Recognition of host-specific chemical stimulants in two sympatric host races of the pea aphid Acyrthosiphon pisum

Abstract.  1. In ecological speciation , adaptation to variation in the external environment provides the crucial push that starts the process of genetic divergence and eventually leads to speciation. This emphasis on the role of ecological specialisation in speciation events has brought with it a renewed interest in its proximate mechanisms in recently diverged groups such as host races. Here, the proximate mechanisms of feeding specialisation are investigated in two host races of the pea aphid Acyrthosiphon pisum .
2. Using alfalfa and clover extracts, enclosed in diet chambers or applied on whole plants, it is shown that feeding specialisation depends on recognition of stimulants specific to the host plant, not on deterrents or toxins specific to the non-host plants.
3. Because pea aphids mate on their host plant, feeding specialisation leads to de facto assortative mating. This study suggests that behavioural recognition of host-specific chemicals, rather than avoidance of deterrents or/and plant toxins, contributes to gene flow restriction between the alfalfa and clover host races.     

Genome scans reveal candidate regions involved in the adaptation to host plant in the pea aphid complex

A major goal in evolutionary biology is to uncover the genetic basis of adaptation. Divergent selection exerted on ecological traits may result in adaptive population differentiation and reproductive isolation and affect differentially the level of genetic divergence along the genome. Genome‐wide scan of large sets of individuals from multiple populations is a powerful approach to identify loci or genomic regions under ecologically divergent selection. Here, we focused on the pea aphid, a species complex of divergent host races, to explore the organization of the genomic divergence associated with host plant adaptation and ecological speciation. We analysed 390 microsatellite markers located at variable distances from predicted genes in replicate samples of sympatric populations of the pea aphid collected on alfalfa, red clover and pea, which correspond to three common host‐adapted races reported in this species complex. Using a method that accounts for the hierarchical structure of our data set, we found a set of 11 outlier loci that show higher genetic differentiation between host races than expected under the null hypothesis of neutral evolution. Two of the outliers are close to olfactory receptor genes and three other nearby genes encoding salivary proteins. The remaining outliers are located in regions with genes of unknown functions, or which functions are unlikely to be involved in interactions with the host plant. This study reveals genetic signatures of divergent selection across the genome and provides an inventory of candidate genes responsible for plant specialization in the pea aphid, thereby setting the stage for future functional studies.     

Parasites may contribute to 'magic trait' evolution in the adaptive radiation of three-spined sticklebacks, Gasterosteus aculeatus (Gasterosteiformes: Gasterosteidae)

In the past decade, there has been a new effort to understand the ecology that drives population divergence and speciation. It is well established in theory that speciation is most likely to occur when a trait that is under divergent natural selection in different populations is also used in mate choice. Such traits have been dubbed 'magic traits' ( Gavrilets, 2004 ) and, although there appears to be good evidence that they exist, the ecological mechanisms that underlie their divergence are not well understood. Size at maturity in three-spined sticklebacks is an archetypal example of a magic trait. The present study documents for the first time that differences in body size at maturity in sympatric species pairs of lacustrine three-spined sticklebacks in British Columbia, Canada, are caused by differences in age at maturity. It is also shown that there are differences between the sympatric species in the patterns of infection with a virulent cestode, Schistocephalus solidus . Although the evidence is circumstantial, these differences in infection are consistent with the hypothesis that they have contributed to the observed divergence in age and size at maturity in these populations.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 425–433.     

Speciation on the Azores islands: congruent patterns in shell morphology, genital anatomy, and molecular markers in endemic land snails (Gastropoda, Leptaxinae)

Morphological data, in combination with molecular data, may provide invaluable insights into speciation processes on archipelagos. Land snails offer ample opportunities to evalutate adaptive and non-adaptive speciation scenarios. However, studies investigating processes of differentiation and speciation on the Azores are scarce. The present study comprises a morphometrical analysis of shell and genital characters in a group of Azorean land snails (Pulmonata, Leptaxinae). Geographical isolation appears to be an important mechanism underlying morphological and molecular differentiation in the Azorean Leptaxini, instead of adaptive radiation through ecological differentiation. Nevertheless, we could not exclude the occurrence of ecological speciation on the oldest island (Santa Maria) where two species that markedly differ in shell-shape co-occur.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 166–176.     

Local adaptation in a plant herbivore interaction depends on the spatial scale

Local adaptation has central importance in the understanding of co-evolution, maintenance of sexual reproduction, and speciation. We investigated local adaptation in the alkaloid-bearing legume Crotalaria pallida and its seed predator, the arctiid moth Utetheisa ornatrix , at different spatial scales. When we studied three populations from south-east Brazil (150 km apart), we did not find evidence of local adaptation, although we did find interpopulational differences in herbivore performance, and a significant interaction between herbivore sex and plant population. These results indicate that both moth and plant populations are differentiated at the regional scale. In a comparison of populations from Brazil and Florida, the herbivore showed local adaptation to its host plant; for both moth populations, the pupae were heavier when the larvae ate the sympatric than the allopatric host population. We discuss the scale dependence of our results and the possible causes for the lack of local adaptation at the regional scale, even in the presence of plant and moth differentiation. The results obtained demonstrate the importance of studying co-evolution and local adaptation at different geographical scales.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 494–502.     

WIDESPREAD HOST‐DEPENDENT HYBRID UNFITNESS IN THE PEA APHID SPECIES COMPLEX

Linking adaptive divergence to hybrid unfitness is necessary to understand the ecological factors contributing to reproductive isolation and speciation. To date, this link has been demonstrated in few model systems, most of which encompass ecotypes that occupy relatively early stages in the speciation process. Here we extend these studies by assessing how host‐plant adaptation conditions hybrid fitness in the pea aphid, Acyrthosiphon pisum. We made crosses between and within five pea aphid biotypes adapted to different host plants and representing various stages of divergence within the complex. Performance of F1 hybrids and nonhybrids was assessed on a “universal” host that is favorable to all pea aphid biotypes in laboratory conditions. Although hybrids performed equally well as nonhybrids on the universal host, their performance was much lower than nonhybrids on the natural hosts of their parental populations. Hence, hybrids, rather than being intrinsically deficient, are maladapted to their parents’ hosts. Interestingly, the impact of this maladaptation was stronger in certain hybrids from crosses involving the most divergent biotype, suggesting that host‐dependent postzygotic isolation has continued to evolve late in divergence. Even though host‐independent deficiencies are not excluded, hybrid maladaptation to parental hosts supports the hypothesis of ecological speciation in this complex.     

Host-based divergence in populations of the pea aphid: insights from nuclear markers and the prevalence of facultative symbionts

In North America, the pea aphid Acyrthosiphon pisum encompasses ecologically and genetically distinct host races that offer an ideal biological system for studies on sympatric speciation. In addition to its obligate symbiont Buchnera, pea aphids harbour several facultative and phylogenetically distant symbionts. We explored the relationships between host races of A. pisum and their symbiotic microbiota to gain insights into the historical process of ecological specialization and symbiotic acquisition in this aphid. We used allozyme and microsatellite markers to analyse the extent of genetic differentiation between populations of A. pisum on pea, alfalfa and clover in France. In parallel, we examined: (i) the distribution of four facultative symbionts; and (ii) the genetic variation in the Buchnera genome across host-associated populations of A. pisum. Our study clearly demonstrates that populations of A. pisum on pea, clover and alfalfa in France are genetically divergent, which indicates that they constitute distinct host races. We also found a very strong association between host races of A. pisum and their symbiotic microbiota. We stress the need for phylogeographic studies to shed light on the process of host-race formation and acquisition of facultative symbionts in A. pisum. We also question the effects of these symbionts on aphid host fitness, including their role in adaptation to a host plant.     

Labile ecotypes accompany rapid cladogenesis in an adaptive radiation of Mandarina (Bradybaenidae) land snails

The endemic land snail genus Mandarina of the oceanic Bonin Islands shows exceptionally diverse morphological and ecological traits. Previous studies have already provided evidence that speciation on different islands of the three main archipelagos was such that similar ecotypes evolved independently in different lineages and islands. Here we present data to show that the same species can have different ecotypes. As most of the characters involved are inherited, then variation between ecotypes must represent genetic differences between populations. We then show that the radiation on the Bonin Islands is derived from a single colonization event, and use a mitochondrial phylogeny to provide evidence for a burst of cladogenesis soon after colonization. As divergent selection has previously been implicated in causing differences between Mandarina species, and theory predicts that most of the speciation should have taken place early in their history, then the study adds to the evidence for an adaptive radiation by ecological speciation in Mandarina . However, while the diversity of ecotypes present at each site is dependent on the regime of natural selection and competition, geography still must have an important role.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 269–282.     

Effects of the maternal and pre‐adult host plant on adult performance and preference in the pea aphid,Acyrthosiphon pisum

Abstract 1. The taxon known as the pea aphid, Acyrthosiphon pisum, is composed of a series of host plant associated populations and is widely used as a model system to explore ecological speciation and the evolution of specialisation. It is thus important to know how maternal and pre‐adult experience influences host plant utilisation in this species. 2. The relative importance of the maternal and pre‐adult host plant for adult fecundity and host preference was investigated using three aphid clones collected from Lathyrus pratensis and maintained on Lathyrus or Vicia faba. 3. No significant effects of the maternal host plant on offspring fecundity were detected. 4. The host plant on which the aphid grew up influenced adult fecundity, although in a complex way that depended on both the adult host plant species and when after transfer to the test plant fecundity was assessed. 5. All three clones preferred to colonise Lathyrus over Vicia, and this preference was stronger for aphids raised on Lathyrus. 6. The significance of the results for studies of the evolution of specialisation and speciation that employ A. pisum is discussed.     

Population differentiation and genetic variation in host choice among pea aphids from eight host plant genera

Habitat choice plays a critical role in the processes of host range evolution, specialization, and ecological speciation. Pea aphid, Acyrthosiphon pisum, populations from alfalfa and red clover in eastern North America are known to be genetically differentiated and show genetic preferences for the appropriate host plant. This species feeds on many more hosts, and here we report a study of the genetic variation in host plant preference within and between pea aphid populations collected from eight genera of host plants in southeastern England. Most host-associated populations show a strong, genetically based preference for the host plant from which they were collected. Only in one case (populations from Vicia and Trifolium) was there little difference in the plant preference spectrum between populations. All populations showed a significant secondary preference for the plant on which all the aphid lines were reared: broad bean, Vicia faba, previously suggested to be a "universal host" for pea aphids. Of the total genetic variance in host preference within our sample, 61% could be attributed to preference for the collection host plant and a further 9% to systematic differences in secondary preferences with the residual representing within-population genetic variation between clones. We discuss how a combination of host plant preference and mating on the host plant may promote local adaptation and possibly ecological speciation, and whether a widely accepted host could oppose speciation by mediating gene flow between different populations.     

REPRODUCTIVE ISOLATION BETWEEN SYMPATRIC RACES OF PEA APHIDS. I. GENE FLOW RESTRICTION AND HABITAT CHOICE

Determining the extent and causes of barriers to gene flow between genetically divergent populations or races of single species is an important complement to post facto analyses of the causes of reproductive isolation between recognized species. Sympatric populations of pea aphids (Acyrthosiphon pisum Harris, Homoptera: Aphididae) on alfalfa and red clover are highly genetically divergent and locally adapted. Here, hierarchical estimates of population structure based on F_st suggest that gene exchange between closely adjacent aphid populations on the two hosts is highly restricted relative to that among fields of the same host plant. Although these host-associated races are presently considered to be the same subspecies, they appear to be significantly reproductively isolated, suggesting incipient speciation. Habitat (host) choice was investigated as the first in a temporal series of factors that could reduce gene exchange between these sympatric populations. Field studies of winged colonists to newly planted fields of each host suggest pronounced habitat fidelity. This result was verified using replicated observations of the host choice behavior of different aphid genotypes for which the relative demographic performance on each host was known. These laboratory observations of behavior revealed a strong genetic correlation between habitat choice (or acceptance) and the relative performance in each habitat. Because mating occurs on the host plant, habitat choice in this system leads to assortative mating and is therefore a major cause of reproductive isolation between the sympatric pea aphid populations on alfalfa and clover. However, the extent of dispersal between hosts estimated from the field study of winged colonists (9–11%) is too great to be consistent with the genetic divergence estimated between the races. This suggests that barriers to gene flow other than host choice also exist, such as selection against migrants or hybrids in the parental environments, hybrid sterility, or hybrid breakdown.     

Meadow pipit (Anthus pratensis) egg appearance in cuckoo (Cuculus canorus) sympatric and allopatric populations

Host populations tend to show less ability to discriminate against parasites when living in their absence. However, comparison of rejection rates among sympatric and allopatric host populations does not allow determination of whether the greater tolerance in allopatric populations reflects a genetic change or phenotypic plasticity. Here we test the existence of changes in a host's adaptation to brood parasitism in the absence of parasitism by studying intraclutch variation in egg appearance, which is a genetically determined component of host defence favouring discrimination of parasitic eggs. We investigated egg phenotypes of a common host of the European cuckoo, Cuculus canorus , in the presence and in the absence of cuckoos. By using objective spectroradiometry techniques of colour assessment we compared intraclutch variation between populations of meadow pipit, Anthus pratensis , sympatric (England) and allopatric (Iceland and Faeroe Islands) with C. canorus . Allopatric populations of A. pratensis showed greater intraclutch variation in egg appearance in the ultraviolet part of the spectrum than did a population sympatric with C. canorus . Two possible alternative mechanisms explaining these findings are discussed.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79 , 543–549.     

Wing shape variation in the medium ground finch (Geospiza fortis): an ecomorphological approach

Wing design in birds is subject to a suite of interacting selective pressures. As different performance traits are favoured in different ecological settings, a tight link is generally expected between variation in wing morphology and variation in ecological parameters. In the present study, we document aspects of variation in wing morphology in the medium ground finch ( Geospiza fortis ) on Isla Santa Cruz in the Galápagos. We compare variation in body size, simple morphometric traits (body mass, last primary length, wing length, wing chord, and wing area) and functional traits (wing loading, aspect ratio and wing pointedness) across years, among populations, and between sexes. Functional traits are found to covary across years with differences in climatic conditions, and to covary among populations with differences in habitat structure. In dry years and arid locations, wing aspect ratios are highest and wings are more pointed, consistent with a need for a low cost of transport. In wet years and cluttered habitats, wing loading is lowest and wings are more rounded, suggesting enhanced capabilities for manoeuvrability. Sexes differ in wing loading, with males having lower wing loadings than females. Superior manoeverability might be favoured in males for efficient territory maintenance. Lastly, in contrast to functional traits, we found little consistent inter-annual or inter-site variation in simple morphometric traits.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 129–138.     

Adult life spans of butterflies (Lepidoptera: Papilionoidea + Hesperioidea): broadscale contingencies with adult and larval traits in multi-species comparisons

Adult life spans of insects are part of supposedly adaptive life-history syndromes, which involve a variety of reproductive strategies with regard to mating, egg maturation and egg laying. Contingencies of life span with many other morphological, behavioural, and ecological traits are to be expected. We used a data-mining approach, incorporating controls for phylogeny, to uncover broad patterns of trait contingencies in a literature-derived data set of adult life spans for 350 butterfly species. We found that adult feeding habit has strong links with life spans, with pollen-feeders living longer than fruit feeders, which in turn live longer than nectar feeders. Furthermore, traits reducing susceptibility to predators (i.e. aposematism, eye-spots) are associated with longer life. Mud-puddling behaviour as a means of gaining access to minerals, and possibly nitrogen, was generally associated with a short life. We also found strong impacts of the region of occurrence of species (i.e. whether they were tropical or temperate taxa), but received conflicting results depending on whether we analysed field data (longer life in the tropics) or data from caged individuals (shorter life in the tropics). Adult longevity was associated with the growth form of larval host plants (higher on lianas as opposed to herbs, shrubs, or trees), whereas other larval host plant traits did not have strong links with species' life spans. We discuss the hypothetical evolutionary mechanisms explaining those patterns and suggest predictions that could be tested in further research.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 166–184.     

Do canal‐cutting behaviours facilitate host‐range expansion by insect herbivores?

According to the escalation–radiation model of co-evolution, insect herbivores that acquire the ability to circumvent a plant defence enter a new adaptive zone and increase in species. How herbivore counter-adaptations to plant defences might lead to speciation is poorly understood. Studies of nymphalid butterflies suggest that the evolution of a broadened host range may be a critical step. This paper examines if leaf-feeding insects capable of deactivating defensive plant canals with canal cutting often have broad host ranges. A total of 94 species of canal-cutting insects were identified from the literature, including eight new canal cutters described in this paper. Only 27% of canal cutters with known host ranges are generalists that feed on plants in multiple families. The proportion of generalist canal cutters is similar or lower than estimates of generalists among phytophagous insects overall. Only five species, at most, of the canal-cutting generalists feed exclusively on plants with secretory canals. The paucity of generalists can be attributed in part to the considerable taxonomic distance separating canal-bearing plant families and to their corresponding chemical distinctiveness. The dependence of many canal-cutting species on host chemicals for defence would also favour specialization.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 715–731.     

Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids

Ecological specialisation on different host plants occurs frequently among phytophagous insects and is normally assumed to have a genetic basis. However, insects often carry microbial symbionts, which may play a role in the evolution of specialisation. The bacterium Regiella insecticola is a facultative symbiont of pea aphids (Acyrthosiphon pisum) where it is found most frequently in aphid clones feeding on Trifolium giving rise to the hypothesis that it may improve aphid performance on this plant. A study in which R. insecticola was eliminated from a single naturally infected aphid clone supported the hypothesis, but a second involving two aphid clones did not find the same effect. We created a series of new pea aphid–R. insecticola associations by injecting different strains of bacteria into five aphid clones uninfected by symbionts. For all aphid clones, the bacteria decreased the rate at which aphids accepted Vicia faba as a food plant and reduced performance on this plant. Their effect on aphids given Trifolium pratense was more complex: R. insecticola negatively affected acceptance by all aphid clones, had no effect on the performance of four aphid clones, but increased performance of a fifth, thus demonstrating genetic variation in the effect of R. insecticola on pea aphid host use. We discuss how these results may explain the distribution and frequency of this symbiont across different aphid populations. Julia Ferrari and Claire L. Scarborough contributed equally to the work.     

No host‐associated differentiation in the gall wasp Diplolepis rosae (Hymenoptera: Cynipidae) on three dog rose species

Differences in quality and quantity of secondary compounds, as well as in leaf traits of host plants, may influence the host choice of herbivores. Different host preferences could lead to host‐associated differentiation, the first step of sympatric speciation. In the present study, we investigated whether the rose gall wasp Diplolepis rosae L. (Hymenoptera: Cynipidae) shows genetic differentiation related to its host plants (Rosa canina L., Rosa corymbifera Borkh., and Rosa rubiginosa L.). These three host species radiated recently and subsequently expanded their range. Therefore, we expected a diversification within the closely‐associated phytophagous insects. The process of genetic differentiation should be intensified in D. rosae by its close relationship to the host plant, as well as by its parthenogenetic reproduction (infection rate by Wolbachia sp. of almost 100%). However, using 106 polymorphic amplified fragment length polymorphism markers, we found no genetic differentiation among the wasps from different host plants. The population structuring between geographical localities was also low, suggesting considerable gene flow between sites. In part, the low genetic differentiation between sites is explained by the wide distribution of host species and hybrids between host plants. Hybrids with intermediate traits may facilitate the gene flow between wasp populations exploiting different host species. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 369–377.     

Microgeographical diversification of threespine stickleback: body shape–habitat correlations in a small, ecologically diverse Alaskan drainage

Adaptive radiations are a major source of evolutionary diversity in nature, and understanding how they originate and how organisms diversify during the early stages of adaptive radiation is a major problem in evolutionary biology. The relationship between habitat type and body shape variation was investigated in a postglacial radiation of threespine stickleback in the upper Fish Creek drainage of Cook Inlet, Alaska. Although small, the upper Fish Creek drainage includes ecologically diverse lakes and streams in close proximity to one another that harbour abundant stickleback. Specimens from ancestral anadromous and derived resident freshwater populations differed substantially and could be distinguished by body shape alone, suggesting that the initial stages of adaptation contribute disproportionately to evolutionary divergence. Body shape divergence among resident freshwater populations was also considerable, and phenotypic distances among samples from freshwater populations were associated with habitat type but not geographical distance. As expected, stream stickleback from slow-moving, structurally complex environments tended to have the deepest bodies, stickleback from lakes with a mostly benthic habitat were similar but less extreme, and stickleback from lakes with a mostly limnetic habitat were the most shallow-bodied, elongate fish. Beyond adapting rapidly to conditions in freshwater environments, stickleback can diversify rapidly over small geographical scales in freshwater systems despite opportunities for gene flow. This study highlights the importance of ecological heterogeneity over small geographical scales for evolutionary diversification during the early stages of adaptive radiation, and lays the foundation for future research on this ecologically diverse, postglacial system.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 139–151.     

Evolutionary history of aphid-plant associations and their role in aphid diversification

Aphids are intimately linked with their host plants that constitute their only food resource and habitat, and thus impose considerable selective pressure on their evolution. It is therefore commonly assumed that host plants have greatly influenced the diversification of aphids. Here, we review what is known about the role of host plant association on aphid speciation by examining both macroevolutionary and population-level studies. Phylogenetic studies conducted at different taxonomic levels show that, as in many phytophagous insect groups, the radiation of angiosperms has probably favoured the major Tertiary diversification of aphids. These studies also highlight many aphid lineages constrained to sets of related host plants, suggesting strong evolutionary commitment in aphids’ host plant choice, but they fail to document cospeciation events between aphid and host lineages. Instead, phylogenies of several aphid genera reveal that divergence events are often accompanied by host shifts, and suggest, without constituting a formal demonstration, that aphid speciation could be a consequence of adaptation to new hosts. Experimental and field studies below the species level support reproductive isolation between host races as partly due to divergent selection by their host plants. Selected traits are mainly feeding performances and life cycle adaptations to plant phenology. Combined with behavioural preference for favourable host species, these divergent adaptations can induce pre- and post-zygotic barriers between host-specialized aphid populations. However, the hypothesis of host-driven speciation is seldom tested formally and must be weighed against overlooked explanations involving geographic isolation and non-ecological reproductive barriers in the process of speciation.