Host preference and larval performance suggest host race formation in Galerucella nymphaeae

In this study we investigated the possibilities for host race formation in Galerucella nymphaeae. This is a chrysomelid beetle feeding on four different hosts, belonging to two different plant families, the Nymphaeaceae and Polygonaceae. Previous results showed that beetles living on the two different host families differ in morphology, i.e., body length, mandibular width, and color of the elytra. In the current study, the preference of G. nymphaeae for four hosts was investigated, together with larval performance on these hosts. In a multichoice experiment, both parents and offspring showed a strong feeding preference for their natal host plant family: between 88-98% of the total consumption consisted of the natal host plant family. Females preferred to lay eggs on their natal host family: 81-100% of the egg clutches were laid on the natal host family. Host preference was accompanied by differences in offspring performance. Offspring survival was 1.2-25 times as high on the host family from which their parents originated than on the hosts of the other plant family. Furthermore, larval development tended to progress faster on the natal than on the other host family. Since the beetles use their host plant as a mating place, positive assortative mating is a likely consequence of the beetles' host preference. Together, these results suggest that there are two host races of G. nymphaeae: one living on Nymphaeaceae and the other on Polygonaceae.     

Divergence in selection of host species and plant parts among populations of a phytophagous insect

The diversification of phytophagous insects is often attributed to diverging processes of host plant specialization onto different, often closely related, host plants. Some insect clades have diversified by specializing not only on different plant species but also on different plant parts of the same hosts. This is the case in Greya moths (Prodoxidae) where both Greya obscura and G. politella are tightly linked to host plants of the genus Lithophragma (Saxifragaceae). We assess how these species differ in their choice of plants and use of plant parts. Previous work showed that strong local host specialization in G. politella is mediated by floral scent variation among Lithophragma species. Here, we identify geographic variation in host plant use in the close relative G. obscura, relate the emerging patterns to previous studies of geographic variation in host use in G. politella and evaluate potential processes underlying the variation among and within species. First, we show that G. obscura also uses floral chemistry to locate hosts but that additional plant cues must be involved in deciding whether to oviposit on a plant, because females did not discriminate against chemically different host species in no-choice trials. We also found that, although all known populations of G. politella oviposit only in flowers, all G. obscura populations examined here distributed their eggs among both floral and scape tissues both in the field and in laboratory experiments. The distribution of eggs among plant parts, however, varied among moth populations, and also depended on the Lithophragma species they attacked. Together, these results show the potential for phytophagous insect species and populations to diverge in use of plant parts as part of the process of speciation and adaptation. These two layers of specialization enhance the potential for subsequent diversification in phytophagous insect lineages.     

Variation in hostplant preference and performance by the milfoil weevil, Euhrychiopsis lecontei Dietz, exposed to native and exotic watermilfoils

Exotic plants often displace native plants and thus alter the availability of native hostplants for specialist herbivorous insects. The submersed aquatic weevil Euhrychiopsis lecontei Dietz is endemic to North America, but there are now source populations on the exotic Eurasian watermilfoil (Myriophyllum spicatum L.) as well as on the weevil's ancestral host, northern watermilfoil (Myriophyllum sibiricum Komarov). This provides an opportunity to examine a host range expansion in progress. To further define the host range of the weevil and to determine how population source and rearing plant influence host plant preference and performance, we conducted a series of preference and rearing experiments with weevils from two source populations reared on northern milfoil, on Eurasian milfoil, switched late in larval development from northern to Eurasian milfoil, and vice versa. We also included two rearing treatments with milfoils on which the weevil has not been documented: the native M. verticillatum L. and the exotic M. aquaticum Verd. Preference by weevils in the switched rearing treatments was similar to preferences exhibited by weevils reared solely on the second (later) milfoil species and an increase in preference for Eurasian milfoil was induced by adult exposure to Eurasian milfoil for 2 weeks. In contrast, sizes and development times of weevils in the switched rearing treatments were similar to sizes and development times exhibited by weevils reared solely on the first (early) milfoil species. These results indicate that preference by the milfoil weevil is determined late in larval development or later and Hopkins' host selection principle is not supported. However, size and development time were most affected by hostplant quality during larval development when larvae must acquire the resources needed for pupation. Oviposition preference in the milfoil weevil was a population attribute, not a fixed individual attribute and there was no significant variation in preference among individuals reared on northern milfoil, but significant variation in preference was detected among weevils reared on Eurasian milfoil. Weevils oviposited on all four milfoil species and completed development on three of them, but did not develop beyond the larval stage on M. aquaticum. Weevils reared on Eurasian milfoil developed faster and reached larger adult sizes than weevils in any other rearing treatment. The smallest sizes and longest development times were for weevils reared on the natives, northern milfoil and M. verticillatum. The milfoil weevil oviposits on an array of milfoil species and is unable to distinguish an unsuitable host (M. aquaticum) within this genus. The influence of rearing plant and adult exposure to Eurasian milfoil on hostplant preference suggests that host range expansion to novel congeners may occur more rapidly than predicted by models which assume that genetic variation is required. Significant variation among individuals in hostplant preference suggests the potential for a host shift to a plant for which E. lecontei appears pre-adapted.     

Parasitoids of grass-feeding chalcid wasps: a comparison of German and British communities

We compared the parasitoid communities associated with grass-feeding herbivores in Germany and Britain to examine geographical consistency in community composition and to test ecological characteristics of the plants and host insects that may explain variability in parasitoid community structure. The parasitoid communities of 16 chalcid wasps feeding on ten grass species were sampled between 1986 and 1989 at 4-11 sites per grass species in southwest Germany. The data were compared to published data from Great Britain, comprising 18 chalcid hosts on ten grass species sampled between 1980 and 1992 at 24 sites in Wales and England. Results showed that many conclusions drawn from patterns in Britain did not hold for Germany, emphasizing the need to repeat analyses in different geographical regions. The parasitoid communities of the Tetramesa hosts included on average 8.1 parasitoid species in Germany, while the British hosts supported only 4.1 parasitoids. The number of monophagous parasitoid species was similar in both areas (2.4 vs 3.2), but German host populations supported many more polyphagous species (5.1 vs 0.9). This difference reinforces the earlier conclusion that parasitoid communities in Britain are highly undersaturated. Increased numbers of parasitoid species in Germany did not result in increased parasitism rates, so the closer species packing was paralleled by reduced impact of each species. In Germany, percent parasitism (range: 5-74%) was closely correlated with log host density, explaining 90% of the variance, while in Great Britain, percent parasitism was less variable (range: 36-76%) and was not related to host density or other host or host plant characteristics. Gallers and non-gallers supported equal numbers of parasitoids in both Germany and Britain, offering support for neither the enemy hypothesis of the adaptive nature of plant galls nor for the finding that galls are often more susceptible to enemy attack than their non-galling relatives. Furthermore, gregarious Tetramesa hosts were not attacked by more parasitoid species than solitary hosts.     

Organic and conventional fertilizer effects on a tritrophic interaction: parasitism,performance and preference of Cotesia vestalis

Interest in sustainable farming methods that rely on alternatives to conventional synthetic fertilizers and pesticides is increasing. Sustainable farming methods often utilize natural populations of predatory and parasitic species to control populations of herbivores, which may be potential pest species. We investigated the effects of several types of fertilizer, including those typical of sustainable and conventional farming systems, on the interaction between a herbivore and parasitoid. The effects of fertilizer type on percentage parasitism, parasitoid performance, parasitoid attack behaviour and responses to plant volatiles were examined using a model Brassica system, consisting of Brassica oleracea var capitata, Plutella xylostella (Lepidoptera) larvae and Cotesia vestalis (parasitoid). Percentage parasitism was greatest for P. xylostella larvae feeding on plants that had received either a synthetic ammonium nitrate fertilizer or were unfertilized, in comparison to those receiving a composite fertilizer containing hoof and horn. Parasitism was intermediate on plants fertilized with an organically produced animal manure. Male parasitoid tibia length showed the same pattern as percentage parasitism, an indication that offspring performance was maximized on the treatments preferred by female parasitoids for oviposition. Percentage parasitism and parasitoid size were not correlated with foliar nitrogen concentration. The parasitoids did not discriminate between hosts feeding on plants in the four fertilizer treatments in parasitoid behaviour assays, but showed a preference for unfertilized plants in olfactometer experiments. The percentage parasitism and tibia length results provide support for the preference–performance hypothesis.     

CHLOROPLAST DNA VARIATION WITHIN AND AMONG GENERA OF THE HEUCHERA GROUP (SAXIFRAGACEAE): EVIDENCE FOR CHLOROPLAST TRANSFER AND PARAPHYLY

The Heuchera group (Saxifragaceae) comprises Bensoniella, Conimitella, Elmera, Heuchera, Lithophragma, Mitella, Tellima, Tiarella, and Totmiea. Earlier studies employing morphology, karyology, and flavonoid chemistry indicated that these genera form a natural group, but failed to resolve relationships among them. Restriction site analysis of chloroplast DNA (cpDNA) suggests that Bensoniella, Tolmiea, and Lithophragma are close allies and form the sister group of a large clade containing the remaining six genera. Mitella and Heuchera are both paraphyletic based on cpDNA data. cpDNA data, in conjunction with morphological and allozyme data, suggest at least four examples of intersectional hybridization and subsequent chloroplast capture in Heuchera. Several of these events may be explained via a stepping stone model in which the chloroplast genome of a species was captured by a second species, and then ultimately by a third taxon. Two well-differentiated groups of Tellima populations were detected: one group has a unique chloroplast genome characterized by nine autapomorphies, and the second group has a chloroplast genome identical to that found in M. trifida and M. diversifolia. cpDNA and allozyme data suggest that some Tellima populations probably obtained their chloroplast genome via intergeneric hybridization with M. trifida, M. diversifolia, or the ancestor of these taxa. The occurrence of intergeneric chloroplast transfer in some populations of Tellima, as well as extensive intersectional chloroplast capture in Heuchera, not only suggests caution in the use of cpDNA restriction site data in phylogenetic reconstruction, but also demonstrates again the importance of adequate sampling of conspecific populations. If the intergeneric relationships in the Heuchera group suggested by cpDNA analysis are accurate, fundamental questions arise regarding the validity of certain morphological traits as good taxonomic characters in Saxifragaceae. Furthermore, significant taxonomic changes at the generic level would be necessary.     

Host plant preference and performance of the vine weevil Otiorhynchus sulcatus

Abstract 1 The relationship between reproductive performance and preference for potential host plants of the vine weevil is investigated, as shown in tests on contact (or feeding) preference, presented herein, and tests on olfactory preference, published elsewhere. 2 Assessment of reproductive performance shows that the host‐plant range of the adult vine weevil Otiorhynchus sulcatus in Europe is limited to one gymnosperm genus (Taxus sp.) and a broad range of angiosperm plants in two subclasses of the Dicotyledonae, namely Dilleniidae and Rosidae. The successful reproduction on very distantly related plant taxa suggests that the original weevil‐ and plant‐habitat has mediated the current host‐plant range of the vine weevil. 3 Contact‐preference tests with equally suitable hosts, such as Aronia, Fragaria, Euonymus and Taxus, and one less suitable host, Humulus, indicate a mismatch between contact preference and performance and, as far as olfactory preferences are known, these match neither the contact preferences nor the performance. This mismatch may arise because (i) host plant species offered do not occur in weevil habitat in Europe (e.g. Aronia and the cultivated Fragaria come from North America) and (ii) predation (or disease) risks differ among host plants, thereby altering effective reproductive performance. 4 With respect to performance on novel hosts (Thuja, Prunus) and bad hosts (Rhododendron), some between‐individual variation is found within a single population, suggesting that local populations harbour (possibly genetic) variation for adaptation to new hosts. How this variation is maintained in the face of strong selection pressures on local populations of flightless and thelytokous weevils, is an important question for understanding the broad host plant range in the vine weevil.     

Geographic variation in feeding preference of a generalist herbivore: the importance of seaweed chemical defenses

The ecological impacts of generalist herbivores depend on feeding preferences, which can vary across and within herbivore species. Among mesoherbivores, geographic variation in host use can occur because host plants have a more restricted geographic distribution than does the herbivore, or there is local evolution in host preference, or both. We tested the role of local evolution using the marine amphipod Ampithoe longimana by rearing multiple amphipod populations from three regions (subtropical Florida, warm-temperate North Carolina and cold-temperate New England) and assaying their feeding preferences toward ten seaweeds that occur in some but not all regions. Six of the ten seaweeds produce anti-herbivore secondary metabolites, and we detected geographic variation in feeding preference toward five (Dictyota menstrualis, Dictyota ciliolata, Fucus distichus, Chondrus crispus and Padina gymnospora, but not Caulerpa sertularioides). Amphipod populations that co-occur with a chemically-rich seaweed tended to have stronger feeding preferences for that seaweed, relative to populations that do not co-occur with the seaweed. A direct test indicated that geographic variation in feeding preference toward one seaweed (D. ciliolata) is mediated by feeding tolerance for lipophilic secondary metabolites. Among the four seaweeds that produce no known secondary metabolites (Acanthophora, Ectocarpus, Gracilaria and Hincksia/Feldmannia spp.), we detected no geographic variation in feeding preference. Thus, populations are more likely to evolve greater feeding preferences for local hosts when those hosts produce secondary metabolites. Microevolution of feeding behaviors of generalist marine consumers likely depends on the availability and identity of local hosts and the strength of their chemical defenses.     

Associative learning of host presence in non‐host environments influences parasitoid foraging

1. Parasitoids are known to utilise learning of herbivore‐induced plant volatiles (HIPVs) when foraging for their herbivorous host. In natural situations these hosts share food plants with other, non‐suitable herbivores (non‐hosts). Simultaneous infestation of plants by hosts and non‐hosts has been found to result in induction of HIPVs that differ from host‐infested plants. Each non‐host herbivore may have different effects on HIPVs when sharing the food plant with hosts, and thus parasitoids may learn that plants with a specific non‐host herbivore also contain the host. 2. This study investigated the adaptive nature of learning by a foraging parasitoid that had acquired oviposition experience on a plant infested with both hosts and different non‐hosts in the laboratory and in semi‐field experiments. 3. In two‐choice preference tests, the parasitoid Cotesia glomerata shifted its preference towards HIPVs of a plant–host–non‐host complex previously associated with an oviposition experience. It could, indeed, learn that the presence of its host is associated with HIPVs induced by simultaneous feeding of its host Pieris brassicae and either the non‐host caterpillar Mamestra brassicae or the non‐host aphid Myzus persicae. However, the learned preference found in the laboratory did not translate into parasitisation preferences for hosts accompanying non‐host caterpillars or aphids in a semi‐field situation. 4. This paper discusses the importance of learning in parasitoid foraging, and debates why observed learned preferences for HIPVs in the laboratory may cancel out under some field experimental conditions.     

Variation in insect herbivory across a salt marsh tidal gradient influences plant survival and distribution

Herbivore damage and impact on plants often varies spatially across environmental gradients. Although such variation has been hypothesized to influence plant distribution, few quantitative evaluations exist. In this study I evaluated patterns of insect herbivory on an annual forb, Atriplex patula var. hastata, across a salt marsh tidal gradient, and performed experiments to examine potential causes and consequences of variation in herbivory. Damage to plants was generally twice as great at mid-tidal elevations, which are more frequently inundated, than at higher, less stressful, elevations at five of six surveyed sites. Field herbivore assays and herbivore preference experiments eliminated the hypothesis that plant damage was mediated by herbivore response to differences in host plants across the gradient. Alternately, greater herbivore densities in the mid-marsh, where densities of an alternate host plant (Salicornia europaea) were high, were associated with greater levels of herbivory on Atriplex, suggesting spillover effects. The effect of insect herbivores on host plant performance varied between the two sites studied more intensively. Where overall herbivore damage to plants was low, herbivory had no detectable effect on plant survival or seed production, and plant performance did not significantly differ between zones. However, where herbivore damage was high, herbivores dramatically reduced both plant survival (>50%) and fruit production (40-70%), and their effects were stronger in the harsher mid-marsh than the high marsh. Thus herbivores likely play a role in maintaining lower Atriplex densities in mid-marsh. Overall, these results suggest that variation in herbivore pressure can be an important determinant of patterns of plant abundance across environmental gradients.     

Local and regional dynamics of a specialist herbivore: overexploitation of a patchily distributed host plant

This study reports a rare example where a native herbivorous insect frequently overexploits local populations of its perennial host. Local dynamics of a flightless weevil (Hadramphus spinipennis, Curculionidae) and its host plant (Aciphylla dieffenbachii, Apiaceae) were assessed for one discrete patch. In this main study site local weevil population structure, dynamics and movement were investigated using a capture-recapture study. Local plant dynamics were studied by mapping plant location, size, sex and the phenological stage of each plant. Regional weevil and plant dynamics were studied for six plant patches using line-transect counts to estimate local weevil numbers and repeated counts of the number of flowering adult plants to assess plant numbers. Dispersal was assessed by regularly searching all plant patches for marked weevils that emigrated from the main study site. Prior to extinction, local weevil abundance, survival and recruitment rates increased continuously. At the same time the feeding damage on the plants increased and the area covered by A. dieffenbachii decreased until no plants were left. An increase in weevil abundance was clearly associated with the extinction of the local host plant population. Weevils stayed in their local host plant patch whilst food was available and dispersed only after local extinction of the plant. Over a 4-year period four local population extinctions were observed. This study was too short to allow explicit conclusions to be drawn about the ratio of extinction to colonisation rates for both the weevil and the host plant populations. However, persistence of this locally unstable system appears possible only in a fragmented habitat where asynchrony in local dynamics is maintained.     

Tempo and mode of recurrent polyploidization in the Carassius auratus species complex (Cypriniformes,Cyprinidae)

Polyploidization is an evolutionarily rare but important mechanism in both plants and animals because it increases genetic diversity. Goldfish of the Carassius auratus species complex can be tetraploids, hexaploids and octaploids. Polyploidization events have occurred repeatedly in goldfish, yet the extent of this phenomenon and its phyletic history are poorly understood. We explore the origin, tempo and frequency of polyploidization in Chinese and Japanese goldfish using both mitochondrial (mtDNA) and nuclear DNA sequences from up to 1202 individuals including the outgroup taxon, Cyprinus carpio. Analyses of de novo nuclear gene data resolve two clusters of alleles and the pattern supports the prior hypothesis of an ancient allotetraploidization for Carassius. Alleles shared by tetraploid and hexaploid individuals indicate recent autoploidizations within the C. auratus complex. Sympatric tetraploids and hexaploids share mtDNA haplotypes and these frequently occur independently within six well-supported lineages and sublineages on a small spatial scale. Gene flow estimates (F_st values) indicate that hexaploids differ only slightly from sympatric tetraploids, if at all. In contrast, allopatric populations of tetraploids and hexaploids differ from one another to a far greater extent. Gene flow between sampled localities appears to be limited. Coalescence-based time estimations for hexaploids reveal that the oldest lineage within any sampled locality is around one million years old, which is very young. Sympatric, recurrent autoploidization occurs in all sampled populations of the C. auratus complex. Goldfish experience polyploidization events more frequently than any other vertebrate.     

Plant ploidy level influences selection by butterfly seed predators

Polyploidization is a common route to plant diversification. Polyploids often differ from their progenitors in size, flower number, flower size and flowering phenology. Such differences may translate into differences in the intensity of interactions with animals. Here we investigated the impact of the ploidy-related differences in tetraploids and octoploids of the perennial herb Cardamine pratensis on pre-dispersal seed predation by the butterfly Anthocharis cardamines . The probability of escaping attack was lower for octoploids than for tetraploids, even after accounting for the fact that octoploids were larger and had fewer flowers than tetraploids. Flower shoot size was correlated with probability of attack in tetraploids but not in octoploids. Differences in plant traits associated with polyploidization can alter interactions with animals, and animal-mediated differences in trait selection between ploidy types can contribute to their further divergence.     

CHLOROPLAST-DNA AND ALLOZYMIC VARIATION IN DIPLOID AND AUTOTETRAPLOID HEUCHERA GROSSULARIIFOLIA (SAXIFRAGACEAE)

Diploid and autotetraploid populations of Heuchera grossulariifolia occur throughout mountainous regions of the Pacific Northwest. Controlled greenhouse crosses indicated that the two cytotypes are largely reproductively isolated. Fourteen diploid and 11 tetraploid populations were analyzed electrophoretically. Individual tetraploid plants expressed up to four alleles per isozyme locus, and tetraploid populations had significantly higher levels of heterozygosity than diploids. Mean observed heterozygosity was 0.159 for tetraploid populations and 0.058 for diploid populations. The patterns of allelic distribution between cytotypes suggested multiple origins of autotetraploids. This hypothesis was supported by restriction-site analysis of chloroplast-DNA (cpDNA) variation which indicated that there had been at least three independent origins of tetraploids. Electrophoretic data, in conjunction with a cpDNA-based phylogeny and geographic distribution of populations, suggest that autopolyploid populations evolved several times as migration of diploids occurred down river systems. This study further supports the contention that autopolyploidy can be a common and successful speciation process in some groups of plants.     

Choosy mothers pick challenging plants: maternal preference and larval performance of a specialist herbivore are not linked

1. Maternal preference is a dynamic process and interactions between preference and performance are fundamental for understanding evolutionary ecology and host association in insect–plant interactions. In the present study, the hypothesis of preference–performance was tested by offering solanaceous specialist Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) larvae and adult females four plant congeners that ranged in suitability. 2. Larval feeding, development, oviposition, plant glycoalkaloids, and headspace volatiles in the four plant species were analysed to examine the extent of variation, which might explain performance–preference differences. 3. It was found that larval performance was mismatched with adult oviposition preferences. Adults laid more eggs on Solanum immite Dunal plants, which were poor hosts for larval development, feeding, and survival, compared to the other three Solanum species. 4. Chemical plant defenses, in general, did not correlate with performance or preference, but some plant volatiles may have played a role in resolving female choice. Glycoalkaloids such as solanine and chaconine were detected in similar amounts in preferred and non‐preferred hosts, but there was significantly more limonene in the headspace of S. immite than in S. tuberosum L. 5. The present findings suggest that we must consider the risk‐spreading hypothesis in cases where preference and performance are not positively correlated, particularly in specialist herbivores that can feed on a diversity of congener plants and may attempt to expand their exploits to other solanaceae species.     

The influence of species identity and herbivore feeding mode on top-down and bottom-up effects in a salt marsh system

In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.     

PREFERENCE HIERARCHIES AND THE ORIGIN OF GEOGRAPHIC SPECIALIZATION IN HOST USE IN SWALLOWTAIL BUTTERFLIES

Four allopatric populations of the widely distributed western anise swallowtail butterfly, Papilio zelicaon, use different plant genera as hosts, but simultaneous choice experiments showed that these populations have diverged only slightly in oviposition preference. Of the four populations—two from southeastern Washington State, one from coastal southwestern Washington State, and one from central California—three use hosts that are not available to any of the others. Although variation for the degree of preference for particular plant species occurred within and among populations, all four populations ranked hosts in the same overall order. Monophagy on a local, low-ranking host outside the range of high-ranking hosts did not necessarily lead to the loss of preference for those high-ranking hosts, thereby indicating that the high-ranking hosts would still be accepted, and in some cases even preferred, if a population encountered them again. Hence, the overall preference hierarchy among P. zelicaon populations appears to be evolutionarily conservative. Analyses of differences among families within the California population indicated that increased preference for some hosts is inversely correlated, whereas preference for other hosts may be uncorrelated. Positive correlations may also occur but were not observed among the plant species tested. Overall, the results indicate local monophagy on different plant species in P. zelicaon has not involved major reorganizations in the preference hierarchy of ovipositing females, even in populations that may have fed on a low-ranking host for many generations. Instead, small increases in preference for local hosts have occurred within an evolutionarily conservative preference hierarchy.     

3种寄主上桃蚜的选择性及形态分化

桃蚜是一种重要的农业害虫,寄主广泛,种下分化复杂。以采自黄土高原旱作区桃树、烟草、甘蓝上的桃蚜为研究对象,通过叶片选择法、传统比较形态测定法研究了3种寄主上桃蚜的选择性及形态分化。结果表明:在3种寄主同时存在的情况下,烟草上的桃蚜嗜食烟草,表现为63.5%的桃蚜选择烟草叶,13.8%选择甘蓝叶,8.2%选择桃叶,而甘蓝和桃树上的桃蚜对原寄主没有表现出明显的嗜好性;从形态指标来看,3种寄主上的桃蚜在体长、触角末节长度、后足腿节长度、触角与体长的比例方面存在显著差异(P0.05),说明这几个特征可以作为区分这3种寄主上桃蚜的依据。综合分析可以初步认为黄土高原旱作区烟草上的桃蚜可能形成了寄主专化型-烟草型。     

In vitro induction and identification of tetraploid plants of <Emphasis Type="Italic">Paulownia tomentosa</Emphasis>

Polyploidization is a major trend in plant evolution that has many advantages over diploid. In particular, the enlargement and lower fertility of polyploids are very attractive traits in forest tree breeding programs. We report here a system for the in vitro induction and identification of tetraploid plants of Paulownia tomentosa induced by colchicine treatment. Embryonic calluses derived from placentas were transferred to liquid Murashige and Skoog (MS) medium containing different concentrations of colchicine (0.01, 0.05, or 0.1%) and incubated for 24, 48, or 72 h on an orbital shaker at 110 rpm. The best result in terms of the production of tetraploid plantlets was obtained in the 48 h + 0.05% colchicine treatment, with more than 100 tetraploid plantlets being produced. The ploidy level of plantlets was verified by chromosome counts, flow cytometry, and morphology. The chromosome number of tetraploids was 2n = 4x = 80 and that of diploid plantlets was 2n = 2x = 40. The relative fluorescence intensity of tetraploids was twofold higher than that of diploids. The tetraploid and diploid plantlets differed significantly in leaf shape, with those of the former being round and those of the latter pentagonal. The mean length of the stomata was longer in tetraploid plants than diploid plants, and stomatal frequency was reduced with the increased ploidy level. The tetraploids had large floral organs that were easily distinguishable from those of diploid plants.     

Vector within‐host feeding preference mediates transmission of a heterogeneously distributed pathogen

1. Ecological theory predicts that vector preference for certain host species or discrimination between infected versus uninfected hosts impacts disease incidence. However, little information exists on the extent to which vector within‐host feeding preference mediates transmission. This may be particularly important for plant pathogens, such as sharpshooter transmission of the bacterium Xylella fastidiosa, which are distributed irregularly throughout hosts. 2. We documented the within‐host distribution of two vector species that differ in transmission efficiency, the leafhoppers Draeculacephala minerva and Graphocephala atropunctata, and which are free to move throughout entirely caged alfalfa plants. The more efficient vector D. minerva fed preferentially at the base of the plant near the soil surface, whereas the less efficient G. atropunctata preferred overwhelming the top of the plant. 3. Next we documented X. fastidiosa heterogeneity in mechanically inoculated plants. Infection rates were up to 50% higher and mean bacterial population densities were 100‐fold higher near the plant base than at the top or in the taproot. 4. Finally, we estimated transmission efficiency of the two leafhoppers when they were confined at either the base or top of inoculated alfalfa plants. Both vectors were inefficient when confined at the top of infected plants and were 20–60% more efficient when confined at the plant base. 5. These results show that vector transmission efficiency is determined by the interaction between leafhopper within‐plant feeding behaviour and pathogen within‐plant distribution. Fine‐scale vector and pathogen overlap is likely to be a requirement generally for efficient transmission of vector‐borne pathogens.