Habitat structure mediates top–down effects of spiders and ants on herbivores

Differences in structural complexity of habitats have been suggested to modify the extent of top–down forces in terrestrial food webs. In order to test this hypothesis, we manipulated densities of generalist invertebrate predators and the complexity of habitat structure in a two-factorial design. We conducted two field experiments in order to study predation effects of ants and spiders and, in particular, of the wasp spider Argiope bruennichi on herbivorous arthropods such as grasshoppers, plant- and leafhoppers in a grassland. Predator densities were manipulated by removal in habitats of higher and lower structural diversity, and the effects on herbivore densities were assessed by suction sampling. Habitat structure was changed by cutting the vegetation to half its height and removing leaf litter.We found a significant negative effect of this assemblage of generalist predators on plant- and leafhoppers, which were 1.6 times more abundant in predator removal plots. This effect was stronger in low-structured (cut) than in uncut vegetation. Densities of the most abundant planthopper Ribautodelphax pungens (Delphacidae) were 2.2 times higher in predator removal plots. Furthermore, adult plant- and leafhoppers responded more strongly than juveniles and epigeic species more strongly than hypergeic species. The presence of predators had a positive effect on plant- and leafhopper species diversity. In a second field experiment, we tested the exclusive impact of Argiope bruennichi on its prey, and found that its effect was also significant, although weaker than the effect of the predator assemblage. This effect was stronger in grass-dominated vegetation compared to structurally more complex mixed vegetation of grasses and herbs. We conclude that habitat structure and in particular vegetation height and architectural complexity strongly modify the strength of top–down forces and indirectly affect the diversity of herbivorous arthropods.     

Species richness and composition patterns across trophic levels of true bugs (Heteroptera) in the agricultural landscape of the lower reach of the Tisza River Basin

River basins are among the most threatened ecosystems. The species diversity of several European river basins decreased seriously during the last decade due to loss of habitats and increasing land use pressure on the remaining habitats. We studied true bug assemblages in various land use types of grassland fragments and dikes as linear grassland habitats in the agricultural landscape of the lower reach of the Tisza River Basin. We tested the effects of the recorded variables of habitat quality, surrounding landscape and land use type on the abundance, species richness and composition of true bugs. Altogether, 5,389 adult Heteroptera individuals representing 149 species in 13 families were collected. The factors which influenced significantly the species richness of different trophic levels (i.e. herbivors, predators) and degrees of food specialization (i.e. generalist and specialist herbivors) were concordant. Contrary to this, the factors which influenced the abundance of the different feeding groups varied strongly. We emphasise the vegetation and land use types as primarily influential factors for insects. Excluding the grass-feeding species, the number of both generalist, specialist herbivorous and predaceous species were lower in agricultural swards, i.e. hay-meadows and pastures than in old field and dike habitats and their number increased with increasing vegetation diversity. Due to the high species richness and abundance observed in dike and old field habitats compared to agricultural swards, we emphasise their importance for conservation of insect diversity and we stress the negative effects of agricultural intensification on the remaining grasslands of the lower reach of the Tisza River Basin.     

Stream mosses as chemically-defended refugia for freshwater macroinvertebrates

Marine and terrestrial studies show that small, sedentary herbivores that utilize plants as both food and habitat can gain enemy-free space by living on hosts that are chemically defended from larger, generalist consumers. Although large herbivores are increasingly recognized as important consumers of macrophytes in freshwater communities, the potential indirect effects of herbivory on plant-associated macroinvertebrates have rarely been studied. Here, we show that the large, generalist consumers in a riverine system, Canada geese, Branta canadensis , and crayfish, Procambarus spiculifer , both selectively consumed riverweed, Podostemum ceratophyllum , over an aquatic moss, Fontinalis novae-angliae, even though moss comprised 89% of the total plant biomass on riverine rocky shoals. Moss supported twice as many plant-associated macroinvertebrates as riverweed, suggesting that it might provide a spatial refuge from consumption by these larger consumers. Bioassay-guided fractionation of moss extracts led to the isolation of a C₁₈ acetylenic acid, octadeca-9,12-dien-6-ynoic acid, that deterred crayfish feeding. In contrast to results with Canada geese and crayfish, both the amphipod Crangonyx gracilis and the isopod Asellus aquaticus consumed significant amounts of moss but rejected riverweed in laboratory feeding assays. Moreover, neither amphipod nor isopod feeding was deterred by the crude organic extract of Fontinalis , suggesting that these mesograzers tolerate or circumvent the chemical defenses that deterred larger consumers. Thus, herbivory by large, generalist herbivores may drive freshwater plant community structure towards chemically defended plants and favor the ecological specialization of smaller, less mobile herbivores on unpalatable hosts that represent enemy-free space.     

Effects of avian grazing on the algal community and small invertebrates in the rocky intertidal zone

In this study of a rocky intertidal habitat in northern Japan, feeding by avian consumers had significant effects on algal assemblages and small herbivorous invertebrates. The effects of the birds on algae were different from those of invertebrate grazers such as urchins and gastropods. The abundance of the dominant algal species decreased during the grazing period, increased again after the grazing period, and indirectly affected algal species richness and evenness. Avian grazing also decreased the density of tube-dwelling amphipods on the dominant alga, but did not change the density of mobile and free-living isopods. These results suggest that avian grazers may act as habitat modifiers rather than exploitative competitors for the small herbivorous crustaceans. Avian herbivores consumed only the upper parts of large algal fronds, apparently reducing the amount of suitable microhabitat for the small herbivorous crustaceans, which are subject to a variety of physical or biological stress. Thus, avian herbivores function as ecosystem engineers, regulating community structure in a manner different to invertebrate herbivores in rocky intertidal habitats.     

Behavioral adaptations increase the value of enemy-free space for Heliothis subflexa,a specialist herbivore

We investigated the importance of specialized behaviors in the use of enemy-free space by comparing the host-use behavior of two closely related moths, Heliothis subflexa Guenee and H. virescens Fabricius. Heliothis subflexa is a specialist on plants in the genus Physalis, whereas H. virescens is an extreme generalist, feeding on plants in at least 14 families. Heliothis subflexa uses the inflated calyx surrounding Physalis fruits as enemy-free space, and field rates of parasitism for H. subflexa on Physalis are much lower than for H. virescens on tobacco and cotton, common hosts found in the same habitat as Physalis. If Physalis, architecture were solely responsible for H. subflexa's low rates of parasitism on Physalis, we predicted that H. virescens larvae experimentally induced to feed on Physalis would experience parasitism rates similar to those of H. subflexa. We found, however, that specialized host-use and host-acceptance behaviors are integral to the use of enemy-free space on Physalis and strongly augment the effects of the structural refuge. In laboratory assays, we found considerable differences between the larval behavior of the specialist. H. subflexa, and the generalist, H. virescens, and these contributed to H. subflexa's superior use of enemy-free space on Physalis. We tested the importance of these behavioral differences in the field by comparing parasitism of H. virescens on Physalis, H. virescens on tobacco, and H. subflexa on Physalis by Cardiochiles nigriceps Vierick, a specialist braconid parasitoid. For H. virescens, a threefold decrease in parasitism occurred when feeding on Physalis (mean parasitism +/- SEM = 13 +/- 4%) rather than tobacco (43 +/- 4%), a difference we attribute to the structural refuge provided by Physalis. However, parasitism of H. virescens on Physalis was more than ten times as great as that of H. subflexa on Pliv.salis (1 +/- 4%), supporting the hypothesis that specialized behaviors have a substantial impact on use of Physalis as enemy-free space. Behavioral adaptations may be central to the use of enemy-free space by phytophagous insects and may act as an important selective force in the evolution of dietary specialization.     

Effects of linear furanocoumarins on an adapted specialist insect (Papilio polyxenes)

Abstract. 1. Examination of phytochemical literature reveals that a disproportionately large number of hostplants for species in the genus Papilio Section II, and in particular the machaon complex, contain linear furanocoumarins.
2. Although the linear furanocoumarin xanthotoxin is known to be toxic to generalist lepidopterous larvae, it failed to affect adversely the growth and survivorship of Papilio polyxenes , a member of the P.machaon complex that feeds primarily on Umbelliferae containing furanocoumarins, when incorporated into an artificial diet.
3. On the contrary, growth rate and weight gain were significantly improved in the presence of xanthotoxin.
4. It is proposed, based on this experimental evidence and on hostplant utilization patterns, that furanocoumarins play a significant role in the behavioural and biochemical adaptation of Papilio species to their umbelliferous hosts.     

Relationship of different feeding groups of true bugs (Hemiptera: Heteroptera) with habitat and landscape features in Pannonic salt grasslands

Eastern European grasslands are still inhabited by a rich arthropod fauna, but the drivers and mechanisms influencing their communities have to be understood to ensure their future survival. Heteroptera communities were studied in 20 plot-pairs in Pannonic salt steppe–salt marsh mosaics in Hungary. The effects of vegetation characteristics, landscape diversity and the proportion of surrounding grasslands on the composition, species richness and abundance of different feeding groups of true bugs (carnivores, specialist and generalist herbivores) were examined using ordinations and mixed-effect models. We found distinct herbivorous assemblages corresponding to microtopography-driven differences in water regime and vegetation between steppe and marsh plots, but this pattern was less pronounced in carnivorous assemblages. A higher species richness of true bugs was found in the more diverse steppe vegetation than in the salt marsh vegetation, while the abundance pattern of true bugs was opposite. Landscape diversity had a positive effect on the species richness and abundance of generalist herbivores and carnivores. Our results suggested that generalist herbivores and carnivores appear to drive diversity patterns in the local landscape due to their high dispersal abilities and the broader range of resources they can utilize. Specialist herbivores strongly influence the local insect biomass in relation to the distribution and density of their host plants. The present study highlights the importance of both habitat and landscape diversity for local insect diversity in Pannonic salt grasslands and suggests that the main threats for arthropod diversity are those processes and activities that homogenize these areas.     

Food webs in tropical Australian streams: shredders are not scarce

1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low‐order streams in the Queensland wet tropics.     

Effects of chlorogenic acid-and tomatine-fed caterpillars on the behavior of an insect predator

If generalist insect predators are a selective force contributing to patterns of feeding specialization by insect herbivores, then predators should be deterred from eating allelochemical-fed prey. The attack and feeding behaviors of naive predators (Podisus maculiventris stinkbugs) reared on control caterpillars (Manduca sexta) fed plain diet were compared to experienced predators reared on caterpillars fed tomato allelochemicals. Tomatine-fed prey were found more quickly by both naive and tomatine-experienced predators, and chlorogenic acid-experienced predators were more stimulated to begin searching for prey. However, experienced predators were less likely to attack both chlorogenic acidfed and tomatine-fed caterpillars than were naive predators. These results indicate that allelochemical-fed prey were easier for predators to locate, but allelochemical-containing prey often deterred predation by experienced predtors.     

Habitat or prey specialization in predatory ladybird beetles: a case study of two closely related species

This study examines the niche and diet breadth of two closely related sympatric aphidophagous ladybirds: Adalia decempunctata and A. bipunctata. The degree of habitat specialization of these species is investigated, and its effect on life history traits of females is explored. The importance of prey quality in determining the diet breadth is also examined. The niches occupied by these species in three countries, the UK, Belgium and southern France, are similar: A. decempunctata is an arboreal habitat specialist with a narrower set of prey than A. bipunctata, which is commonly found on several types of vegetation. The niches of the two species overlap on trees. Experiments indicate that habitat specialization has resulted in A. decempunctata investing more in each of its offspring than A. bipunctata. A. decempunctata females lay, relative to their body size, heavier eggs than those of the more generalist A. bipunctata, which results in A. decempunctata having bigger larvae. In addition, A. decempunctata larvae are better at surviving starvation than A. bipunctata larvae. In contrast to the expected pattern in food specialization, our study failed to demonstrate a better efficiency of the specialist when fed its usual prey and a detrimental effect when fed on prey that it is unlikely to encounter in the field. The reproductive performance of the specialist ladybird was better when fed an aphid that it was unlikely to regularly feed on in the field. Therefore, the narrow diet of the specialist ladybird is most likely a consequence of it occupying a narrow habitat rather than the quality of the prey. Although further studies on specialization in predatory insects are needed, the results indicate that unlike the role of plant quality in host specialization in herbivorous insects, prey quality has not been the main determinant of ecological specialization in these predatory insects.     

The importance of the ontogenetic niche in resource-associated divergence: evidence from a generalist grasshopper

Geographic variation in resource use can produce locally adapted populations that exhibit genetic and phenotypic divergence. In the bird-winged grasshopper (Schistocerca emarginata = [lineata]), we investigate whether genetic data exist in accordance with geographic variation in resource (host) use and coloration. In Texas, juvenile grasshoppers feed almost exclusively on one of two host plants, Rubus trivialis (Rosaceae) or Ptelea trifoliata (Rutaceae), whereas adults of both forms are dietary generalists and consume many plants from unrelated families. Along with differences in juvenile feeding, differences in a density-dependent color polyphenism are concordant with genetic (mitochondrial DNA) variation among eight populations of the bird-winged grasshopper. Forms feeding on R. trivialis and those feeding on P. trifoliata represent monophyletic lineages according to phylogenetic analysis and maximum-likelihood tests of two alternative phylogeographic hypotheses for geographic variation in host use. Character-state optimization of host-plant acceptability on a phylogeny containing S. emarginata and outgroup taxa indicates that populations consuming R. trivialis gave rise to populations consuming P. trifoliata. Juvenile grasshoppers that consume P. trifoliata acquire deterrence against predation, suggesting that enemy-free space facilitated this host shift. In extant populations, adaptations stemming from alternative resource use during ontogeny present possible barriers to gene exchange. This study represents the first demonstration of resource-associated divergence in an otherwise generalist insect that exhibits temporal variation in resource use, characterized as developmental changes in host specialization. Our findings suggest that exploitation of different resources may have unexplored significance for generalist species that compartmentalize specialization to particular life stages.     

Influence of surrounding vegetation on insect herbivory: A matter of spatial scale and herbivore specialisation

The diversity of surrounding vegetation is thought to modify the interactions between a focal plant and its herbivores, disrupting (associational resistance) or enhancing (associational susceptibility) host plant location and colonisation. We compared the effects of host plant concentration on herbivory by generalist and specialist insects feeding on oak seedlings by increasing local concentration of seedlings. We also assessed the effects of the composition and structure of surrounding vegetation, both at stand and local levels. The damage caused by generalist leaf-feeding insects depended on the structure of plant communities at stand level, and increased with tree cover. By contrast, infestation by specialist leaf miners was affected by local understorey vegetation surrounding oak seedlings, and decreased with increasing shrub cover and stratification diversity. Leaf mine abundance was higher at higher oak seedling density, giving support to the host concentration hypothesis. However, the abundance of these specialist herbivores was also negatively correlated with damage caused by the generalist external leaf-feeders, suggesting competitive interactions.     

Nest and foraging‐site selection in Yellowhammers Emberiza citrinella: implications for chick provisioning

Capsule Vegetation structure and invertebrate abundance interact to influence both foraging sites and nestling provisioning rate; when invertebrate availability is low, adults may take greater risks to provide food for their young.

Aims To investigate nesting and foraging ecology in a declining farmland bird whose fledging success is influenced by the availability of invertebrate prey suitable for feeding to offspring, and where perceived predation risk during foraging can be mediated by vegetation structure.

Methods Provisioning rates of adult Yellowhammers feeding nestlings were measured at nests on arable farmland. Foraging sites were compared with control sites of both the same and different microhabitats; provisioning rate was related to habitat features of foraging‐sites.

Results Foraging sites had low vegetation density, probably enhancing detection of predators, or high invertebrate abundance at high vegetation density. Parental provisioning rate decreased with increasing vegetation cover at foraging sites with high invertebrate abundance; conversely, where invertebrate abundance was low, provisioning rate increased with increasing vegetation cover.

Conclusions Vegetation structure at foraging sites suggests that a trade‐off between predator detection and prey availability influences foraging site selection in Yellowhammers. Associations between parental provisioning rate and vegetation variables suggest that where invertebrate abundance is high birds increase time spent scanning for predators at higher vegetation densities; however, when prey are scarce, adults may take more risks to provide food for their young.     

Phytophagous insect fauna tracks host plant responses to exotic grass invasion

The high dependence of herbivorous insects on their host plants implies that plant invaders can affect these insects directly, by not providing a suitable habitat, or indirectly, by altering host plant availability. In this study, we sampled Asteraceae flower heads in cerrado remnants with varying levels of exotic grass invasion to evaluate whether invasive grasses have a direct effect on herbivore richness independent of the current disturbance level and host plant richness. By classifying herbivores according to the degree of host plant specialization, we also investigated whether invasive grasses reduce the uniqueness of the herbivorous assemblages. Herbivorous insect richness showed a unimodal relationship with invasive grass cover that was significantly explained only by way of the variation in host plant richness. The same result was found for polyphagous and oligophagous insects, but monophages showed a significant negative response to the intensity of the grass invasion that was independent of host plant richness. Our findings lend support to the hypothesis that the aggregate effect of invasive plants on herbivores tends to mirror the effects of invasive plants on host plants. In addition, exotic plants affect specialist insects differently from generalist insects; thus exotic plants affect not only the size but also the structural profile of herbivorous insect assemblages.     

Sexual and local divergence in host exploitation in the marine herbivore Idotea baltica (Isopoda)

We studied sexual and habitat-specific variation in herbivores' host exploitation patterns and tested for a hypothesis on sex-specific adaptation in replicated landscapes. The hypothesis has not to our knowledge been previously tested either in a marine environment or for the host exploitation traits of an herbivore. The hypothesis states, first, that populations may show different adaptations due to spatially differing selective environments, and second, that males and females that differ ecologically (e.g. in reproductive behavior, in host use patterns) will respond to selective environments in distinct ways. We investigated possible differences in the host exploitation patterns of the marine generalist crustacean grazer Idotea baltica between the sexes or among populations originating from different habitats: plant assemblages dominated by either a brown alga or an angiosperm species. We determined the preferences for both the structure and nutritive quality of five common host species and compared these to the actual performance on the hosts. We found that performance on the hosts differed between the sexes and differently so in the two habitat types, supporting the hypothesis of sex-specific adaptation between distinct selective environments. Habitat-specific preference for the structural host further supported ecological divergence between habitat types. The different approaches for testing host exploitation resulted in a very different rank order of the hosts, indicating the complexity of the factors involved. The herbivore differentiates for the chemical quality of the hosts, but this alone does not explain host exploitation; rather it supports the theory of enemy-free space as a determinant of host choice. We also discuss the chemical characteristics of the host associated with the exploitation patterns found.     

Coexistence of permanently territorial cichlids of the genus Petrochromis through male-mating attack

Coexistence mechanisms of permanently territorial fishes proposed hitherto have been mainly based on interspecific competition. To test the hypothesis of the coexistence through male-mating aggression (Kohda, 1995a), spacing patterns of feeding territories of three coexisting herbivorous cichlids, Petrochromis polyodon, P. trewavasae and P. famula, were investigated in a homogeneous habitat at a shallow rocky shore in Lake Tanganyika. All three species maintained individual feeding territories that were defended against both conspecifics and congenerics and rarely overlapped either intra- or interspecifically. Territories of three species were scattered and covered almost all rocky bottoms. Territorial attacks usually occurred near the border of feeding territories. But male P. polyodon, the largest and socially most dominant fish, attacked conspecific males far beyond their feeding territories, which were widely separated. Removal of some territory owners suggested that such attacks result in the separate distribution of male P. polyodon territories. The interstitial space between P. polyodon feeding territories resulting from their mating aggression can be occupied by subordinate congeneric fishes. Male-mating attack of dominant males of Petrochromis will facilitate the coexistence of other congeners.     

Larval feeding induced defensive responses in tobacco: comparison of two sibling species of <Emphasis Type="Italic">Helicoverpa</Emphasis> with different diet breadths

Plants respond differently to damage by different herbivorous insects. We speculated that sibling herbivorous species with different host ranges might also influence plant responses differently. Such differences may be associated with the diet breadth (specialization) of herbivores within a feeding guild, and the specialist may cause less intensive plant responses than the generalist. The tobacco Nicotinana tabacum L. is the common host plant of a generalist Helicoverpa armigera (Hübner) and a specialist H. assulta Guenée (Lepidoptera, Noctuidae). The induced responses of tobacco to feeding of these two noctuid herbivores and mechanical wounding were compared. The results showed that the feeding of the specialist H. assulta and the generalist H. armigera resulted in the same inducible defensive system, but response intensity of plants was different to these two species. Inductions of jasmonic acid (JA), lipoxygenase (LOX), and proteinase inhibitors (PIs) were not significantly different concerning these two species, but H. assulta caused the less intensive foliar polyphenol oxidase (PPO) increase, more intensive nicotine and peroxidase (POD) increases in tobacco than H. armigera. The defensive response of plant to herbivores with different diet breadth seems to be more complicated than we expected, and the specialist does not necessarily cause less intensive plant responses than the generalist.     

Enemy release but no evolutionary loss of defence in a plant invasion: an inter-continental reciprocal transplant experiment

Plant chemical defenses and escape from natural enemies have been postulated to select for dietary specialization in herbivorous insects. In field and laboratory bioassays, we evaluated the effectiveness of intact and chemically modified larval shield defenses of the generalist Chelymorpha alternans and the specialists Acromis sparsa and Stolas plagiata (Chrysomelidae: Cassidinae) against three natural predators, using larvae reared on two morning glory (Convolvulaceae) species. We assessed whether: (1) specialists were better defended than generalists when both were fed and assayed on the same plant; (2) larval shield defenses were chemical, physical, or both; and (3) specialists exploit chemistry better than generalists. Live specialist larvae survived at higher rates than did generalists in predator bioassays with the bug Montina nigripes (Reduviidae), but there were no differences among groups against two species of Azteca ants (Hymenoptera: Dolichoderinae). Solvent leaching by H₂O or MeOH significantly reduced shield efficacy for all species compared to larvae with intact shields. In contrast, freshly killed specialist larvae exhibited significantly lower capture rates and frequencies than the generalists. Although solvent leaching significantly reduced overall shield efficacy for freshly killed larvae of all species, the pattern of leaching effects differed between specialists and generalists, with H₂O-leaching having a greater impact on the specialists. The overall vulnerability of the generalists appears due to lower chemical protection, which is ameliorated by increased escape behaviors, suggesting a selective trade-off between these defensive components. These experiments indicate that shield defenses are essential for larval survival and that specialists are superior at exploiting plant compounds residing in the aqueous fraction. Our results support the hypothesis that diet-specialized herbivorous insects have more effective defenses than generalists when both feed on the same plant due to the differential ability to exploit defensive precursors obtained from the host. The evolution of dietary specialization may therefore confer the advantage of enhanced enemy-free space.     

Host‐associated divergence in sympatric host races of the leaf beetle Lochmaea capreae: implications for local adaptation and reproductive isolation

Ecological specialization is widely recognized as a major determinant of the emergence and maintenance of biodiversity. We studied two critical facets of specialization – local adaptation and habitat choice – in the host races of the leaf beetle Lochmaea capreae on willow and birch. Our results revealed that there is asymmetric disruptive selection for host use traits, and host races achieved different adaptive sets of life history traits through association with their host plant. Beetles from each host race exhibited food and oviposition preference for their own host plant. Reciprocal transplant displayed significant variation in host acceptance and performance: all families from the willow race rejected the alternative host plant before initiation of feeding and all died on this host plant. By contrast, all families from the birch race accepted willow for feeding, but they consumed less and performed less well. Intriguingly, families that performed well on birch also performed well on willow, suggesting positive genetic correlation rather than genetic trade‐offs. Our results suggest that the major proximal determinant of host specialization in the willow race is the behavioural acceptance of a plant rather than the toxicity of the food resource. However, in the birch race a combination of behavioural host acceptance and performance may play a role in specialization. Our study sheds light on the mechanisms by which divergent host adaptation might influence the evolution of reproductive isolation between herbivorous populations.     

Differential patterns of hybridization and introgression between the swallowtails Papilio machaon and P. hospiton from Sardinia and Corsica islands (Lepidoptera,Papilionidae)

Proportions of hybridization and introgression between the swallowtails Papilio hospiton, endemic to Sardinia and Corsica, and the holarctic Papilio machaon, were characterized using nine fully diagnostic and two differentiated allozyme loci and a mitochondrial DNA marker. Very low frequencies of F1 hybrids were detected in both Sardinia (0-4%, average 1.4%) and Corsica (0-3%, average 0.5%), as well as of first generation backcrosses (B1). No F2 were observed, in agreement with the hybrid breakdown detected in laboratory crosses. In spite of this minimal current gene exchange, specimens carrying introgressed alleles were found in high proportions in P. machaon but in lower proportions in P. hospiton. Introgression apparently occurred through past hybridization and repeated backcrossing, as evidenced by hybrid index scores and Bayesian assignment tests. Levels of introgression were low (0-1%) at two sex-linked loci and mitochondrial DNA, limited (0.4-2%) at three autosomal loci coding for dimeric enzymes, and high (up to 43%) at four autosomal loci coding for monomeric enzymes. Accordingly, selective filters are acting against foreign alleles, with differential effectiveness depending on the loci involved. The low levels of introgression at sex-linked loci and mitochondrial DNA are in agreement with Haldane's rule and suggest that introgression in P. machaon proceeds mainly through males, owing to a lower fitness of hybrid females. Papilio machaon populations showed higher levels of introgression in Sardinia than in Corsica. The role of reinforcement in the present reproductive isolation between P. machaon and P. hospiton is examined, as well as the evolutionary effects of introgressive hybridization between the two species.