Host‐associated genetic differentiation in pecan leaf phylloxera

Host‐associated differentiation (HAD) is the formation of genetically distinct host‐associated populations. One of the genotypic signatures of HAD is that populations exhibit stronger differentiation by host‐plant species than by geographic isolation. HAD, as a mechanism promoting ecological speciation, has been invoked to explain phytophagous insect diversity. Two traits proposed to promote HAD are endophagy and parthenogenesis. Using amplified fragment length polymorphisms (AFLPs), we tested for the presence of HAD in pecan leaf phylloxera, Phylloxera notabilis Pergande (Hemiptera: Phylloxeridae), an endophagous, gall inducing, and cyclically parthenogenetic insect on sympatric pecan and water hickory at a geographic mesoscale. This species shows strong HAD. Whereas the effect of collecting site was significant, accounting for 7.3% of molecular variation, host‐plant species identity accounted for 63.5%. In addition, a choice test indicated that pecan leaf phylloxera originating from water hickory showed weak but significant preference for leaflets of the natal host, whereas pecan leaf phylloxera originating from pecan did not. This is the first such study of a species of arboreal Phylloxeridae, a poorly known insect group. This is also the first endophage and the second parthenogen shared by these two hickory species to show evidence of HAD. This hickory system could be a good parthenogen‐rich counterpoint to the goldenrod system in the study of HAD in insect communities.     

Differences in bacterial diversity of host-associated populations of Phylloxera notabilis Pergande (Hemiptera: Phylloxeridae) in pecan and water hickory

Host‐associated differentiation (HAD) is the presence of genetically divergent, host‐associated populations. It has been suggested that microbial symbionts of insect herbivores may play a role in HAD by allowing their insect hosts to use different plant species. The objective of this study was to document if host‐associated populations of Phylloxera notabilis Pergande (Hemiptera: Phylloxeridae) in pecan and water hickory corresponded with differences in the composition of their associated bacteria. To test this hypothesis, we characterized the symbionts present in P. notabilis associated with these two tree species through metagenomic analyses using 454 sequencing. Differences in bacterial diversity were found between P. notabilis populations associated with pecan and water hickory. The bacteria, Pantoea agglomerans and Serratia marcescens, were absent in the P. notabilis water hickory population, whereas both species accounted for more than 69.72% of bacterial abundance in the pecan population.     

Distribution of the black pecan aphid,Melanocallis caryaefoliae,on the upper and lower surface of pecan foliage

Three aphid species regularly feed on pecan [Carya illinoinensis (Wangenh.) K. Koch (Juglandaceae)] foliage: the black pecan aphid, Melanocallis caryaefoliae (Davis), the yellow pecan aphid, Monelliopsis pecanis Bissell, and the blackmargined aphid, Monellia caryella (Fitch) (all Hemiptera: Aphididae). Adults of M. caryaefoliae and both the nymphs and adults of M. pecanis and M. caryella mainly feed on the lower surface of leaves. Nymphs of M. caryaefoliae appear unique by frequently feeding on the upper surface of pecan leaves. This is risky behavior given the environmental hazards (e.g., rain, solar radiation, and dislodgement) associated with the upper surface. Thus, we determined the leaf surface distribution of M. caryaefoliae on trees in an orchard and on pecan seedlings in the laboratory. A pecan orchard survey found all three aphid species and stages predominantly on the lower leaf surface, except for the nymphs of M. caryaefoliae, which were evenly distributed between upper and lower leaf surfaces. This survey also found aphidophagous lacewing (Neuroptera) larvae predominantly on the lower leaf surface, whereas ladybird beetle (Coleoptera: Coccinellidae) larvae were more evenly distributed between upper and lower surfaces. Laboratory experiments using single or multiple pecan aphid species revealed M. caryaefoliae distribution on pecan seedlings similar to orchard data. Nymphal M. caryaefoliae require nearly 2 days to elicit chlorotic feeding lesions on leaves; without these lesions, nymphal development is hindered. The similar distribution of nymphs of M. caryaefoliae on both leaf surfaces likely reflects a strategy of predator avoidance allowing a proportion of the population to survive.     

Host‐associated genomic differentiation in congeneric butterflies: now you see it,now you do not

Ecotypic variation among populations may become associated with widespread genomic differentiation, but theory predicts that this should happen only under particular conditions of gene flow, selection and population size. In closely related species, we might expect the strength of host‐associated genomic differentiation (HAD) to be correlated with the degree of phenotypic differentiation in host‐adaptive traits. Using microsatellite and Amplified Fragment Length Polymorphism (AFLP) markers, and controlling for isolation by distance between populations, we sought HAD in two congeneric species of butterflies with different degrees of host plant specialization. Prior work on Euphydryas editha had shown strong interpopulation differentiation in host‐adapted traits, resulting in incipient reproductive isolation among host‐associated ecotypes. We show here that Euphydryas aurinia had much weaker host‐associated phenotypic differentiation. Contrary to our expectations, we detected HAD in Euphydryas aurinia, but not in E. editha. Even within an E. aurinia population that fed on both hosts, we found weak but significant sympatric HAD that persisted in samples taken 9 years apart. The finding of significantly stronger HAD in the system with less phenotypic differentiation may seem paradoxical. Our findings can be explained by multiple factors, ranging from differences in dispersal or effective population size, to spatial variation in genomic or phenotypic traits and to structure induced by past histories of host‐adapted populations. Other infrequently measured factors, such as differences in recombination rates, may also play a role. Our result adds to recent work as a further caution against assumptions of simple relationships between genomic and adaptive phenotypic differentiation.     

Immigrant inviability in yellow pecan aphid

1. Host‐associated differentiation (HAD) may be an important driver of parasite biodiversity. The sympatric occurrence of host‐associated populations features prominently in studies documenting HAD but this neither confirms nor denies that the lineages arose in sympatry. It does raise the question of what maintains such structure despite the proximity of populations in space and time. 2. We tested for immigrant inviability in yellow pecan aphid Monelliopsis pecanis Bissell, a species previously shown to be host associated, using a laboratory reciprocal transplant experiment on pecan and water hickory. 3. Immigrant inviability on the alternative host is present in yellow pecan aphid. Immigrants to both hosts were significantly less fecund than residents and their nymphs were significantly less viable. Future studies should establish the genetic and non‐genetic (maternal effects) basis of immigrant inviability in yellow pecan aphid in order to be certain about its contribution to pre‐mating reproductive isolation. 4. Since aphids are cyclic parthenogens, inviability of both immigrants and their offspring has the potential to contribute to total pre‐mating reproductive isolation. This is different than the case of obligate sexually reproducing organisms where offspring viability is a post‐mating reproductive isolating mechanism. We discuss the implications of this for estimating the contributions of immigrant and offspring inviability to total reproductive isolation in aphids.     

Exploring host‐associated differentiation in the North American native cranberry fruitworm,Acrobasis vaccinii,from blueberries and cranberries

The factors explaining host‐associated differentiation (HAD) have not yet been fully characterized, especially in agricultural systems. It is thought that certain characteristics within a system may increase the probability for HAD to occur. These characteristics include relatively long‐standing evolutionary relationships between insects and their host plants, endophagy, and allochrony in host‐plant phenologies. We assessed the status of these characteristics as well as the presence of HAD in the cranberry fruitworm, Acrobasis vaccinii Riley (Lepidoptera: Pyralidae), a pest associated with blueberry and cranberry in eastern North America. We reveal the occurrence of two distinct populations of A. vaccinii that are allochronically isolated by the phenological stage of their respective host plants (cranberries or blueberries). Laboratory‐reared A. vaccinii adults collected from blueberries emerge at least 1 week earlier than adults from cranberries and the antennal sensitivity of adults to host‐plant volatiles differs between A. vaccinii collected from blueberry and cranberry. Despite finding characteristics indicative of HAD, we did not detect a genetic signature of HAD in A. vaccinii. These findings suggest that HAD may occur through behavioral and phenological mechanisms before there is sufficient genetic variation to be detected.     

Host‐associated differentiation in a highly polyphagous,sexually reproducing insect herbivore

Insect herbivores may undergo genetic divergence on their host plants through host‐associated differentiation (HAD). Much of what we know about HAD involves insect species with narrow host ranges (i.e., specialists) that spend part or all their life cycle inside their hosts, and/or reproduce asexually (e.g., parthenogenetic insects), all of which are thought to facilitate HAD. However, sexually reproducing polyphagous insects can also exhibit HAD. Few sexually reproducing insects have been tested for HAD, and when they have insects from only a handful of potential host‐plant populations have been tested, making it difficult to predict how common HAD is when one considers the entire species' host range. This question is particularly relevant when considering insect pests, as host‐associated populations may differ in traits relevant to their control. Here, we tested for HAD in a cotton (Gossypium hirsutum) pest, the cotton fleahopper (CFH) (Pseudatomoscelis seriatus), a sexually reproducing, highly polyphagous hemipteran insect. A previous study detected one incidence of HAD among three of its host plants. We used Amplified fragment length polymorphism (AFLP) markers to assess HAD in CFH collected from an expanded array of 13 host‐plant species belonging to seven families. Overall, four genetically distinct populations were found. One genetically distinct genotype was exclusively associated with one of the host‐plant species while the other three were observed across more than one host‐plant species. The relatively low degree of HAD in CFH compared to the pea aphid, another hemipteran insect, stresses the likely importance of sexual recombination as a factor increasing the likelihood of HAD.     

Testing host‐associated differentiation in a quasi‐endophage and a parthenogen on native trees

Host‐associated differentiation (HAD) is the formation of genetically divergent host‐associated sub‐populations. Evidence of HAD has been reported for multiple insect herbivores to date, but published studies testing more than one herbivore for any given host‐plant species pair is limited to herbivores on goldenrods. This limits the number of pair‐wise comparisons that can be made about insect life‐history traits that might facilitate or inhibit host‐race development in general. Two traits previously proposed to facilitate HAD include endophagy and parthenogenesis. We tested for HAD in two herbivores, a quasi‐endophagous caterpillar and a parthenogenetic aphid, feeding on two closely related species of hickories. We found that the quasi‐endophage is panmictic, whereas the parthenogen exhibits HAD on their sympatric host plants, pecan and water hickory, at a geographic mesoscale. This is an important first step in the characterization of HAD in multiple insect herbivores using North American hickories, a host‐plant system with many shared parthenogens.     

Genomic evidence for host-associated differentiation in an animal parasite,Dermacentor variabilis,the American dog tick

The process responsible for the formation of genetically distinct populations associated with different host species is known as host-associated differentiation (HAD). Many insect parasites of plants have been shown to exhibit HAD but there have been fewer studies of HAD in parasites of vertebrate animals. Previous to this study, HAD has been documented in at least three species of ticks. The American dog tick, Dermacentor variabilis (Say) (Acari: Ixodidae) was chosen as the focal species for this study due to its importance as the vector of tularemia and Rocky Mountain spotted fever. Previous population genetic studies of this tick found the existence of various haplotypes but the tick’s host origins were unknown. In this study, ticks were collected from 15 vertebrate host species to test for HAD using single nuclear polymorphisms (SNPs). In total, 136 individual D. variabilis ticks were sequenced using ddRADseq. Genomic evidence was found to point to D. variabilis exhibiting HAD on eight different hosts. A STRUCTURE analysis showed that the highest posterior probability was obtained with a population size of eight and these populations correlated with host species. Pairwise F_ST values were as high as 0.622 and indicated a range of genetic distinction between host groups. In addition, ticks collected from the vegetation appeared as one homogenous distinct genotype suggesting the existence of nidicolous (nest dwelling) and non-nidicolous genotypes. The identification of host race formation occurring in this animal parasite has implications for the understanding of D. variabilis pathogen transmission and targeted control efforts because genetically distinct populations can differ in traits relevant to these applications.     

Testing for host‐associated differentiation in two egg parasitoids of a forest herbivore

Several studies underline the importance of ecological barriers and differential selection in driving sympatric speciation. Host‐associated differentiation (HAD) has been proposed as one of the mechanisms leading to sympatric speciation. However, it is still unclear how common HAD is or which are the factors that could promote it. In particular, not much is known about HAD in predators and parasitoids of herbivorous insects. One of the characteristics postulated to pre‐dispose insects to HAD is parthenogenesis as it may favour adaptive responses to particular environments, amplifying selected gene complexes. In this study, we used amplified fragment length polymorphism (AFLP) markers to determine whether HAD is present in two parthenogenetic egg parasitoids attacking the same herbivore species – the pine processionary moth, Thaumetopoea pityocampa (Denis & Schiffermüller) (Lepidoptera: Notodontidae) – on two host Pinus species. A total of 100 loci for 59 individuals sampled in four populations of Baryscapus servadeii (Domenichini) (Hymenoptera: Eulophidae), a specialist parasitoid, and 106 loci for 117 individuals sampled in six populations of Ooencyrtus pityocampae Mercet (Hymenoptera: Encyrtidae), a generalist parasitoid, were analysed. Levels of genetic differentiation were also assessed with an outlier analysis, checking for alleles associated to host plants. No evidence of HAD was detected in any of the two parasitoid species. We hypothesize that both the lack of strict parthenogenetic reproduction and the ectophagous nature of the insect host could explain the absence of HAD. The genetic variation observed in the generalist parasitoid responded to a pattern of local adaptation, whereas no relationship with either host or geography was found in the specialist parasitoid.     

Development of SSR Markers in Hickory (Carya cathayensis Sarg.) and Their Transferability to Other Species of Carya

Hickory (Carya cathayensis Sarg.), an important nut-producing species in Southeastern China, has high economic value, but so far there has been no cultivar bred under species although it is mostly propagated by seeding and some elite individuals have been found. It has been found recently that this species has a certain rate of apomixis and poor knowledge of its genetic background has influenced development of a feasible breeding strategy. Here in this paper we first release SSR (Simple sequence repeat) markers developed in this species and their transferability to other three species of the same genus, Carya. A total of 311 pairs of SSR primers in hickory were developed based on sequenced cDNAs of a fruit development-associated cDNA library and RNA-seq data of developing female floral buds and could be used to distinguish hickory, C. hunanensis Cheng et R. H. Chang ex R. H. Chang et Lu, C. illinoensis K. Koch (pecan) and C. dabieshanensis M. C. Liu et Z. J. Li, but they were monomorphic in both hickory and C. hunanensis although multi-alleles have been identified in all the four species. There is a transferability rate of 63.02% observed between hickory and pecan and the markers can be applied to study genetic diversity of accessions in pecan. When used in C. dabieshanensis, it was revealed that C. dabieshanensis had the number of alleles per locus ranging from 2 to 4, observed heterozygosity from 0 to 0.6667 and expected heterozygosity from 0.333 to 0.8667, respectively, which supports the existence of C. dabieshanensis as a separate species different from hickory and indicates that there is potential for selection and breeding in this species.     

Geographic pattern of host‐associated differentiation in the cotton fleahopper,Pseudatomoscelis seriatus

Host‐associated differentiation (HAD) is the occurrence of genetically distinct, host‐associated lineages. Most of the cases of HAD in phytophagous insects have been documented in specialist insects inhabiting feral ecosystems or in generalist parthenogens in agroecosystems. Herein we report HAD in the cotton fleahopper, Pseudatomoscelis seriatus (Reuter) (Hemiptera: Miridae), a native, generalist, non‐parthenogenetic insect feeding on native wild hosts [horsemint, Monarda punctata L. (Lamiaceae) and woolly croton, Croton capitatus Michx. (Euphorbiaceae)] and on cotton [Gossypium hirsutum L. (Malvaceae)] in the USA. Examination of genome‐wide genetic variation with AFLP markers and Bayesian analyses of P. seriatus associated with three different host plant species at five locations in Texas revealed a geographic pattern of HAD. The geographic pattern of HAD corresponded with differences in precipitation among the locations studied. In three locations, two distinct lineages of P. seriatus were found in association with horsemint and cotton/woolly croton, whereas in two other locations, populations associated with the different host plants studied were panmictic. We suggest that precipitation differences among locations translate into heterogeneity in vegetation distribution, composition, and phenology, which altogether may contribute to the observed geographic pattern of HAD.     

Do differences in life‐history traits and the timing of peak mating activity between host‐associated populations of Chilo suppressalis have a genetic basis?

The development of host races, genetically distinct populations of the same species with different hosts, is considered to be the initial stage of ecological speciation. Ecological and biological differences consistent with host race formation have been reported between water‐oat and rice‐associated populations of Chilo suppressalis. In order to confirm whether these differences have a genetic basis, we conducted experiments to determine the extent to which various life‐history traits and the time of peak mating activity of these populations were influenced by the species of host plant larvae were raised on. Individuals from each population were reared for three consecutive generations on either water‐oat fruit pulp or rice seedlings. Descendants of both populations had higher larval survival rates, shorter larval developmental periods, higher pupal weight, and longer adult forewings, when reared on water‐oats than when reared on rice. The time of peak of mating activity differed between the descendants of each population, irrespective of whether they were raised on water‐oats or rice. These results indicate that although some life‐history traits of host‐associated populations of C. suppressalis are influenced by the host plant larvae are raised on, time of peak mating activity is not. Because it is a stable, objective, phenotypic trait, further research on difference in the time of peak mating activity between host‐associated populations of C. suppressalis should be conducted to clarify the mechanism responsible for host race formation in this species.     

Cryptic diversity,reproductive isolation and cytoplasmic incompatibility in a classic biological control success story

Molecular genetics and symbiont diagnostics have revolutionized our understanding of insect species diversity, and the transformative effects of bacterial symbionts on host life history. Encarsia inaron is a parasitoid wasp that has been shown to harbour two bacterial endosymbionts, Wolbachia and Cardinium. Known then as E. partenopea, it was introduced to the USA in the late 1980s from populations collected in Italy and Israel for the biological control of an ornamental tree pest, the ash whitefly, Siphoninus phillyreae. We studied natural populations from sites in the USA, the Mediterranean and the Middle East as well as from a Cardinium‐infected laboratory culture established from Italy, with the aims of characterizing these populations genetically, testing reproductive isolation, determining symbiont infection status in their native and introduced range, and determining symbiont role. The results showed that the two Encarsia populations introduced to the USA are genetically distinct, reproductively isolated, have different symbionts and different host–symbiont interactions, and can be considered different biological species. One (‘E. inaron’) is doubly infected by Wolbachia and Cardinium, while only Cardinium is present in the other (‘E. partenopea’). The Cardinium strains in the two species are distinct, although closely related, and crossing tests indicate that the Cardinium infecting ‘E. partenopea’ induces cytoplasmic incompatibility. The frequency of symbiont infection found in the native and introduced range of these wasps was similar, unlike the pattern seen in some other systems. These results also lead to a retelling of a successful biological control story, with several more characters than had been initially described.     

Meloidogyne partityla on Pecan Isozyme Phenotypes and Other Host

Meloidogyne sp. from five pecan (Carya illinoensis) orchards in Texas were distinctive in host range and iszoyme profiles from common species of Meloidogyne but were morphologically congruent with Meloidogyne partityla Kleynhans, a species previously known only in South Africa. In addition to pecan, species of walnut (Juglans hindsii and J. regia) and hickory (C. ovata) also were hosts. No reproduction was observed on 15 other plant species from nine families, including several common hosts of other Meloidogyne spp. Three esterase phenotypes and two malate dehydrogenase phenotypes of M. partityla were identified by polyacrylamide gel electrophoresis. Each of these isozyme phenotypes was distinct from those of the more common species M. arenaria, M. hapla, M. incognita, and M. javanica.     

Unique haplotypes in ant‐attended aphids and widespread haplotypes in non‐attended aphids

Aphid species within the genus Tuberculatus Mordvilko (Hemiptera: Aphididae) exhibit a variety of interactions with ants, ranging from close associations to non‐attendance. A previous study indicated that despite wing possession, ant‐attended Tuberculatus species exhibited low dispersal rates compared with non‐attended species. This study examined if presence or absence of mutualistic interactions and habitat continuity of host plants affected intraspecific genetic diversity and genetic differentiation in mitochondrial DNA cytochrome oxidase I (COI) sequences. Sympatric ant‐attended Tuberculatus quercicola (Matsumura) (Hemiptera: Aphididae) and non‐attended Tuberculatus paiki Hille Ris Lambers (Hemiptera: Aphididae) were collected from the daimyo oak Quercus dentata Thunberg (Fagales: Fagaceae) in Japan and examined for haplotype variability. Seventeen haplotypes were identified in 568 T. quercicola individuals representing 23 populations and seven haplotypes in 425 T. paiki representing 19 populations. Haplotype diversity, which indicates the mean number of differences between all pairs of haplotypes in the sample, and nucleotide diversity were higher in T. quercicola than T. paiki. Analysis of molecular variance (AMOVA) showed higher genetic differentiation among populations within groups of T. quercicola (39.8%) than T. paiki (22.6%). The effects of attendant ant species on genetic differentiation in T. quercicola were not distinguishable from geographic factors. Despite low dispersal rates, host plant habitat continuity might facilitate widespread dispersal of a T. quercicola haplotype in Hokkaido. These results suggested that following T. quercicola colonization, gene flow among populations was limited, resulting in genetic drift within populations. However, frequent T. paiki dispersal is clearly evident by low genetic differentiation among populations within groups, resulting in lower haplotype diversity.     

Postglacial colonization history reflects in the genetic structure of natural populations of Festuca rubra in Europe

We conducted a large‐scale population genetic survey of genetic diversity of the host grass Festuca rubra s.l., which fitness can be highly dependent on its symbiotic fungus Epichloë festucae, to evaluate genetic variation and population structure across the European range. The 27 studied populations have previously been found to differ in frequencies of occurrence of the symbiotic fungus E. festucae and ploidy levels. As predicted, we found decreased genetic diversity in previously glaciated areas in comparison with nonglaciated regions and discovered three major maternal genetic groups: southern, northeastern, and northwestern Europe. Interestingly, host populations from Greenland were genetically similar to those from the Faroe Islands and Iceland, suggesting gene flow also between those areas. The level of variation among populations within regions is evidently highly dependent on the postglacial colonization history, in particular on the number of independent long‐distance seed colonization events. Yet, also anthropogenic effects may have affected the population structure in F. rubra. We did not observe higher fungal infection rates in grass populations with lower levels of genetic variability. In fact, the fungal infection rates of E. festucae in relation to genetic variability of the host populations varied widely among geographical areas, which indicate differences in population histories due to colonization events and possible costs of systemic fungi in harsh environmental conditions. We found that the plants of different ploidy levels are genetically closely related within geographic areas indicating independent formation of polyploids in different maternal lineages.     

Quantifying aphid behavioral responses to environmental change

Aphids are the most common vector of plant viruses, and their feeding behavior is an important determinant of virus transmission. Positive effects of global change on aphid performance have been documented, but effects on aphid behavior are not known. We assessed the plant‐mediated behavioral responses of a generalist aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), to increased CO₂ and nitrogen when feeding on each of three host species: Amaranthus viridis L. (Amaranthaceae), Polygonum persicaria L. (= Persicaria maculosa Gray) (Polygonaceae), and Solanum dulcamara L. (Solanaceae). Via a family of constrained Markov models, we tested the degree to which aphid movements demonstrate preference among host species or plants grown under varying environmental conditions. Entropy rates of the estimated Markov chains were used to further quantify aphid behavior. Our statistical methods provide a general tool for assessing choice and quantitatively comparing animal behavior under different conditions. Aphids displayed strong preferences for the same host species under all growth conditions, indicating that CO₂‐ and N‐induced changes in plant chemistry have minimal effects on host preference. However, entropy rates increased in the presence of non‐preferred hosts, even when preferred hosts were available. We conclude that the presence of a non‐preferred host species affected aphid‐feeding behavior more than changes in plant leaf chemistry when plants were grown under elevated CO₂ and increased N availability.     

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.     

Molecular Phylogeny and Evolutionary Relationships between the Ciliate Genera Peniculistoma and Mytilophilus (Peniculistomatidae,Pleuronematida)

Peniculistoma mytili and Mytilophilus pacificae are placed in the pleuronematid scuticociliate family Peniculistomatidae based on morphology and ecological preference for the mantle cavity of mytiloid bivalves. We tested this placement with sequences of the small subunit rRNA (SSUrRNA) and cytochrome c oxidase subunit 1 (cox1) genes. These species are very closely related sister taxa with no distinct genetic difference in the SSUrRNA sequence but about 21% genetic difference for cox1, supporting their placement together but separation as distinct taxa. Using infection frequencies, M. pacificae, like its sister species P. mytili, does not interact with Ancistrum spp., co‐inhabitants of the mantle cavity. On the basis of these ecological similarities, the fossil record of host mussels, and features of morphology and stomatogenesis of these two ciliates, we argue that M. pacificae derived from a Peniculistoma‐like ancestor after divergence of the two host mussels. Our phylogenetic analyses of pleuronematid ciliates includes the SSUrRNA gene sequence of Sulcigera comosa, a Histiobalantium‐like ciliate from Lake Baikal. We conclude: (i) that the pleuronematids are a monophyletic group; (ii) that the genus Pleuronema is paraphyletic; and (iii) that S. comosa is a Histiobalantium species. We transfer S. comosa to Histiobalantium and propose a new combination Histiobalantium comosa n. comb.