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.     

Evidence for sexual isolation as a prezygotic barrier to gene flow between morphologically divergent species of Rhagoletis fruit flies

1. Certain groups of fruit flies in the genus Rhagoletis (Diptera: Tephritidae) are exemplars for sympatric speciation via host plant shifting. Flies in these species groups are morphologically similar and overlap in their geographic ranges, yet attack different, non‐overlapping sets of host plants. Ecological adaptations related to differences in host choice and preference have been shown to be important prezygotic barriers to gene flow between these taxa, as Rhagoletis flies mate on or near the fruit of their respective host plants. Non‐host‐related assortative mating is generally absent or present at low levels between these sympatrically diverging fly populations. 2. However, some Rhagoletis taxa occasionally migrate to ‘non‐natal’ plants that are the primary hosts of other, morphologically differentiated fly species in the genus. These observations raise the question of whether sexual isolation may reduce courtship and copulation between morphologically divergent species of Rhagoletis flies, contributing to their prezygotic isolation along with host‐specific mating. 3. Using reciprocal multiple‐choice mating trials, we measured sexual isolation among nine species pairs of morphologically differentiated Rhagoletis flies. Complete sexual isolation was observed in eight of the nine comparisons, while partial sexual isolation was observed in the remaining comparison. 4. We conclude that sexual isolation can be an effective prezygotic barrier to gene flow contributing to substantial reproductive isolation between many morphologically distinct Rhagoletis species, even in the absence of differential host plant choice and host‐associated mating.     

Rampant host- and defensive phenotype-associated diversification in a goldenrod gall midge

Natural selection can play an important role in the genetic divergence of populations and their subsequent speciation. Such adaptive diversification, or ecological speciation, might underlie the enormous diversity of plant-feeding insects that frequently experience strong selection pressures associated with host plant use as well as from natural enemies. This view is supported by increasing documentation of host-associated (genetic) differentiation in populations of plant-feeding insects using alternate hosts. Here, we examine evolutionary diversification in a single nominal taxon, the gall midge Asteromyia carbonifera (O.S.), with respect to host plant use and gall phenotype. Because galls can be viewed as extended defensive phenotypes of the midges, gall morphology is likely to be a reflection of selective pressures by enemies. Using phylogenetic and comparative analyses of mtDNA and nuclear sequence data, we find evidence that A. carbonifera populations are rapidly diversifying along host plant and gall morphological lines. At a broad scale, geography explains surprisingly little genetic variation, and there is little evidence of strict co-cladogenesis with their Solidago hosts. Gall morphology is relatively labile, distinct gall morphs have evolved repeatedly and colonized multiple hosts, and multiple genetically and morphologically distinct morphs frequently coexist on a single host plant species. These results suggest that Asteromyia carbonifera is in the midst of an adaptive radiation driven by multitrophic selective pressures. Similar complex community pressures are likely to play a role in the diversification of other herbivorous insect groups.     

The adaptive significance of colour pattern polymorphism in the Australian scincid lizard Lampropholis delicata

Females of Lampropholis delicata are dimorphic for colour pattern, the difference between morphs being the presence or absence of a distinct white mid-lateral stripe. A less distinct striped morph occurs also in males. We evaluated alternative hypotheses for the maintenance of this polymorphism by examining temporal and spatial variation in morph frequency, testing for differential selection among morphs using data on body size and reproductive traits from preserved specimens, and experimentally manipulating colour pattern in free-ranging lizards of both sexes, to assess the influence of the lateral stripe on survival rates. We found that the relative frequency of striped individuals varied among populations and decreased from north to south in both sexes, coincident with an increasing incidence of regenerated tails. Morph frequencies did not change through time within a population. Striped gravid females appeared to survive better and produced larger clutches than did non-striped females. In our experimental study, the relationship between survival and colour morph differed between the two sexes; males painted with a white lateral stripe had lower survival than control (brown stripe) males, but survival did not differ between striped and control females. The different response in the two sexes may be due partly to differences in temperature and microhabitat selection. We propose that the white lateral stripe decreases susceptibility to predators in gravid females but increases risk of predation in males, especially in combination with low temperatures. The polymorphism might be maintained by: (1) opposing fitness consequences of the stripe in males and females; (2) sex-specific habitat selection; and (3) gene flow in combination with spatial variation in relative fitness of the two morphs.     

Host plant genotype influences survival of hybrids between Eurosta solidaginis host races

Extrinsic, host-associated environmental factors may influence postmating isolation between herbivorous insect populations and represent a fundamentally ecological cause of speciation. We investigated this issue in experiments on hybrids between the host races of Eurosta solidaginis, a fly that induces galls on the goldenrods Solidago altissima and S. gigantea. To do so, we measured the performance of parental host races and their hybrids on five genotypes of S. gigantea and nine genotypes of S. altissima to test hypotheses about how variation in plant genotype affects performance (i.e., fitness) and potentially influences gene flow between these host races. We found that rates of gall induction and of survival to adult emergence by hybrid larvae were significantly lower than those of both parental host races on both host species, adding support to the hypothesis that there is partial postmating isolation between the host races. Hybrid flies significantly varied in their performance across plant genotypes of both host species. A significant interaction between the effects of plant genotype and mating treatment (parental vs. hybrid crosses) on larval performance indicated that the relative suitability of particular plant genotypes differed between the parental host races and their hybrids. These patterns illustrate a poor correspondence between optimal parental and hybrid environments, consistent with the hypothesis that these host races are partially isolated due to extrinsic (ecological) factors. Based on these findings, we discuss the possibility that plant genotypes in which hybrid performance is high can facilitate hybridization and gene flow between partially reproductively isolated populations of herbivorous insects, thus affecting the dynamics of ecological speciation.     

Evidence of genetic and reproductive isolation between two morphs of subtropical-dominant coral Acropora solitaryensis in the non-reef region of Japan

We reveal the existence of a cryptic species of Acropora solitaryensis ( Veron and Wallace, 1984 ), a dominant species in high-latitude coral communities. Although some morphs, such as arborescent table (AR), solid plate (PL), and intermediate (IM) forms, had been known in this species, it was unclear whether these are reproductively isolated from one another. Here, potential reproductive exchange between two representative morphs, AR and PL, were examined using genetic and reproductive methods. Molecular phylogenetic analyses using both mitochondrial and nuclear molecular markers (mitochondrial control region and mini-collagen intron, respectively) indicated that AR is clearly distinct from PL, suggesting that gene flow between the morphs is absent. In cross-fertilization experiments, gametic compatibility between AR and PL was extremely low, suggesting prezygotic isolation of these morphs. These results strongly suggest that AR and IM forms are variations of A. solitaryensis, whereas PL form may be an undescribed species. In addition, AR was closely related genetically to A. pruinosa, which is a high-latitude species with arborescent form, and AR and A. pruinosa were able to hybridize, although with lower fertilities than observed in intra-specific crosses. The two species are thus likely to have speciated not in tropical regions, but in non-reef regions due to habitat segregation.     

Abundance of Gall Midges on Poulsenia armata (Moraceae): Importance of Host Plant Size and Light Environment in Tropical Rain Forests1

We studied the effects of contrasting light environments on the relationship between the host plant size of Poulsenia armata and the abundance of two gall midges in a tropical rain forest in Veracruz, Mexico. The number and density of two gall morphs (i.e., laminar and vein‐petiole galls) were positively correlated with plant size only in trees found in the forest but not in gaps. The availability of foliar area of P. armata trees was greater in forest gaps than in the forest. The foliar area was positively correlated with the abundance of laminar galls in trees in forest sites, but not with vein‐petiole galls. We concluded that the abundance of two morphs of gall midges on P. armata was associated with host plant size only in the forest trees. Larger plants had more galls than small plants, although this relationship was affected by local light environments.     

Choice of host plant as a factor in reproductive isolation of the aphid genus Cryptomyzus (Homoptera, Aphididae)

Abstract. 1. Host plant preference experiments were conducted with closely related taxa of the aphid genus Cryptomyzus. Males, and presexual morphs (sexuparae and gynoparae), were used to determine the impact of host plant choice on reproductive isolation. In the case of host-alternating species these morphs are migratory and so will select the host plant.
2. Host plant preference of two closely related taxa of C. alboapicalis (Theobald) was found to promote their reproductive isolation. The preference of sexuparae of these monoecious taxa was more pronounced than that of the males.
3. Host plant preference and subsequent production of oviparae showed that C. galeopsidis (Kaltenbach) consists of two host races restricted to Ribes rubrum L. and R. nigrum L., respectively. The existence of clones, intermediate in their preference and reproductive performance on these plants, suggests that hybridization occurs.     

Host plant‐related genomic differentiation in the European cherry fruit fly,Rhagoletis cerasi

Elucidating the mechanisms and conditions facilitating the formation of biodiversity are central topics in evolutionary biology. A growing number of studies imply that divergent ecological selection may often play a critical role in speciation by counteracting the homogenising effects of gene flow. Several examples involve phytophagous insects, where divergent selection pressures associated with host plant shifts may generate reproductive isolation, promoting speciation. Here, we use ddRADseq to assess the population structure and to test for host‐related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi (L., 1758) (Diptera: Tephritidae). This tephritid is distributed throughout Europe and western Asia, and has adapted to two different genera of host plants, Prunus spp. (cherries) and Lonicera spp. (honeysuckle). Our data imply that geographic distance and geomorphic barriers serve as the primary factors shaping genetic population structure across the species range. Locally, however, flies genetically cluster according to host plant, with consistent allele frequency differences displayed by a subset of loci between Prunus and Lonicera flies across four sites surveyed in Germany and Norway. These 17 loci display significantly higher F_ST values between host plants than others. They also showed high levels of linkage disequilibrium within and between Prunus and Lonicera flies, supporting host‐related selection and reduced gene flow. Our findings support the existence of sympatric host races in R. cerasi embedded within broader patterns of geographic variation in the fly, similar to the related apple maggot, Rhagoletis pomonella, in North America.     

Hybridization between pollination syndromes as an ecological and evolutionary resource

In plants, pollination syndromes (the correlated presence of many features of relevance to pollination mode, for instance pollination by a particular animal clade) are a striking feature of plant biodiversity, providing great floral phenotypic diversity (Fenster et al. 2004 ). Adaptation to a particular animal pollinator provides an explanation for why recently diverged plants can have such extreme differentiation in floral form. One might expect such elaborate adaptations to provide a high degree of pollinator specificity and hence reproductive isolation, but there are many cases where substantial gene flow exists between extreme floral morphs (see Table 1), and the resulting hybrids may be highly fertile. This gene flow provides tremendous opportunities to study the genetics and biology of the pollination syndromes by providing intermediate forms and segregating genotypes. If it is true that pollination syndromes result from adaptation under strong selection, we will expect such flowers to be crucibles of natural selection. If strong selection for particular floral phenotypes can be shown, then this, when coupled with hybridization, will give us one of the most valuable of all experimental systems for evolutionary research: gene flow and selection in balance. In this issue of Molecular Ecology, the paper of Milano et al. ( 2016 ) delivers this. It shows that in populations of the Ipomopsis aggregata complex, gene flow between pollination morphs is high and selection to stabilize those morphs is also high: a probable case of gene flow–selection balance.     

Flickers of speciation: Sympatric colour morphs of the arc‐eye hawkfish,Paracirrhites arcatus,reveal key elements of divergence with gene flow

One of the primary challenges of evolutionary research is to identify ecological factors that favour reproductive isolation. Therefore, studying partially isolated taxa has the potential to provide novel insight into the mechanisms of evolutionary divergence. Our study utilizes an adaptive colour polymorphism in the arc‐eye hawkfish (Paracirrhites arcatus) to explore the evolution of reproductive barriers in the absence of geographic isolation. Dark and light morphs are ecologically partitioned into basaltic and coral microhabitats a few metres apart. To test whether ecological barriers have reduced gene flow among dark and light phenotypes, we evaluated genetic variation at 30 microsatellite loci and a nuclear exon (Mc1r) associated with melanistic coloration. We report low, but significant microsatellite differentiation among colour morphs and stronger divergence in the coding region of Mc1r indicating signatures of selection. Critically, we observed greater genetic divergence between colour morphs on the same reefs than that between the same morphs in different geographic locations. We hypothesize that adaptation to the contrasting microhabitats is overriding gene flow and is responsible for the partial reproductive isolation observed between sympatric colour morphs. Combined with complementary studies of hawkfish ecology and behaviour, these genetic results indicate an ecological barrier to gene flow initiated by habitat selection and enhanced by assortative mating. Hence, the arc‐eye hawkfish fulfil theoretical expectations for the earliest phase of speciation with gene flow.     

Occurrence of five distinct clades of mermithid nematodes (Nematoda: Mermithidae) infecting black fly larvae (Diptera: Simuliidae) in tropical streams in Malaysia

Mermithids are the most common parasites of black flies and are associated with host feminization and sterilization in infected hosts. However, information on the species / lineage of black fly mermithids in Southeast Asia, including Malaysia requires further elucidation. In this study, mermithids were obtained from black fly larvae collected from 138 freshwater stream sites across East and West Malaysia. A molecular approach based on nuclear-encoded 18S ribosomal RNA (18S rRNA) gene was used to identify the species identity / lineage of 77 nematodes successfully extracted and sequenced from the specimens collected. Maximum likelihood and neighbor-joining phylogenetic analyses demonstrated five distinct mermithid lineages. Four species delimitation analyses: automated simultaneous analysis phylogenetics (ASAP), maximum likelihood Poisson tree processes with Bayesian inferences (bPTP_ML), generalized mixed yule coalescent (GMYC) and single rate Poisson tree processes (PTP) were applied to delimit the species boundaries of mermithid lineages in this data set along with genetic distance analysis. Data analysis supports five distinct lineages or operational taxonomic units for mermithids in the present study, with two requiring further investigation as they may represent intraspecific variation or closely related taxa. One mermithid lineage was similar to that previously observed in Simulium nigrogilvum from Thailand. Co-infection with two mermithids of different lineages was observed in one larva of Simulium trangense. This study represents an important first step towards exploring other aspects of host – parasite interactions in black fly mermithids.     

The control of morph development in the parasitic nematode Strongyloides ratti.

The parasitic nematode Strongyloides ratti has a complex life cycle. The progeny of the parasitic females can develop into three distinct morphs, namely directly developing infective third-stage larvae (iL3s), free-living adult males and free-living adult females. We have analysed of the effect of host immune status (an intra-host factor), environmental temperature (an extra-host factor) and their interaction on the proportion of larvae that develop into these three morphs. The results are consistent with the developmental decision of larvae being controlled by at least two discrete developmental switches. One is a sex-determination event that is affected by host immune status and the other is a switch between alternative female morphs that is affected by both host immune status and environmental temperature. These findings clarify the basis of the life cycle of S. ratti and demonstrate how such complex life cycles can result from a combination of simple developmental switches.     

THE EFFECTS OF THE RELATIVE GEOGRAPHIC SCALES OF GENE FLOW AND SELECTION ON MORPH FREQUENCIES IN THE WALKING-STICK TIMEMA CRISTINAE

Gene frequencies in large populations are determined by a balance between selection and gene flow between neighborhoods of different selection regimes. This balance is affected by the area of the patches of a given selection regime relative to the gene-flow distance. If patches are small relative to gene-flow distance, similarity in the total area occupied by different patch types is a crucial condition for the stability of polymorphisms. However, if patches are larger than the gene-flow distance, then the relative area of different patch types is less important because of reduced gene flow resulting from isolation by distance. Two morphs (striped and unstriped) of the walking-stick Timema cristinae were each strongly associated with patches of distinct species of food plants on which they are most cryptic. The frequency of a morph was high on the plant on which it is most cryptic when either: (1) the area occupied by the food plant (patch) was very large; (2) the patch was completely isolated from other patches; or (3) the patch was larger than adjacent patches. Results (1) and (2) are consistent with isolation-by-distance models, and result (3) is consistent with Levene's multiple-niche polymorphism model.     

Immune function, parasitization and extended phenotypes in colour polymorphic pygmy grasshoppers

Ecological and evolutionary consequences of host–parasite interactions have attracted considerable attention from evolutionary biologists. Previous studies have suggested that immune responsiveness may be genetically or developmentally linked with colour pattern, such that the evolution of animal colour patterns may be influenced by correlated responses to selection for parasite resistance. We studied interactions between the endoparasitic fly Leiophora innoxia (Meigen) (Diptera: Tachinidae) and its colour polymorphic pygmy grasshopper host Tetrix undulata (Sow.) (Orthoptera: Tetrigidae) to test for morph‐specific differences in parasitization and immune defence, and host‐induced variation in parasite phenotypes. Our results revealed that c. 2 and 30% of adult grasshoppers collected from the same natural population in two subsequent years, respectively were parasitized. Parasite prevalence was independent of host sex and colour morph. Pupae were larger if the parasite had developed in a female than in a male host, possibly reflecting host resource value or a physical constraint on larval growth imposed by host body size. Pupal size was also associated with host colour morph, with individuals that had developed in dark morphs being smaller at pupation compared to those that developed in paler morphs. However, immune defence, measured as the encapsulation response to a novel antigen, did not differ among individuals belonging to alternative colour morphs or sexes. Darker morphs warm up more quickly and prefer higher body temperatures than paler ones. Encapsulation was not influenced by maintenance temperature (15 vs. 30 °C), however, suggesting that indirect effects of coloration on parasite resistance mediated via differential body temperature are unlikely. The dependence of parasite body size on host colour morph may thus reflect plasticity of growth and development of the larvae in response to differential host body temperature, rather than variable host immune defence. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 373–383.     

Genetic and morphological differentiation in Tephritis bardanae (Diptera: Tephritidae): evidence for host-race formation

The fruit fly Tephritis bardanae infests flower heads of two burdock hosts, Arctium tomentosum and A. minus. Observations suggest host-associated mating and behavioural differences at oviposition indicating host-race status. Previously, flies from each host plant were found to differ slightly in allozyme allele frequencies, but these differences could as well be explained by geographical separation of host plants. In the present study, we explicitly test whether genetic and morphological variance among T. bardanae are explained best by host-plant association or by geographical location, and if this pattern is stable over a 10-year period. Populations of A. tomentosum flies differed significantly from those of A. minus flies in (i) allozyme allele frequencies at the loci Pep-A and Pgd, (ii) mtDNA haplotype frequencies and (iii) wing size. In contrast, geographical location had no significant influence on the variance estimates. While it remains uncertain whether morphometric differentiation reflects genotypic variability or phenotypic plasticity, allozyme and mtDNA differentiation is genetically determined. This provides strong evidence for host-race formation in T. bardanae. However, the levels of differentiation are relatively low indicating that the system is in an early stage of divergence. This might be due to a lack of time (i.e. the host shift occurred recently) or due to relatively high gene flow preventing much differentiation at loci not experiencing selection.     

SEX CHROMOSOME LINKAGE OF MATE PREFERENCE AND COLOR SIGNAL MAINTAINS ASSORTATIVE MATING BETWEEN INTERBREEDING FINCH MORPHS

Assortative mating is a key aspect in the speciation process because it is important for both initial divergence and maintenance of distinct species. However, it remains a challenge to explain how assortative mating evolves when diverging populations are undergoing gene flow (e.g., during hybridization). Here I experimentally test how assortative mating is maintained with frequent gene flow between diverged head‐color morphs of the Gouldian finch (Erythrura gouldiae). Contrary to the predominant view on the development of sexual preferences in birds, cross‐fostered offspring did not imprint on the phenotype of their conspecific (red or black morphs) or heterospecific (Bengalese finch) foster parents. Instead, the mating preferences of F₁ and F₂ intermorph‐hybrids are consistent with inheritance on the Z chromosomes, which are also the location for genes controlling color expression and the genes causing low fitness of intermorph‐hybrids. Genetic associations between color signal and preference loci on the sex chromosomes may prevent recombination from breaking down these associations when the morphs interbreed, helping to maintain assortative mating in the face of gene flow. Although sex linkage of reproductively isolating traits is theoretically expected to promote speciation, social and ecological constraints may enforce frequent interbreeding between the morphs, thus preventing complete reproductive isolation.     

Divergence among generalist herbivores: the <Emphasis Type="Italic">Frankliniella schultzei</Emphasis> species complex in Australia (Thysanoptera: Thripidae)

Understanding and interpreting the host plant interactions of “generalist” herbivorous insects requires that species limits are accurately defined, as such taxa frequently harbour cryptic species with restricted host use. We tested for the presence of cryptic species across different host plant species in Australian Frankliniella schultzei using a combination of gene sequencing and newly developed microsatellite markers. We detect deep divergence between three colour morphs (black, brown and yellow) but no discordance between mitochondrial and nuclear genes in areas of sympatry, indicating the presence of at least three species in Australia (and potentially six globally). Microsatellite markers were developed for the brown species but could not be amplified in the black or yellow species because the divergence between them is too great. When applied to six populations across Queensland and New South Wales the microsatellites showed high levels of gene flow across thrips collected from Gossypium hirsutum (cotton), Hibiscus rosa-sinensis and Malvaviscus arboreus, and over distances of at least 950 km, indicating considerable movement by these insects and no host-associated genetic differentiation in the brown species. Significantly, the divergence between the three species in Australia was not associated with any noticeable host specialisation. The substantial overlap in geographical distribution and host plant range raises questions about the process of speciation in generalist insects. Our results provide the basis from which detailed quantification of relative host use can be conducted for each species within the F. schultzei complex; this next step is crucial to fully understanding the host plant relationships of each and, thus, the basis of their speciation.     

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.