Divergent host plant preference causes assortative mating between sympatric host races of the ladybird beetle,Henosepilachna diekei

Divergent host preference (i.e. host fidelity) plays a significant role in the speciation process in phytophagous insects. However, how and to what extent this divergence reduces gene flow between populations has rarely been measured. Here, we estimated the intensity of assortative mating caused solely by host fidelity in two host races of the phytophagous ladybird beetle Henosepilachna diekei, specialized on Mikania micrantha (Asteraceae) and Leucas lavandulifolia (Lamiaceae) in West Java, Indonesia. These host races mated randomly in the absence of host plants under laboratory conditions, but demonstrated nearly complete assortative mating in field cages with the two host plants, by spending almost all of their time on their respective host plants. The frequency of assortative mating in the field cages was not affected drastically by host plant patch structure. These results suggest that fidelity to the different host plants yields directly almost complete reproductive isolation between the host races by limiting the habitat on the respective host plant. In addition, the high host fidelity also ensures female oviposition on the original host plant. As larvae cannot survive on non‐host plants, a positive association between female oviposition preference and larval performance on the host plant on which the beetles are specialized will further facilitate the evolution of host fidelity. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 606–614.     

CONTRIBUTION OF MULTIPLE ISOLATING BARRIERS TO REPRODUCTIVE ISOLATION BETWEEN A PAIR OF PHYTOPHAGOUS LADYBIRD BEETLES

Reproductive isolation between species may often be attained by multiple isolating barriers, but the components are rarely studied in animal taxa. To elucidate the nature of multiple isolating barriers, we quantified the strength of three premating barriers, including ecologically based ones (seasonal, habitat, and sexual), two postmating–prehatching barriers (reduced egg hatchability and conspecific sperm precedence [CSP]), and one posthatching barrier, including four components of F₁ hybrid reduced fitness, between two phytophagous ladybird beetles, Henosepilachna vigintioctomaculata and H. pustulosa . We detected five positive barriers (habitat isolation, sexual isolation, reduced egg hatchability, CSP, and reduced egg hatchability in backcrosses of F₁ hybrids). None of these barriers entirely prevents gene exchange when it acts alone, but jointly they generate nearly complete reproductive isolation even between sympatric populations. Host fidelity contributed most strongly to reproductive isolation by reducing interspecific hybridization through several important types of ecological isolation, including microspatial, habitat, and seasonal isolation. The existence of multiple isolating barriers likely helps keep reproductive isolation stable and robust, by complementing changes in the strength of leaky barriers. This complementarity of multiple isolating barriers yields the concept of robustness of reproductive isolation, which is important when considering the long-term maintenance of species boundaries in coexisting species pairs.     

Genetics, experience, and host-plant preference in Eurosta solidaginis: implications for host shifts and speciation

Host-associated mating is crucial in maintaining the partial reproductive isolation between the host races of Eurosta solidaginis (Diptera: Tephritidae), a fly that forms galls on Solidago altissima and S. gigantea. (We refer to flies reared from S. gigantea as gigantea flies and those reared from S. altissima as altissima flies.) We measured the host preference of males and females of both host races, F1 hybrids between the host races, F2, and backcrosses to both host races. Male and female altissima flies and female gigantea flies had high host fidelity, whereas male gigantea flies had low host fidelity. This result suggests that there may be gene flow between the host races due to nonassortative mating that occurs when male gigantea mate with altissima females on S. altissima. This indicates assortative-mating mechanisms in addition to host-associated mating are required to produce the partial reproductive isolation between the host races that has been observed. Nongenetic factors had no influence on host preference. Larval conditioning did not influence host preference: reciprocal F1 hybrids reared in S. altissima and S. gigantea both preferred S. gigantea. Adult experience had no impact on host preference: females preferred their natal host plant regardless of which host they encountered first as an adult. The hypothesis that maternal effects influence preferences was rejected because male and female flies did not show a consistent preference for the host plant of their mother. We also found no evidence that preference was a sex-linked trait because F1 and backcrosses to the host races with different combinations of X chromosomes from the two host races preferred S. gigantea. Our results indicate that host preference is not determined by a large number of genes because preference of hybrids did not correspond to the proportion of the genome derived from each host race. The strength of the ovipuncture preference for S. gigantea by gigantea females, the females of both reciprocal F1 hybrids, the backcross to gigantea, and F2s indicates that preference is inherited nonadditively at a limited number of loci. The F1 female hybrids, however, had a weaker host preference for S. gigantea than the pure gigantea host race, indicating that there may be incomplete dominance or modifier loci. Males had different host preference patterns than females, with individual male gigantea and male F1 hybrids usually exhibiting preference exclusively for S. gigantea or S. altissima. One hypothesis explaining the difference in host preference between males and females is that the same gene influences both female and male host preference, but it is a sex-influenced gene. Thus, males carrying the gene for S. gigantea preference have an intermediate host preference, whereas females have a strong host preference to S. gigantea. In summary, we found that the host preference that produces host-associated mating is inherited nonadditively at a relatively small number of loci on autosomal genes. This mode of inheritance meets the assumptions of models of sympatric speciation, indicating that the host races could have evolved in sympatry.     

Host‐associated pre‐mating reproductive isolation between host races of Acrocercops transecta: mating site preferences and effect of host presence on mating

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Extremely divergent host plant acceptance with gene flow between sympatric host races of the phytophagous ladybird beetle Henosepilachna diekei

Ecological divergence alone can prevent the majority of gene flow in the absence of other forms of reproductive isolation. Although the importance of ecological divergence in promoting reproductive isolation has been broadly recognized, its net impact on speciation has rarely been estimated in the wild. The phytophagous ladybird beetle Henosepilachna diekei Jadwiszczak & Wegrzynowicz includes two sympatric host races that are reproductively isolated solely by extreme specialization to either of the host plants Mikania micrantha Kunth (Asteraceae) or Leucas lavandulifolia Sm. (Lamiaceae) in West Java, Indonesia. To investigate the impact of differential host use as an isolating barrier, we carried out adult host acceptance tests and molecular population genetic analyses based on mitochondrial ND2 and nucleic ITS2 gene sequences using 13 wild populations of the host races, including four sympatric population pairs. Almost all individuals of these host races persistently accepted only the original host plant. We detected restricted but a degree of gene flow between these host races. A migration event occurred only in very recent time compared to their divergence time, indicating recent secondary contact of these host races in the surveyed area. These results reveal the remarkably large impact of host‐plant shift over almost the entire process of speciation and illustrate that ecological divergence has been maintained even under the presence of a certain degree of gene flow.     

Predicting the occurrence of host-associated differentiation in parasitic arthropods: a quantitative literature review

Parasite populations associated with different host species can encounter a variety of isolating reproductive barriers, leading to each population independently accumulating genome-wide genetic differences due to their host associations. This phenomenon is called host-associated differentiation (HAD) and has been proposed as an indicator of early diversification among parasitic arthropods. Although many parasite–host case study systems have been tested for the genetic signature of HAD (e.g., F_ST≥0.15 between sympatric, host-associated populations in the absence of allopatry), it is unknown which isolating reproductive barriers best predict the general occurrence of HAD. HAD development has been attributed to biological and ecological factors that either directly generate reproductive isolation between parasites living on different hosts, such as ‘immigrant inviability’ (i.e., lower fitness of immigrants in non-native environments), or that promote the accumulation of host-specific genetic adaptations, such as the gallmaking feeding mode. In fact, some of these factors are shared across multiple case studies, suggesting that the occurrence of HAD is generalizable and can be predicted based on the incidence of significant biological and ecological factors. By means of a discriminant function analysis (DFA), this research assessed 108 arthropod parasite–host case studies for ecological and biological factors significantly correlated with the occurrence of HAD and whether these factors could be used to distinguish the presence of HAD from its absence. The DFA demonstrated that case studies that developed HAD could be distinguished from case studies that did not develop HAD. The results of the DFA were corroborated by a ‘non-iterative partial least squares’ (NIPALS) discriminant model and a nominal logistic regression. Case studies with HAD could be robustly separated from case studies without HAD based on the incidence of these predictive factors: immigrant inviability, gallmaking, endophagy, recent range invasions of either hosts or parasites, differential host phenology, and differential parasite morphology. These results were used in an infinite random forest analysis to generate a hierarchy of conditional probabilities that separated HAD presence from absence. The results provide researchers with a tool for reliably predicting which untested parasite–host system would likely develop HAD. Immigrant inviability, gallmaking, and their combination were strongly correlated with the presence of HAD, which indicated parasite–host systems with these traits were highly likely to develop HAD. Contrary to expectation, endophagous feeding was negatively correlated with HAD presence, which indicated phytophagous endophagous feeders (excepting gallmakers) were highly unlikely to develop HAD. Furthermore, parasitoids were shown to be just as likely to develop HAD as not. Unfortunately, potentially significant predictive factors (e.g., allochrony) were excluded from analysis because too few case studies have been specifically tested for these factors. Furthermore, this analysis was biased by the lack of ‘negative’ publication results and the overrepresentation of research laboratories that primarily study HAD. Future research should accumulate novel HAD case studies that specifically test for allochrony, differential microbial associations, and morphological differentiation.     

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.     

Genetic isolation between two sympatric host-plant races of the European corn borer,Ostrinia nubilalis Hübner. I. Sex pheromone,moth emergence timing,and parasitism

Adaptation to different environments may be a powerful source of genetic differentiation between populations. The biological traits selected in each environment can pleiotropically induce assortative mating between individuals of these genetically differentiated populations. This situation may facilitate sympatric speciation. Successful host shifts in phytophagous insects provide some of the best evidence for the ecological speciation that occurs, or has occurred, in sympatry. The European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae), colonized maize after its introduction into Europe by humans about 500 years ago. In northern France, two sympatric host races feed on maize (Zea mays) and mugwort (Artemisia vulgaris), respectively. We investigated the factors involved in the genetic isolation of these two races at a field site near Paris, France. We identified two biological differences that might make a significant contribution to the genetic divergence between sympatric populations feeding on the two host plants. First, assortative mating may be due to differences in the moth emergence pattern between the two races: mugwort-race moths emerged on average 10 days earlier than maize-race moths. In addition, the males emerged earlier than females in both races. Hence, the likelihood of mating between maize-race males and mugwort-race females was higher than that of mating between mugwort-race males and maize-race females. Second, the females feeding on mugwort and maize produced sex pheromones with different E/Z isomeric ratios of delta-11-tetradecenyl acetate. This difference in mate recognition systems reinforces the potential for assortative mating in the two races. During the experiment, overwintering mortality was much lower on maize than on mugwort. This difference was due to a braconid parasitoid wasp, Macrocentrus cingulum, that killed more than 50% of the larvae overwintering on mugwort but did not infest larvae diapausing on maize. Hence, by colonizing maize, European corn borer populations probably escaped from numerous predators, competitors, and parasitoids, such as M. cingulum. This decrease in host-associated selection may have favored the colonization of this new host. Finally, throughout this experiment we observed selection at two allozyme loci (or at linked loci): Tpi and Mpi. The Tpi locus is tightly linked with the genes involved in the response of the male to the sex pheromone and in developmental timing. The location of these traits on the Z chromosome may play a role in shortening the time required for the evolution of premating barriers.     

Natural selection and divergence in mate preference during speciation

Sexual isolation can evolve due to natural selection against hybrids (reinforcement). However, many different forms of hybrid dysfunction, and selective processes that do not involve hybrids, can contribute to the evolution of sexual isolation. Here we review how different selective processes affect the evolution of sexual isolation, describe approaches for distinguishing among them, and assess how they contribute to variation in sexual isolation among populations of Timema cristinae stick-insects. Pairs of allopatric populations of T. cristinae living on different host-plant species exhibit greater sexual isolation than those on the same host, indicating that some sexual isolation has evolved due to host adaptation. Sexual isolation is strongest in regions where populations on different hosts are in geographic contact, a pattern of reproductive character displacement that is indicative of reinforcement. Ecological costs to hybridization do occur but traits under ecological selection (predation) do not co-vary strongly with the probability of between-population mating such that selection on ecological traits is not predicted to produce a strong correlated evolutionary response in mate preference. Moreover, F1 hybrid egg inviability is lacking and the factors contributing to reproductive character displacement require further study. Finally, we show that sexual isolation involves, at least in part, olfactory communication. Our results illustrate how understanding of the evolution of sexual isolation can be enhanced by isolating the roles of diverse ecological and evolutionary processes.     

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.     

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.     

Ecologically dependent and intrinsic genetic signatures of postzygotic isolation between sympatric host races of the leaf beetle Lochmaea capreae

The fitness of hybrids might be compromised as a result of intrinsic isolation and/or because they fall between ecological niches due to their intermediate phenotypes (“extrinsic isolation”). Here, we present data from several crosses (parental crosses, F1, F2, and backcrosses) between the two host races of Lochmaea capreae on willow and birch to test for extrinsic isolation, intrinsic isolation, and environmentally dependent genetic incompatibilities. We employed a reciprocal transplant design in which offspring were raised on either host plant and their survival was recorded until adulthood. We also applied joint‐scaling analysis to determine the genetic architecture of hybrid inviability. The relative fitness of the backcrosses switched between environments; furthermore, the additive genetic–environment interaction was detected as the strongest effect in our analysis. These results provide strong evidence that divergent natural selection has played a central role in the evolution of hybrid dysfunction between host races. Joint‐scaling analysis detected significant negative epistatic effects that are most evident in the poor performance of F2‐hybrids on willow, indicating signs of intrinsic isolation. We did not find any evidence that genetic incompatibilities are manifested independently of environmental conditions. Our findings suggest the outcome of natural hybridization between these host races is mainly affected by extrinsic isolation and a weak contribution of intrinsic isolation.     

Evolutionary ecology of the relationship between oviposition preference and performance of offspring in phytophagous insects

The relationship between oviposition preference and growth, survival, and reproduction of offspring is the crux of the problem in the evolution of host associations between phytophagous insects and plants. Observed relationships between oviposition preference and performance of offspring range from good to poor. At least four hypotheses have been suggested to explain observed use of particular host plants that may not result in the fastest growth rates or greatest pupal masses: time, patch dynamics, parasite versus grazer lifestyles, and enemy-free space. Our current understanding of these relationships, however, is hampered by an almost complete lack of data on how preference and performance are related genetically. These data are needed to understand the origins of covariance between preference and performance and constraints on the evolution of host associations.     

Host specialization involving attraction,avoidance and performance,in two phytophagous moth species

Host specialization plays a key role in the extreme diversification of phytophagous insects. Whereas proximate mechanisms of specialization have been studied extensively, their consequences for species divergence remain unclear. Preference for, and performance on hosts are thought to be a major source of divergence in phytophagous insects. We assessed these major components of specialization in two moth species, the European corn borer (ECB) and the Adzuki bean borer (ABB), by testing their oviposition behaviour in different conditions (choice or no‐choice set‐ups) and their performances, by reciprocal transplant at the larval stage on the usual host and an alternative host plant. We demonstrated that both ABB and ECB have a strong preference for their host plants for oviposition, but that relative larval performances on the usual host and an alternative host differed according to the experiment and the trait considered (weight or survival). Finally, we show for the first time that the preference for maize in ECB conceals a strong avoidance of mugwort. The differences in performance, attraction and avoidance between ECB and ABB are discussed in the light of the underlying mechanisms and divergence process.     

The genetic architecture of a niche: variation and covariation in host use traits in the Colorado potato beetle

The genetic basis of host plant use by phytophagous insects can provide insight into the evolution of ecological niches, especially phenomena such as specialization and phylogenetic conservatism. We carried out a quantitative genetic analysis of multiple host use traits, estimated on five species of host plants, in the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae). Mean values of all characters varied among host plants, providing evidence that adaptation to plants may require evolution of both behavioral (preference) and post-ingestive physiological (performance) characteristics. Significant additive genetic variation was detected for several characters on several hosts, but not in the capacity to use the two major hosts, a pattern that might be caused by directional selection. No negative genetic correlations across hosts were detected for any 'performance' traits, i.e. we found no evidence of trade-offs in fitness on different plants. Larval consumption was positively genetically correlated across host plants, suggesting that diet generalization might evolve as a distinct trait, rather than by independent evolution of feeding responses to each plant species, but several other traits did not show this pattern. We explored genetic correlations among traits expressed on a given plant species, in a first effort to shed light on the number of independent traits that may evolve in response to selection for host-plant utilization. Most traits were not correlated with each other, implying that adaptation to a novel potential host could be a complex, multidimensional 'character' that might constrain adaptation and contribute to the pronounced ecological specialization and the phylogenetic niche conservatism that characterize many clades of phytophagous insects.     

The Role of Adaptation to Host Plants in the Evolution of Reproductive Isolation: Negative Evidence from Tetranychus Urticae Koch

Reproductive isolation between demes of a phytophagous arthropod population that use different host plant species could evolve in two different ways. First, adaptation to different host species might result in reproductive isolation as a pleiotropic by-product. Second, if adaptation to one host species strongly reduces fitness on others, selection could favour mechanisms, such as host fidelity and assortative mating, that restrict gene flow between host-adapted demes. A laboratory selection experiment on the broadly polyphagous spider mite Tetranychus urticae gave information on these possibilities. A population allowed to adapt to tomato plants showed increased survival, development rate and fecundity on tomato relative to the base population from which it was derived. In spite of the large difference between the tomato-adapted and base populations in performance on tomato plants, the two populations showed no evidence of reproductive isolation, as measured by the hatching rate of eggs laid by F1 hybrids between the lines. Furthermore, a genetically variable population formed by hybridizing the tomato-adapted and base populations did not show evidence for a decline in ability to survive on tomato after more than ten generations of mass rearing on lima bean, indicating that tomato-adapted genotypes suffered little or no selective disadvantage on bean. These results give no support for the role of host plants in the evolution of reproductive isolation in T. urticae.     

BEHAVIORAL EVIDENCE FOR HOST-RACE FORMATION IN EUROSTA SOLIDAGINIS

We report behavioral evidence that Eurosta solidaginis, a stem-galling tephritid fly, has formed host races on its two goldenrod hosts, Solidago altissima and S. gigantea. Previous work has shown that flies from each host plant differ electrophoretically at the level of host races. The two host-associated populations were truly sympatric and were frequently found on host plants of the two species growing interdigitated with each other. Each host-associated population demonstrated a strong preference for ovipuncturing its own host. The S. gigantea–associated population emerged 10 to 14 d earlier than the S. altissima–associated population, contributing to the reproductive isolation between populations. Partial reproductive isolation is also maintained by a preference for mating on the host from which the fly emerged. The populations meet the criteria established for host races, suggesting that they may be in an intermediate stage of sympatric speciation.     

Perspective: Reproductive isolation caused by natural selection against immigrants from divergent habitats

The classification of reproductive isolating barriers laid out by Dobzhansky and Mayr has motivated and structured decades of research on speciation. We argue, however, that this classification is incomplete and that the unique contributions of a major source of reproductive isolation have often been overlooked. Here, we describe reproductive barriers that derive from the reduced survival of immigrants upon reaching foreign habitats that are ecologically divergent from their native habitat. This selection against immigrants reduces encounters and thus mating opportunities between individuals from divergently adapted populations. It also reduces the likelihood that successfully mated immigrant females will survive long enough to produce their hybrid offspring. Thus, natural selection against immigrants results in distinctive elements of premating and postmating reproductive isolation that we hereby dub "immigrant inviability." We quantify the contributions of immigrant inviability to total reproductive isolation by examining study systems where multiple components of reproductive isolation have been measured and demonstrate that these contributions are frequently greater than those of traditionally recognized reproductive barriers. The relevance of immigrant inviability is further illustrated by a consideration of population-genetic theory, a review of selection against immigrant alleles in hybrid zone studies, and an examination of its participation in feedback loops that influence the evolution of additional reproductive barriers. Because some degree of immigrant inviability will commonly exist between populations that exhibit adaptive ecological divergence, we emphasize that these barriers play critical roles in ecological modes of speciation. We hope that the formal recognition of immigrant inviability and our demonstration of its evolutionary importance will stimulate more explicit empirical studies of its contributions to speciation.     

Sympatric host races of the European corn borer: adaptation to host plants and hybrid performance

The European corn borer (ECB), Ostrinia nubilalis, is a major pest of maize crops. In Europe, two sympatric host races are found: one feeds on maize (Zea mays) and the other mainly on mugwort (Artemisia vulgaris). The two host races are genetically differentiated, seldom crossing in the laboratory or in the field, and females preferentially lay eggs on their native host species. We conducted two independent experiments, in field and greenhouse conditions, to determine whether the two host races are locally adapted to their host species. The effect of larval density and the performance of hybrids were also investigated. Despite some differences in overall larval feeding performance, both experiments revealed consistent patterns of local adaptation for survival and for larval weight in males. In females the same trend was observed but with weaker statistical support. F1 hybrids did not seem to be disadvantaged compared with the two parental races. Overall, our results showed that both host races are physiologically adapted to their native host. The fitness trade-off between the two host plants provides a potential driving force for ecological speciation in this species.     

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.