Evolutionary radiation of Asteromyia carbonifera (Diptera: Cecidomyiidae) gall morphotypes on the goldenrod Solidago altissima (Asteraceae)

Population divergence of phytophagous insects is often coupled to host‐plant shifts and is frequently attributed to the divergent selective environments associated with alternative host‐plants. In some cases, however, divergence is associated with the use of alternative host‐plant organs of a single host species. The basis of within‐host radiations such as these remains poorly understood. In the present stusy, we analysed the radiation of Asteromyia gall midges occurring both within one host plant species and within a single organ on that host. In this system, four morphologically distinct Asteromyia gall forms (morphs) coexist on the leaves of goldenrod Solidago altissima. Our analyses of amplified fragment length polymorphism and DNA sequence data confirm the genetic differentiation among midges from three gall morphs and reveal evidence of a genetically distinct fourth gall morph. The absence of clear gall morph related clades in the mitochondrial DNA derived phylogenies is indicative of incomplete lineage sorting or recent gene flow, suggesting that population divergence among gall forms is recent. We assess the likely history of this radiation and use the results of phylogenetic analyses along with ecological data on phenology and parasitism rates to evaluate potential hypotheses for the mode of differentiation. These preliminary analyses suggest that diversification of the Asteromyia gall morphs is likely shaped by interactions between the midge, a symbiotic fungus, and parasitoid enemies. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 840–858.     

Symbiont‐mediated phenotypic variation without co‐evolution in an insect–fungus association

Recent studies have shown that symbionts can be a source of adaptive phenotypic variation for their hosts. It is assumed that co‐evolution between hosts and symbionts underlies these ecologically significant phenotypic traits. We tested this assumption in the ectosymbiotic fungal associate of the gall midge Asteromyia carbonifera. Phylogenetic analysis placed the fungal symbiont within a monophyletic clade formed by Botryosphaeria dothidea, a typically free‐living (i.e. not associated with an insect host) plant pathogen. Symbiont isolates from four divergent midge lineages demonstrated none of the patterns common to heritable microbial symbioses, including parallel diversification with their hosts, substitution rate acceleration, or A+T nucleotide bias. Amplified fragment length polymorphism genotyping of the symbiont revealed that within‐lineage genetic diversity was not clustered along host population lines. Culture‐based experiments demonstrated that the symbiont‐mediated variation in gall phenotype is not borne out in the absence of the midge. This study shows that symbionts can be important players in phenotypic variation for their hosts, even in the absence of a co‐evolutionary association.     

Dissecting the association between a gall midge,Asteromyia carbonifera,and its symbiotic fungus,Botryosphaeria dothidea

The Ambrosia gall midge [Asteromyia carbonifera (Osten Sacken) (Diptera: Cecidomyiidae: Alycaulini)] consists, in part, of a complex of genetically differentiated populations that have diverged in gall morphology on the host plant Solidago altissima L. (Asteraceae). This divergence appears to be an incipient adaptive radiation that may be driven by parasitoid pressure. Understanding the mechanisms driving this genetic and phenotypic diversification requires a close examination of the relationship between the midge and its fungal associate Botryosphaeria dothidea (Moug.) Ces. & De Not. (Ascomycota: Dothideomycetes), whose mycelia actually form the protective gall structure. We used manipulative experiments to test the degree of interdependency of the fungus and the midge, and we employed field and laboratory studies to gain insight into the source of fungal conidia, which our data and observations indicate are collected by females and stored in specialized pockets (mycangia) on the ovipositor. Manipulative experiments demonstrate that fungal proliferation on the host plant is dependent on the midge larvae and larvae exhibit significant growth on the fungus alone. Field observations and experiments were unable to identify the source of mycangial conidia; however, analyses of conidia shape suggest a biotrophic source. We conclude that this association is an obligatory mutualism with respect to successful gall formation. These findings corroborate recent findings that the primary food source of the midge is the gall fungus.     

Lack of evidence for reproductive isolation among ecologically specialised lycaenid butterflies

Abstract 1. The evolution of reproductive isolation between recently diverged or incipient species is a critical component of speciation and a major focus of speciation models. In phytophagous insects, host plant fidelity (the habit of mating and ovipositing on a single host species) can contribute to assortative mating and reproductive isolation between populations adapting to alternative hosts. The potential role of host plant fidelity in the evolution of reproductive isolation was examined in a pair of North American blue butterfly species, Lycaeides idas and L. melissa .
2. These species are morphologically distinct and populations of each species utilise different host plants; however they share 410 bp haplotypes of the mitochondrial cytochrome oxidase subunit I (COI) gene, indicating recent divergence.
3. Some populations using native hosts exhibited strong fidelity for their natal host plant over the hosts used by nearby populations. Because these butterflies mate on or near the host plant, the development of strong host fidelity may create reproductive isolation among populations on different hosts and restrict gene flow.
4. Tests of population differentiation using allozyme allele frequency data did not provide convincing evidence of restricted gene flow among populations. Based on morphological differences, observed ecological specialisation, and the sharing of genetic markers, these butterflies appear to be undergoing adaptive radiation driven at least partially by host shifts. Neutral genetic markers may fail to detect the effects of very recent host shifts in these populations due to gene flow and/or the recency of divergence and shared ancestral polymorphism.     

Genetic divergence among sympatric colour morphs of the Dalmatian wall lizard (<Emphasis Type="Italic">Podarcis melisellensis</Emphasis>)

If alternative phenotypes in polymorphic populations do not mate randomly, they can be used as model systems to study adaptive diversification and possibly the early stages of sympatric speciation. In this case, non random mating is expected to support genetic divergence among the different phenotypes. In the present study, we use population genetic analyses to test putatively neutral genetic divergence (of microsatellite loci) among three colour morphs of the lizard Podarcis melisellensis, which is associated with differences in male morphology, performance and behaviour. We found weak evidence of genetic divergence, indicating that gene flow is somewhat restricted among morphs and suggesting possible adaptive diversification.     

Ecological niche dimensionality and the evolutionary diversification of stick insects

The degree of phenotypic divergence and reproductive isolation between taxon pairs can vary quantitatively, and often increases as evolutionary divergence proceeds through various stages, from polymorphism to population differentiation, ecotype and race formation, speciation, and post-speciational divergence. Although divergent natural selection promotes divergence, it does not always result in strong differentiation. For example, divergent selection can fail to complete speciation, and distinct species pairs sometimes collapse ('speciation in reverse'). Widely-discussed explanations for this variability concern genetic architecture, and the geographic arrangement of populations. A less-explored possibility is that the degree of phenotypic and reproductive divergence between taxon pairs is positively related to the number of ecological niche dimensions (i.e., traits) subject to divergent selection. Some data supporting this idea stem from laboratory experimental evolution studies using Drosophila, but tests from nature are lacking. Here we report results from manipulative field experiments in natural populations of herbivorous Timema stick insects that are consistent with this 'niche dimensionality' hypothesis. In such insects, divergent selection between host plants might occur for cryptic colouration (camouflage to evade visual predation), physiology (to detoxify plant chemicals), or both of these niche dimensions. We show that divergent selection on the single niche dimension of cryptic colouration can result in ecotype formation and intermediate levels of phenotypic and reproductive divergence between populations feeding on different hosts. However, greater divergence between a species pair involved divergent selection on both niche dimensions. Although further replication of the trends reported here is required, the results suggest that dimensionality of selection may complement genetic and geographic explanations for the degree of diversification in nature.     

How common is ecological speciation in plant-feeding insects? A 'Higher' Nematinae perspective

Background  

Ecological speciation is a process in which a transiently resource-polymorphic species divides into two specialized sister lineages as a result of divergent selection pressures caused by the use of multiple niches or environments. Ecology-based speciation has been studied intensively in plant-feeding insects, in which both sympatric and allopatric shifts onto novel host plants could speed up diversification. However, while numerous examples of species pairs likely to have originated by resource shifts have been found, the overall importance of ecological speciation in relation to other, non-ecological speciation modes remains unknown. Here, we apply phylogenetic information on sawflies belonging to the 'Higher' Nematinae (Hymenoptera: Tenthredinidae) to infer the frequency of niche shifts in relation to speciation events.     

A Comparative Analysis of Genetic Differentiation across Six Shared Willow Host Species in Leaf- and Bud-Galling Sawflies

Genetic divergence and speciation in plant-feeding insects could be driven by contrasting selection pressures imposed by different plant species and taxa. While numerous examples of host-associated differentiation (HAD) have been found, the overall importance of HAD in insect diversification remains unclear, as few studies have investigated its frequency in relation to all speciation events. One promising way to infer the prevalence and repeatability of HAD is to estimate genetic differentiation in multiple insect taxa that use the same set of hosts. To this end, we measured and compared variation in mitochondrial COI and nuclear ITS2 sequences in population samples of leaf-galling Pontania and bud-galling Euura sawflies (Hymenoptera: Tenthredinidae) collected from six Salix species in two replicate locations in northern Fennoscandia. We found evidence of frequent HAD in both species complexes, as individuals from the same willow species tended to cluster together on both mitochondrial and nuclear phylogenetic trees. Although few fixed differences among the putative species were found, hierarchical AMOVAs showed that most of the genetic variation in the samples was explained by host species rather than by sampling location. Nevertheless, the levels of HAD measured across specific pairs of host species were not correlated in the two focal galler groups. Hence, our results support the hypothesis of HAD as a central force in herbivore speciation, but also indicate that evolutionary trajectories are only weakly repeatable even in temporally overlapping radiations of related insect taxa.     

Host-driven diversification of gall-inducing Acacia thrips and the aridification of Australia

Background  

Insects that feed on plants contribute greatly to the generation of biodiversity. Hypotheses explaining rate increases in phytophagous insect diversification and mechanisms driving speciation in such specialists remain vexing despite considerable attention. The proliferation of plant-feeding insects and their hosts are expected to broadly parallel one another where climate change over geological timescales imposes consequences for the diversification of flora and fauna via habitat modification. This work uses a phylogenetic approach to investigate the premise that the aridification of Australia, and subsequent expansion and modification of arid-adapted host flora, has implications for the diversification of insects that specialise on them.     

Butterfly host plant range: an example of plasticity as a promoter of speciation?

Mary Jane West-Eberhard has suggested that plasticity may be of primary importance in promoting evolutionary innovation and diversification. Here, we explore the possibility that the diversification of phytophagous insects may have occurred through such a process, using examples from nymphalid butterflies. We discuss the ways in which host plant range is connected to plasticity and present our interpretation of how West-Eberhard’s scenario may result in speciation driven by plasticity in host utilization. We then review some of the evidence that diversity of plant utilization has driven the diversification of phytophagous insects and finally discuss whether this suggests a role for plasticity-driven speciation. We find a close conceptual connection between our theory that the diversification of phytophagous insects has been driven by oscillations in host range, and our personal interpretation of the most efficient way in which West-Eberhard’s theory could account for plasticity-driven speciation. A major unresolved issue is the extent to which a wide host plant range is due to adaptive plasticity with dedicated modules of genetic machinery for utilizing different plants.     

REVIEW Evolution of phytophagy in trombidiform mites

This paper reviews the evolutionary aspects of obligate phytophagy (excluding mycophagy and phycophagy) in the mite suborder Trombidiformes. Phytophagy in the other acariform suborder, Sarcoptiformes, is limited to just a few species, amidst otherwise saprophagous or fungivorous taxa, that attack the living tissues of higher plants. The phylogenetic relationships of lineages that contain taxa of plant-feeding mites are reviewed briefly, to facilitate hypotheses about the number of times that phytophagy has arisen within the Trombidiformes. The relationship between the two most important plant-feeding taxa, the Tetranychoidea and Eriophyoidea, is so distant that their obligate phytophagy represents independent events. Outgroup comparisons allow an estimate of the relative ages when phytophagy arose. This background facilitates analyses of the evolutionary patterns of attributes relevant to phytophagy as a way of life. Styliform modifications of chelate chelicerae for predation or fungivory were fundamental pre-adaptations for effective phytophagy. Dispersal among the major lineages of phytophagous mites seems generally passive, with little evidence of phoretic behaviour. Continued individual mobility seems to be needed during ontogeny and adulthood, such that no scale-like or sac-like instars have arisen. Trends towards physogastric reproduction and ovoviviparity are not evident. Arrhenotokous sex determination predominates among lineages of phytophagous mites. The primary sex ratios are not usually highly female biased. Direct sperm transfer does not seem to have been advantageous or disadvantageous to adaptive radiations of plant-feeding lineages. Adaptive trends towards thelytoky are scattered and do not seem to have played major roles in speciation, diversification or trends towards increasing host specificity in lineages. Alternate asexual and sexual generations and life cycles on different species of hosts, as occur among families of aphid and scale insects, are not known. Among unrelated lineages of trombidiform mites, there appears to have been convergent evolution of attributes, such as those noted above, in response to similar selective pressures for a phytophagous way of life. The patterns of attributes discussed need experimental analysis and detailed documentation to test their accuracy and generality and to understand the selective pressures that have formed them.     

Evidence for sympatric speciation by host shift in the sea

The genetic divergence and evolution of new species within the geographic range of a single population (sympatric speciation) contrasts with the well-established doctrine that speciation occurs when populations become geographically isolated (allopatric speciation). Although there is considerable theoretical support for sympatric speciation, this mode of diversification remains controversial, at least in part because there are few well-supported examples. We use a combination of molecular, ecological, and biogeographical data to build a case for sympatric speciation by host shift in a new species of coral-dwelling fish (genus Gobiodon). We propose that competition for preferred coral habitats drives host shifts in Gobiodon and that the high diversity of corals provides the source of novel, unoccupied habitats. Disruptive selection in conjunction with strong host fidelity could promote rapid reproductive isolation and ultimately lead to species divergence. Our hypothesis is analogous to sympatric speciation by host shift in phytophagous insects except that we propose a primary role for intraspecific competition in the process of speciation. The fundamental similarity between these fishes and insects is a specialized and intimate relationship with their hosts that makes them ideal candidates for speciation by host shift.     

The origin and genetic differentiation of the socially parasitic aphid Tamalia inquilinus

Social and brood parasitisms are nonconsumptive forms of parasitism involving the exploitation of the colonies or nests of a host. Such parasites are often related to their hosts and may evolve in various ecological contexts, causing evolutionary constraints and opportunities for both parasites and their hosts. In extreme cases, patterns of diversification between social parasites and their hosts can be coupled, such that diversity of one is correlated with or even shapes the diversity of the other. Aphids in the genus Tamalia induce galls on North American manzanita (Arctostaphylos) and related shrubs (Arbutoideae) and are parasitized by nongalling social parasites or inquilines in the same genus. We used RNA sequencing to identify and generate new gene sequences for Tamalia and performed maximum‐likelihood, Bayesian and phylogeographic analyses to reconstruct the origins and patterns of diversity and host‐associated differentiation in the genus. Our results indicate that the Tamalia inquilines are monophyletic and closely related to their gall‐forming hosts on Arctostaphylos, supporting a previously proposed scenario for origins of these parasitic aphids. Unexpectedly, population structure and host‐plant‐associated differentiation were greater in the non‐gall‐inducing parasites than in their gall‐inducing hosts. RNA‐seq indicated contrasting patterns of gene expression between host aphids and parasites, and perhaps functional differences in host‐plant relationships. Our results suggest a mode of speciation in which host plants drive within‐guild diversification in insect hosts and their parasites. Shared host plants may be sufficient to promote the ecological diversification of a network of phytophagous insects and their parasites, as exemplified by Tamalia aphids.     

The evolution of host plant manipulation by insects: molecular and ecological evidence from gall-forming aphids on Pistacia

One of the most striking characteristics of gall-forming insects is the variability in gall position, morphology, and complexity. Our knowledge of the driving forces behind the evolutionary divergence of gall types is limited. Natural enemies, competition, and behavioral constraints might be involved. We present a cladogram, based on sequences of COI and COII (1952bp), of mitochondrial DNA for the evolution of 14 species of gall-forming aphids (Fordinae). These insects induce five gall types with remarkable morphological variation on Pistacia spp. hosts. The parsimony cladogram divides the Fordinae into three lineages, Fordini and Baizongiini, and a third (new) sister group including the previously Fordini member, Smynthurodes betae (West). We then use ecological data to trace and explain the evolution of gall morphology. The aphids seem to have evolved gradually towards better ability to manipulate their host plant, induce stronger sinks, and gain higher reproductive success. We suggest that the ancestral gall type was a simple, open, "pea"-sized gall located on the leaflet midvein. Some Fordini and S. betae evolved a two-gall life cycle, inducing a new gall type on the leaflet margin. The Baizongiini improved the manipulation of their host by inducing larger galls near the midvein, with stronger sinks supporting thousands of aphids. Similar gall types are induced at similar sites on different Pistacia hosts suggesting control of the aphids on gall morphology and frequent host shifts. Thus, even extreme specialization (specific gall and host) is flexible.     

Partitioning of herbivore hosts across time and food plants promotes diversification in the Megastigmus dorsalis oak gall parasitoid complex

Communities of insect herbivores and their natural enemies are rich and ecologically crucial components of terrestrial biodiversity. Understanding the processes that promote their origin and maintenance is thus of considerable interest. One major proposed mechanism is ecological speciation through host‐associated differentiation (HAD), the divergence of a polyphagous species first into ecological host races and eventually into more specialized daughter species. The rich chalcid parasitoid communities attacking cynipid oak gall wasp hosts are structured by multiple host traits, including food plant taxon, host gall phenology, and gall structure. Here, we ask whether the same traits structure genetic diversity within supposedly generalist parasitoid morphospecies. We use mitochondrial DNA sequences and microsatellite genotypes to quantify HAD for Megastigmus (Bootanomyia) dorsalis, a complex of two apparently generalist cryptic parasitoid species attacking oak galls. Ancient Balkan refugial populations showed phenological separation between the cryptic species, one primarily attacking spring galls, and the other mainly attacking autumn galls. The spring species also contained host races specializing on galls developing on different host‐plant lineages (sections Cerris vs. Quercus) within the oak genus Quercus. These results indicate more significant host‐associated structuring within oak gall parasitoid communities than previously thought and support ecological theory predicting the evolution of specialist lineages within generalist parasitoids. In contrast, UK populations of the autumn cryptic species associated with both native and recently invading oak gall wasps showed no evidence of population differentiation, implying rapid recruitment of native parasitoid populations onto invading hosts, and hence potential for natural biological control. This is of significance given recent rapid range expansion of the economically damaging chestnut gall wasp, Dryocosmus kuriphilus, in Europe.     

Carnivores and carotenoids are associated with adaptive behavioural divergence in a radiation of gall midges

1. Adaptive divergence in sympatry is supposed to be inhibited by the homogenizing role of gene flow. However, studies continue to uncover examples of sympatric divergence. In this study, two divergent phenotypes in a complex of four syntopic gall midge morphotypes [nominally Asteromyia carbonifera Osten Saken, Diptera: Cecidomyiidae: Alycaulini] are characterised. The first is a behavioural phenotype governing within‐host tissue preference and the second is a trait governing accessory‐gland carotenoid quality and quantity. 2. One gall morphotype (crescents) lay most of their eggs on mature tissue whereas the other three gall morphotypes oviposit only on young emerging leaves. Ecological maintenance of this divergent trait appears to be driven by enemy‐reduced space. That is, nearly 40% of the crescent morphotype galls that develop high on the plant are attacked by the egg parasitoid Platygaster solidaginis Ashmed, whereas those low on the plant are relatively protected. 3. All morphotypes contain carotenoids in their accessory glands, but the quality and quantity of these pigments differs significantly between the morphotypes and is positively associated with the probability of parasitism by P. solidaginis. 4. Larval salivary glands also contain carotenoids and the plant hormone abscisic acid, which in plants is synthesized from carotenoid precursors and is involved in regulating plant defences. This hormone may facilitate gall development and influence gall morphology. 5. Ecological fitness trade‐offs between carotenoids, parasitoid attack, and plant resistance may be fostering adaptive divergence in ovipositional phenotypes and sympatric speciation in this complex of gall midge morphotypes.     

Host races in plant-feeding insects and their importance in sympatric speciation

The existence of a continuous array of sympatric biotypes - from polymorphisms, through ecological or host races with increasing reproductive isolation, to good species - can provide strong evidence for a continuous route to sympatric speciation via natural selection. Host races in plant-feeding insects, in particular, have often been used as evidence for the probability of sympatric speciation. Here, we provide verifiable criteria to distinguish host races from other biotypes: in brief, host races are genetically differentiated, sympatric populations of parasites that use different hosts and between which there is appreciable gene flow. We recognize host races as kinds of species that regularly exchange genes with other species at a rate of more than ca. 1% per generation, rather than as fundamentally distinct taxa. Host races provide a convenient, although admittedly somewhat arbitrary intermediate stage along the speciation continuum. They are a heuristic device to aid in evaluating the probability of speciation by natural selection, particularly in sympatry. Speciation is thereby envisaged as having two phases: (i) the evolution of host races from within polymorphic, panmictic populations; and (ii) further reduction of gene flow between host races until the diverging populations can become generally accepted as species. We apply this criterion to 21 putative host race systems. Of these, only three are unambiguously classified as host races, but a further eight are strong candidates that merely lack accurate information on rates of hybridization or gene flow. Thus, over one-half of the cases that we review are probably or certainly host races, under our definition. Our review of the data favours the idea of sympatric speciation via host shift for three major reasons: (i) the evolution of assortative mating as a pleiotropic by-product of adaptation to a new host seems likely, even in cases where mating occurs away from the host; (ii) stable genetic differences in half of the cases attest to the power of natural selection to maintain multilocus polymorphisms with substantial linkage disequilibrium, in spite of probable gene flow; and (iii) this linkage disequilibrium should permit additional host adaptation, leading to further reproductive isolation via pleiotropy, and also provides conditions suitable for adaptive evolution of mate choice (reinforcement) to cause still further reductions in gene flow. Current data are too sparse to rule out a cryptic discontinuity in the apparently stable sympatric route from host-associated polymorphism to host-associated species, but such a hiatus seems unlikely on present evidence. Finally, we discuss applications of an understanding of host races in conservation and in managing adaptation by pests to control strategies, including those involving biological control or transgenic parasite-resistant plants.     

Adaptive radiation of gall-inducing insects within a single host-plant species

Speciation of plant-feeding insects is typically associated with host-plant shifts, with subsequent divergent selection and adaptation to the ecological conditions associated with the new plant. However, a few insect groups have apparently undergone speciation while remaining on the same host-plant species, and such radiations may provide novel insights into the causes of adaptive radiation. We used mitochondrial and nuclear DNA to infer a phylogeny for 14 species of gall-inducing Asphondylia flies (Diptera: Cecidomyiidae) found on Larrea tridentata (creosote bush), which have been considered to be monophyletic based on morphological evidence. Our phylogenetic analyses provide strong support for extensive within-host plant speciation in this group, and it demonstrates that diversification has involved numerous shifts between different plant organs (leaves, buds, flowers, and stems) of the same host-plant species. Within-plant speciation of Asphondylia is thus apparently facilitated by the opportunity to partition the plant ecologically. One clade exhibits temporal isolation among species, which may have facilitated divergence via allochronic shifts. Using a novel method based on Bayesian reconstruction, we show that the rate of change in an ecomorphological trait, ovipositor length, was significantly higher along branches with inferred shifts between host-plant organs than along branches without such shifts. This finding suggests that Larrea gall midges exhibit close morphological adaptation to specific host-plant parts, which may mediate ecological transitions via disruptive selection.     

Cascading host-associated genetic differentiation in parasitoids of phytophagous insects

The extraordinary diversity of phytophagous insects may be attributable to their narrow specialization as parasites of plants, with selective tradeoffs associated with alternate host plants driving genetic divergence of host-associated forms via ecological speciation. Most phytophagous insects in turn are attacked by parasitoid insects, which are similarly specialized and may also undergo host-associated differentiation (HAD). A particularly interesting possibility is that HAD by phytophagous insects might lead to HAD in parasitoids, as parasitoids evolve divergent lineages on the new host plant-specific lineages of their phytophagous hosts. We call this process 'cascading host-associated differentiation' (cascading HAD). We tested for cascading HAD in parasitoids of two phytophagous insects, each of which consists of genetically distinct host-associated lineages on the same pair of goldenrods (Solidago). Each parasitoid exhibited significant host-associated genetic divergence, and the distribution and patterns of divergence are consistent with divergence in sympatry. Although evidence for cascading HAD is currently limited, our results suggest that it could play an important role in the diversification of parasitoids attacking phytophagous insects. The existence of cryptic host-associated lineages also suggests that the diversity of parasitoids may be vastly underestimated.