by combining forage from open grassland with either forage from under or around tree canopies. Forage quality was also higher around dead trees than in the open field. So forage quality does not reduce immediately after trees die which explains why negative effects of reduced tree numbers probably go initially unnoticed. In conclusion our results suggest that continued destruction of large trees could affect future numbers of large herbivores in African savannas and better protection of large trees is probably necessary to sustain high animal densities in these ecosystems.     

Plant regeneration from a cryopreserved embryogenic cell suspension of a commercial sugarcane hybrid (Saccharum sp.)

Efficient plant regeneration was obtained from a cryopreserved embryogenic cell suspension of sugarcane established from leaf derived callus. Pregrowing the cells for three days in MS basal medium supplemented with 0.33 M sorbitol was essential to the process. The cells were cooled at a rate of 0.5°C/min to –40°C and then stored in liquid nitrogen. Thawing was carried out rapidly in water at +40°C, and the cells were then plated without washing onto filter paper discs placed on a semi-solid regeneration medium (MS basal + 3% sucrose + 0.13 mg/1 2,4-D +0.25 mg/1 BAP + 0.25 mg/1 kinetin + 0.25 mg/1 zeatin). The filter paper discs, along with the cells, were transferred to the same, fresh medium after five hours. After 24 hours the cells were scraped off, placed on fresh semi-solid medium and incubated at 28°C in the dark for two weeks before transfer to light. A regeneration efficiency of 92% was obtained (regenerated plants, expressed as a percent of unfrozen control). Plants regenerated from cryopreserved cells, and grown to maturity in the greenhouse, were morphologically identical to regenerated control plants.Abbreviations DMSO dimethyl sulfoxide - PEG polyethylene glycol - 2,4-D 2,4-dichlorophenoxyacetic acid - BAP benzyl aminopurine - TTC 2,3,5-triphenyl tetrazolium chloride     

Molecular genetics of growth and development in Populus. III. A genetic linkage map of a hybrid poplar composed of RFLP,STS, and RAPD markers

We have evaluated three DNA-based marker types for linkage map construction in Populus: RFLPs detected by Southern blot hybridization, STSs detected by a combination of PCR and RFLP analysis, and RAPDs. The mapping pedigree consists of three generations, with the F₁ produced by interspecific hybridization between a P. trichocarpa female and a P. deltoides male. The F₂ generation was made by inbreeding to the maximum degree permitted by the dioecious mating system of Populus. The applicability of STSs and RAPDs outside the mapping pedigree has been investigated, showing that these PCR-based marker systems are well-suited to breeding designs involving interspecific hybridization. A Populus genome map (343 markers) has been constructed from a combination of all three types. The length of the Populus genome is estimated to be 2400–2800 cM.Abbreviations RFLP restriction fragment length polymorphism - STS sequence-tagged site - PCR polymerase chain reaction - RAPD random amplified polymorphic DNA     

Nested subset structure of larval mycophagous fly assemblages: nestedness in a non-island system

Nested subset structure has been studied in archipelagoes and fragmented habitats, and has been attributed to differential colonization and extinction rates among species and nested environmental tolerances. In this experiment, we tested for nestedness in assemblages of mycophagous fly larvae. Twenty mushrooms in each of three size classes (4.8–6.0 g, 10–15 g, 21–32 g) were placed on moist potting soil in experimental cups. The cups were placed in oak and pine forests in Greenville, S.C., USA for 5 days, where they were available to ovipositing flies. Upon collection, the mushrooms were incubated in the laboratory for 3 weeks and all emerging flies were sorted by species, counted, and weighed. A random placement analysis was conducted to determine whether the species richness pattern was a sampling artifact of the species abundance distributions. The actual species richness pattern did not conform to the random placement model; most mushrooms contained significantly fewer species than predicted by random sampling. The communities were strongly nested as measured by two different indices, and the nestedness pattern was related to mushroom size. Small mushrooms usually produced no flies or a single species, Dohrniphora sp. (Phoridae). Medium and large mushrooms typically produced more species-rich communities that usually contained the phorid and Drosophila putrida, D. tripunctata, and Leucophenga varia. This core guild was nested within a more diverse assemblage that included D. falleni, Mycodrosophila dimidiata, a muscid, and two Leptocera sp. (sphaeroceridae). These patterns are tentatively explained in the context of nested desiccation tolerances, mediated by differences in mushroom size.     

Resource heterogeneity and community structure: A case study inHeliconia imbricata Phytotelmata

Summary Complex or non-additive differences in the distribution and abundance of arthropod species inhabiting the water-filled bracts ofHeliconia imbricata can be created by simple manipulations of resource levels. The primary resources for these assemblages are the corollas of the flowers that accumulate in the bracts. Removing or adding corollas to individual bracts changes the pattern in the abundance of arthropod species within each bract such that bracts with different treatments ultimately differ in composition and numerical associations among species. These results suggest that direct and indirect resource-mediated factors can structure or significantly affect the distribution and abundance of species in these and perhaps other assemblages. Thus, in natural communities, if resources are heterogeneous among patches (such as among the bracts in this study) structure in a given patch may be a function of the resource level of that patch and can differ significantly from neighboring patches that provide different resource levels.     

Interactions of plant stress and herbivory: intraspecific variation in the susceptibility of a palatable versus an unpalatable seaweed to sea urchin grazing

Summary Interactions among environmental stresses, plant defensive characteristics, and plant nutrient status may significantly affect an alga's susceptibility to herbivores. Following desiccation, the palatable seaweed Gracilaria tikvahiae was less susceptible to grazing by the sea urchin Arbacia punctulata while the unpalatable alga Padina gymnospora became more susceptible. Increased grazing on desiccated Padina appeared to result from a loss of chemical defenses following desiccation. Palatable plants treated with organic extracts from desiccated Padina plants were consumed at more than twice the rate of plants treated with extracts from undesiccated plants. Increased susceptibility of Padina did not correlate with changes in protein content of the alga; reduced grazing on desiccated Gracilaria was associated with a decrease in protein content. When Padina was grazed by Arbacia or mechanically damaged to mimic urchin grazing, its susceptibility to Arbacia decreased within 1 to 5 days. These results demonstrate that history of physical or biological stress may affect a plant's susceptibility to herbivory. We hypothesize that urchins cue primarily on attractiveness features (e.g. nutrient content) of highpreference algae and deterrent features (e.g. chemical defenses) of low-preference algae. Stresses may therefore increase, decrease, or not affect a plant's susceptibility to herbivory depending upon the primary feeding cues used by the herbivore, the defensive mechanisms used by the plant, and the way these are altered by various environmental stresses.     

Seasonal shifts in habitat selection of a large herbivore and the influence of human activity

Failure to recognize factors contributing to variation in habitat models like resource selection functions (RSFs) can affect their application for projecting probabilities of occurrence, and thereby limit their relevance for conservation and management. We compared seasonal RSFs (2006–2008) for 16 adult female moose (Alces alces) with home ranges located in western Algonquin Provincial Park (APP), Ontario, Canada, to those of 14 adult females located in provincial Wildlife Management Unit (WMU) 49, 40 km west of the protected area. Wildlife and habitat management practices differed between regions: hunting was higher in WMU 49 compared to APP, and APP preserved large tracts of old growth forest rarely found in WMU 49. Seasonal RSFs projected expected similarities in moose resource use between regions (e.g., responses to wetlands and stands of eastern hemlock, Tsuga canadensis [in winter]); however, we also observed differences consistent with the hypothesis that animals, through effects of hunting, would shift habitat use seasonally and in response to roads. We further observed evidence of functional responses in habitat selection due to underlying differences in forestry practices (e.g., responses to stands of old-growth hemlock forest). Given the close proximity and shared biogeographic region between study areas, we believe that observed spatial dynamics in RSFs were ultimately reflective of divergent management strategies between areas and ensuing differences in predation and hunting mortality risk, and functional habitat.