Evidence for widespread cryptic sexual generations in apparently purely asexual Andricus gallwasps

Modelling suggests Wolbachia-induced cytoplasmic incompatibility in oak gall wasps with cyclical parthenogenesis

Oak gall wasps typically exhibit a life cycle with one sexual and one asexual generation each year. These wasps can carry various endosymbionts, one of which is the maternally inherited bacterium Wolbachia that can induce several reproductive manipulations on its host. Cytoplasmic incompatibility (CI) has been described as the most prominent of these manipulations. CI leads to embryonic mortality in the hosts' offspring when infected males mate with either uninfected females or with females that harbour different Wolbachia strains. It has been hypothesized that Wolbachia can induce CI in oak gall wasps. To address this hypothesis, we derived a mathematical model to investigate the spread of a bacterial infection in naive populations and to determine the plausibility of CI occurrence. To validate our model, we used published data from Wolbachia-infected Belonocnema kinseyi populations in two approaches. Our first approach uses measurements of infection frequencies and maternal transmission in the sexual generation. For the second approach, we extended the model to compare predictions to estimates of mtDNA-haplotypes, which, like Wolbachia, are maternally inherited, and can therefore be associated with the infection. Both approaches indicate that CI is present in these populations. Our model can be generalized to investigate the occurrence of CI not only for oak gall wasps but also for other species.

The experimental life cycle closure of Andricus truncicolus (Giraud, 1859) (Hymenoptera: Cynipidae: Cynipini) and taxonomic description of its sexual generation

The life cycle of Andricus truncicolus (Giraud, 1869), a species previously known only from its asexual generation, is closed. Laboratory rearing and field sampling have revealed the sexual generation of this species, which develops in a hypertrophic degeneration of the terminal or lateral shoot buds appearing as a muddled agglomerate of leaves on Quercus, section Cerris oaks. We describe for the first time the newly discovered sexual generation of A. truncicolus and provide illustrations and diagnosis for adults and galls of both generations, further observations on biology and information on the species distribution.

A One-Step Multiplex PCR Method to Rapidly Distinguish Two Strains of Diglyphus wani (Hymenoptera: Eulophidae) Against Agromyzid Leafminers

Hymenopteran parasitoids generally show a haplo-diploid sex determination system. Haploid males are produced from unfertilized eggs, whereas diploid females develop from fertilized eggs (arrhenotokous). In some cases, diploid females develop from unfertilized eggs (thelytokous). Diglyphus wani (Hymenoptera: Eulophidae) is a biological control agent for agromyzid leafminers and have arrhenotokous and thelytokous strains. However, the morphological characteristics of two strains of D. wani are so similar that it is difficult to accurately distinguish them based on morphology. Here, a rapid molecular identification method was developed based on the mitochondrial gene cytochrome c oxidase I (COI) and one-step multiplex PCR. Two primer combinations, PC1 (Ar-F1/Th-F1/WR2) and PC2 (Ar-F1/Th-F4/WR2), were designed and repeatedly screened to distinguish two strains simultaneously, of which two special forward primers Th-F1/Th-F4 were used for the thelytokous strain and one special forward primer Ar-F1 was used for the arrhenotokous strain. In addition, a common reverse primer, WR2, was used for both strains. The PC1 and PC2 PCR assays were effective in distinguishing the two strains at different developmental stages and field colonies. This method provides a reliable, highly sensitive, and cost-effective tool for the rapid identification of the two strains of D. wani.

The Paths of the Galls: Differences in the Ecology and Distribution of Two European Oak Gall Wasps Andricus dentimitratus and Andricus pictus

Andricus dentimitratus (Rejtõ, 1887) and Andricus pictus (Hartig, 1856) are two European gall wasps (Hymenoptera, Cynipidae) that induce galls on species of Quercus. The distribution and ecological niches of these species have not been studied in detail, though they are known to have a different distribution pattern in the Iberian Peninsula in Europe. To investigate this difference and its potential relationship with climate and host species distribution, we analysed the potential distribution of both species in the Iberian Peninsula using six algorithms and a consensus model based on 600 iterations for each species. We compared the models obtained for each species with the distribution of their host Quercus species. The results show that A. dentimitratus and A. pictus have a complementary distribution delimited by the Ebro valley, with A. dentimitratus occurring northeast of the valley and A. pictus southwest. The observed distribution patterns might be due to differences in the climatic requirements of each species or to the distribution of their host species given that A. dentimitratus is specific to Q. humilis and Q. cerris (except in the northeastern Iberian Peninsula) and A. pictus, to marcescent Mediterranean oaks (Q. faginea and Q. pyrenaica) and Q. suber. We propose two hypotheses to explain the nonoverlapping distribution of the two gall wasp species in the Iberian Peninsula: in the first scenario, A. dentimitratus arrived to the to the Iberian Peninsula from the eastern Palearctic by way of Europe and A. pictus, from the north coast of Africa; in the second, their distribution is a result of their speciation in different glacial refugia: A. dentimitratus in the Italian Peninsula and A. pictus in the Iberian Peninsula.

Speciation in Nearctic oak gall wasps is frequently correlated with changes in host plant, host organ, or both

Quantifying the frequency of shifts to new host plants within diverse clades of specialist herbivorous insects is critically important to understand whether and how host shifts contribute to the origin of species. Oak gall wasps (Hymenoptera: Cynipidae: Cynipini) comprise a tribe of ∼1000 species of phytophagous insects that induce gall formation on various organs of trees in the family Fagacae-primarily the oaks (genus Quercus; ∼435 sp.). The association of oak gall wasps with oaks is ancient (∼50 my), and most oak species are galled by one or more gall wasp species. Despite the diversity of both gall wasp species and their plant associations, previous phylogenetic work has not identified the strong signal of host plant shifting among oak gall wasps that has been found in other phytophagous insect systems. However, most emphasis has been on the Western Palearctic and not the Nearctic where both oaks and oak gall wasps are considerably more species rich. We collected 86 species of Nearctic oak gall wasps from most of the major clades of Nearctic oaks and sequenced >1000 Ultraconserved Elements (UCEs) and flanking sequences to infer wasp phylogenies. We assessed the relationships of Nearctic gall wasps to one another and, by leveraging previously published UCE data, to the Palearctic fauna. We then used phylogenies to infer historical patterns of shifts among host tree species and tree organs. Our results indicate that oak gall wasps have moved between the Palearctic and Nearctic at least four times, that some Palearctic wasp clades have their proximate origin in the Nearctic, and that gall wasps have shifted within and between oak tree sections, subsections, and organs considerably more often than previous data have suggested. Given that host shifts have been demonstrated to drive reproductive isolation between host-associated populations in other phytophagous insects, our analyses of Nearctic gall wasps suggest that host shifts are key drivers of speciation in this clade, especially in hotspots of oak diversity. Although formal assessment of this hypothesis requires further study, two putatively oligophagous gall wasp species in our dataset show signals of host-associated genetic differentiation unconfounded by geographic distance, suggestive of barriers to gene flow associated with the use of alternative host plants.

Andricus cydoniae Giraud, 1859 Junior Synonym of Cynips conifica Hartig, 1843, as Experimentally Demonstrated (Hymenoptera: Cynipidae: Cynipini)

We demonstrated the life cycle closure of Cynips conifica Hartig, 1843 (presently Andricus conificus), previously supposed on the basis of molecular data, and the identity of the sexual generation, through laboratory experiments. As a consequence, Andricus cydoniae Giraud, 1859 became a junior synonym of A. conificus (Hartig, 1843). We provide illustrations and a diagnosis for adults and galls, observations on biology, and information on distribution. Moreover, as sexual galls of A. conificus cannot be distinguished from those of Andricus multiplicatus, a detailed comparison between sexual galls and adults of these two species is reported.

New species of Nearctic oak gall wasps (Hymenoptera: Cynipidae, Cynipini)

Twenty nine new species of cynipid oak gall wasps from the Nearctic region (America north of Mexico) are described: Andricus archboldi Melika & Abrahamson, sp. nov., A. catalinensis Melika, Nicholls & Stone, sp. nov., A. chapmanii Melika & Abrahamson, sp. nov., A. chiricahuensis Melika, Nicholls & Stone, sp. nov., A. coconinoensis Melika, Nicholls & Stone, sp. nov., A. columbiensis Melika, Nicholls & Stone, sp. nov., A. cooki Melika, Nicholls & Stone, sp. nov., A. fitzpatricki Melika & Abrahamson, sp. nov., A. highlandensis Melika, Nicholls & Stone, sp. nov., A. mellificus Nicholls, Stone & Melika, sp. nov., A. menkei Melika & Abrahamson, sp. nov., A. mogollonensis Melika, Nicholls & Stone, sp. nov., A. nichollsi Melika & Stone, sp. nov., A. schickae Nicholls, Melika & Stone, sp. nov., A. torreyaensis Melika & Abrahamson, sp. nov., A. williami Melika, Nicholls & Stone, sp. nov., Antron lovellae Melika, Nicholls & Stone, sp. nov., A.tomkursari Melika, Nicholls & Stone, sp. nov., Dryocosmus archboldi Melika & Abrahamson, sp. nov., Loxaulus virginianae Melika & Buss, sp. nov., Neuroterus alexandrae Nicholls & Melika, sp. nov., N. aliceae Melika, Nicholls & Stone, sp. nov., N. bussae Melika & Nicholls, sp. nov., N. oblongifoliae Nicholls, Stone & Melika, sp. nov., N. quaili Melika, Nicholls & Stone, sp. nov., N. rosieae Melika, Nicholls & Stone, sp. nov., N. stonei Melika & Nicholls, sp. nov., Zapatella abrahamsoni Melika, sp. nov., Z. brooksvillei Melika & Abrahamson, sp. nov.. Alternate asexual and sexual generations are described for four species, Andricus archboldi Melika & Abrahamson, sp. nov., A. fitzpatricki Melika & Abrahamson, sp. nov., A. schickae Nicholls, Melika & Stone, sp. nov., Neuroterus aliceae Melika, Nicholls & Stone, sp. nov.. Descriptions, diagnoses, plus information on biology and host associations are given for all new species. All taxa are supported by morphological data; matching of generations is established using DNA sequence data. We also demonstrate that Neuroterus niger var. alimas Kinsey should be considered as a nomen dubium.  

Development of microsatellite markers for an outbreaking species of oak gall wasp, Zapatella davisae (Hymenoptera: Cynipidae), in the northeastern United States

Populations of the recently described black oak gall wasp, Zapatella davisae Buffington (Hymenoptera: Cynipidae), have been identified as the cause of extensive tree damage and mortality to black oaks, Quercus velutina Lamarck (Fagales: Fagaceae), in the northeastern United States. Relatively little is known, however, about the distribution, phylogenetic placement, and lifecycle of this important tree pest. Therefore, we conducted next-generation sequencing using the Ion Torrent™ PGM (ThermoFisher Scientific, Inc.) platform to develop genomic resources for the study of Z. davisae and for other closely related species of oak gall wasps. Individual sequence reads were aligned, assembled into unique contigs, and the contigs were then utilized for the in silico isolation and development of microsatellite markers. In total, we screened 36 candidate microsatellite loci, of which 23 amplified consistently (five polymorphic and 18 monomorphic). We then examined whether the polymorphic loci could be used to infer whether populations of Z. davisae from Cape Cod and Nantucket are sexual or asexual by calculating several metrics of genetic diversity that might indicate the mode of reproduction. These included testing for statistical deviations from Hardy-Weinberg equilibrium (HWE) and for linkage disequilibrium (LD), observations for the presence of the Meselson effect, and by calculating the probability that clonal individuals are more prevalent than would be expected in a randomly mating population. While we found significant deviations from HWE and more clonal individuals than expected, our estimates of the Meselson effect were inconclusive due to limited sampling, and we found no evidence of LD. Therefore, the sexual/asexual status of Z. davisae populations remains uncertain.

Tracking of Host Defenses and Phylogeny During the Radiation of Neotropical Inga-Feeding Sawflies (Hymenoptera; Argidae)

Coevolutionary theory has long predicted that the arms race between plants and herbivores is a major driver of host selection and diversification. At a local scale, plant defenses contribute significantly to the structure of herbivore assemblages and the high alpha diversity of plants in tropical rain forests. However, the general importance of plant defenses in host associations and divergence at regional scales remains unclear. Here, we examine the role of plant defensive traits and phylogeny in the evolution of host range and species divergence in leaf-feeding sawflies of the family Argidae associated with Neotropical trees in the genus Inga throughout the Amazon, the Guiana Shield and Panama. Our analyses show that the phylogenies of both the sawfly herbivores and their Inga hosts are congruent, and that sawflies radiated at approximately the same time, or more recently than their Inga hosts. Analyses controlling for phylogenetic effects show that the evolution of host use in the sawflies associated with Inga is better correlated with Inga chemistry than with Inga phylogeny, suggesting a pattern of delayed host tracking closely tied to host chemistry. Finally, phylogenetic analyses show that sister species of Inga-sawflies are dispersed across the Neotropics, suggesting a role for allopatric divergence and vicariance in Inga diversification. These results are consistent with the idea that host defensive traits play a key role not only in structuring the herbivore assemblages at a single site, but also in the processes shaping host association and species divergence at a regional scale.

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.

Can asexuality confer a short-term advantage? Investigating apparent biogeographic success in the apomictic triploid fern Myriopteris gracilis

PREMISE OF THE STUDY

Although asexual taxa are generally seen as evolutionary dead ends, asexuality appears to provide a short-term benefit in some taxa, including a wider geographic distribution compared to sexual relatives. However, this may be an illusion created by multiple, morphologically cryptic, asexual lineages, each occupying a relatively small area. In this study we investigate the role of multiple lineages in the biogeography of Myriopteris gracilis Fée (Pteridaceae), a North American apomictic triploid fern species with a particularly large range.

METHODS

Range-wide asexuality was assessed by counting spores/sporangium in 606 Myriopteris gracilis specimens from across the species range, and lineage structure was assessed with both plastid DNA sequence and Genotyping By Sequencing (GBS) SNP datasets.

KEY RESULTS

Spore counting of >600 specimens identified no sexual populations, establishing that Myriopteris gracilis is exclusively asexual. The plastid data estimated the crown age of M. gracilis at ca. 2.5 mya and identified two lineages, each largely confined to the eastern or western portions of the range. These groups were further subdivided by the GBS data, revealing at least seven asexual lineages of varying geographic distributions, each occupying a relatively small portion of the total range of M. gracilis.

CONCLUSIONS

Although maintained exclusively through asexual reproduction, the broad distribution of Myriopteris gracilis is a compilation of numerous, independently formed asexual lineages. Since no single asexual lineage occupies the full extent of the species distribution, recurrent lineage formation should be considered when evaluating the short-term benefit of asexuality in this taxon and others.

Checklist of British and Irish Hymenoptera - Cynipoidea

BACKGROUND

The British and Irish checklist of Cynipoidea is revised, considerably updating the last complete checklist published in 1978. Disregarding uncertain identifications, 220 species are now known from Britain and Ireland, comprising 91 Cynipidae (including two established non-natives), 127 Figitidae and two Ibaliidae.

NEW INFORMATION

One replacement name is proposed, Kleidotoma thomsoni Forshage, for the secondary homonym Kleidotoma tetratoma Thomson, 1861 (nec K. tetratoma (Hartig, 1841)).

Barucynips panamensis , a new genus and species of oak gallwasps (Hymenoptera, Cynipidae, Cynipini) from Panama, and description of one new species of Coffeikokkos

Barucynips panamensis Medianero & Nieves-Aldrey, a new genus and species of oak gallwasps (Hymenoptera: Cynipidae: Cynipini), is described from adults reared from galls on Quercus bumelioides in Panama. The new genus is taxonomically close to the recently described Coffeikokkos from Costa Rica, but differs from it and all of the described genera of Cynipini, by the shape and setation of the projecting part of the ventral spine of the hypopygium and by the sculpture of the propodeum. A new species of Coffeikokkos is also described from the same area, the Volcán Barú in Panama. Diagnostic characters, gall description, distribution, and biological data of the new genus and the two new species are given. The new genus is the first genus of oak gallwasps of the tribe Cynipini described in Panama.

Host tracking or cryptic adaptation? Phylogeography of Pediobius saulius (Hymenoptera, Eulophidae), a parasitoid of the highly invasive horse-chestnut leafminer

Classical biological control is often advocated as a tool for managing invasive species. However, accurate evaluations of parasitoid species complexes and assessment of host specificity are impeded by the lack of morphological variation. Here, we study the possibility of host races/species within the eulophid wasp Pediobius saulius, a pupal generalist parasitoid that parasitize the highly invasive horse-chestnut leaf-mining moth Cameraria ohridella. We analysed the population genetic structure, host associations and phylogeographic patterns of P. saulius in Europe using the COI mitochondrial gene. This marker strongly supports a division into at least five highly differentiated parasitoid complexes, within two of which clades with differing degrees of host specialization were found: a Balkan clade that mainly (but not only) attacks C. ohridella and a more generalist European group that attacks many hosts, including C. ohridella. The divergence in COI (up to 7.6%) suggests the existence of cryptic species, although this is neither confirmed by nuclear divergence nor morphology. We do not find evidence of host tracking. The higher parasitism rates observed in the Balkans and the scarcity of the Balkan–Cameraria haplotypes out of the Balkans open the possibility of using these Balkan haplotypes as biological control agents of C. ohridella elsewhere in Europe.

Clonal genetic variation in a Wolbachia-infected asexual wasp: horizontal transmission or historical sex?

Wolbachia are endocellular bacteria known for manipulating the reproductive systems of many of their invertebrate hosts. Wolbachia are transmitted vertically from mother to offspring. In addition, new infections result from horizontal transmission between different host species. However, to what extent horizontal transmission plays a role in the spread of a new infection through the host population is unknown. Here, we investigate whether horizontal transmission of Wolbachia can explain clonal genetic variation in natural populations of Leptopilina clavipes, a parasitoid wasp infected with a parthenogenesis-inducing Wolbachia. We assessed variance of markers on the nuclear, mitochondrial and Wolbachia genomes. The nuclear and mitochondrial markers displayed significant and congruent variation among thelytokous wasp lineages, showing that multiple lineages have become infected with Wolbachia. The alternative hypothesis in which a single female became infected, the daughters of which mated with males (thus introducing nuclear genetic variance) cannot account for the presence of concordant variance in mtDNA. All Wolbachia markers, including the hypervariable wsp gene, were invariant, suggesting that only a single strain of Wolbachia is involved. These results show that Wolbachia has transferred horizontally to infect multiple female lineages during the early spread through L. clavipes. Remarkably, multiple thelytokous lineages have persisted side by side in the field for tens of thousands of generations.

The occurrence and phylogenetic implications of wing interference patterns in Cynipoidea (Insecta:Hymenoptera)

Wing interference patterns (WIPs) are a potentially rich source of taxonomic data. We surveyed a broad range of Cynipoidea (Hymenoptera) for the presence of these patterns. Further, we used phylogeny to investigate the evolutionary patterns of WIPs throughout the superfamily. We also documented that WIPs occur across nearly all species sampled, in both the hindwing and forewing; in many instances, the forewing WIP is carried over to the hindwing. Only Austrocynips mirabilis Riek lacked WIPs. Among species that possess WIPs, we were able to recognise four distinct categories: radiform (radiating), striatiform (longitudinally striate), campiform (continuous field) and galactiform (amorphous mixture of colours). Wing interference patterns are not found in infuscate wings or areas of infuscation on otherwise hyaline wings, and are reduced on large wings. Within Figitidae, WIPs are more phylogenetically conservative, especially at the tribal level: within Cynipidae the phylogenetic conservatism of WIPs is somewhat less clear, though all Synergini sampled had a very consistent WIPs. For certain genera, such as Ganaspidium and Andricus, WIPs may prove useful for species-level discrimination.

Phylogeny and DNA barcoding of inquiline oak gallwasps (Hymenoptera: Cynipidae) of the Western Palaearctic

We examine phylogenetic relationships within the Synergus complex of herbivorous inquiline gallwasps (Hymenoptera; Cynipidae; Synergini) associated with cynipid host galls on oak, a biologically diverse group whose genus-level morphological taxonomy has long been considered stable but whose species level taxonomy is problematic. We incorporate data for over 70% of recognised Western Palaearctic species in five morphology-based genera (Ceroptres, Saphonecrus, Synergus, Synophrus, Ufo), comprising sequence for two mitochondrial loci (coxI, cytb) and one nuclear locus (28S D2). In particular, we assess the evidence for monophyly of two long-established, morphology-defined sections within the genus Synergus that differ in a range of biological traits. To aid analyses of ecological interactions within oak cynipid communities, we also consider the utility of cytochrome oxidase I (coxI) DNA barcodes in the oak inquilines. In this assessment, we do not assume that species are delineated at a single threshold value of sequence divergence for a single gene, but examine concordance in the composition of molecular operational Taxonomic units (MOTUs) across a range of sequence divergences in each gene and across genes. We also assess the impact of sampling effort on MOTU stability. Phylogenetic reconstructions for all three loci support monophyly for Synergus and Synophrus, but reject monophyly for Saphonecrus and for the two sections within Synergus. The suites of traits associated with the two sections of the genus Synergus are thus homoplasious. All three loci also reject monophyly for three Synergus species (S. hayneanus, S. pallipes, S. umbraculus). Sequences for each locus identify robust MOTUs that are largely concordant across loci for a range of cut-off values. Though many MOTU's correspond to recognised Linnean species, there is significant, multigene disagreement between groupings supported by morphology and sequence data, with both allocation of different morphospecies to the same MOTU and allocation of the same morphospecies to multiple MOTUs, regardless of cut-off value. Our results imply that while DNA barcoding has considerable utility within this group, morphology-based identification needs major revision at both genus and species levels. Further, lifehistory traits currently attributed to single morphospecies probably confound attributes of multiple lineages. Revealing patterns of character state evolution in Synergus requires collection of new host association and life history data explicitly linked to DNA barcode data for the specimens concerned.

Host niches and defensive extended phenotypes structure parasitoid wasp communities

Oak galls are spectacular extended phenotypes of gallwasp genes in host oak tissues and have evolved complex morphologies that serve, in part, to exclude parasitoid natural enemies.

Parasitoids and their insect herbivore hosts have coevolved to produce diverse communities comprising about a third of all animal species. The factors structuring these communities, however, remain poorly understood. An emerging theme in community ecology is the need to consider the effects of host traits, shaped by both natural selection and phylogenetic history, on associated communities of natural enemies. Here we examine the impact of host traits and phylogenetic relatedness on 48 ecologically closed and species-rich communities of parasitoids attacking gall-inducing wasps on oaks. Gallwasps induce the development of spectacular and structurally complex galls whose species- and generation-specific morphologies are the extended phenotypes of gallwasp genes. All the associated natural enemies attack their concealed hosts through gall tissues, and several structural gall traits have been shown to enhance defence against parasitoid attack. Here we explore the significance of these and other host traits in predicting variation in parasitoid community structure across gallwasp species. In particular, we test the “Enemy Hypothesis,” which predicts that galls with similar morphology will exclude similar sets of parasitoids and therefore have similar parasitoid communities. Having controlled for phylogenetic patterning in host traits and communities, we found significant correlations between parasitoid community structure and several gall structural traits (toughness, hairiness, stickiness), supporting the Enemy Hypothesis. Parasitoid community structure was also consistently predicted by components of the hosts' spatiotemporal niche, particularly host oak taxonomy and gall location (e.g., leaf versus bud versus seed). The combined explanatory power of structural and spatiotemporal traits on community structure can be high, reaching 62% in one analysis. The observed patterns derive mainly from partial niche specialisation of highly generalist parasitoids with broad host ranges (>20 hosts), rather than strict separation of enemies with narrower host ranges, and so may contribute to maintenance of the richness of generalist parasitoids in gallwasp communities. Though evolutionary escape from parasitoids might most effectively be achieved via changes in host oak taxon, extreme conservatism in this trait for gallwasps suggests that selection is more likely to have acted on gall morphology and location. Any escape from parasitoids associated with evolutionary shifts in these traits has probably only been transient, however, due to subsequent recruitment of parasitoid species already attacking other host galls with similar trait combinations.

Herbivorous insects, such as the wasps that induce trees to make galls, and the parasitoids that attack (and ultimately kill) the wasps comprise about a third of all animal species, but it remains unclear what determines the structure of these complex coevolving communities. Here, we analyzed 48 parasitoid communities attacking different cynipid wasps that live and feed on oak trees. These communities are diverse and “closed,” with each centered upon the characteristic gall induced by a given cynipid wasp species. The often spectacular and complex galls are extended phenotypes of gallwasp genes and have been suggested to evolve as gallwasp defenses against their parasitoid enemies—“the Enemy Hypothesis.” Our analysis showed that similar parasitoid communities occurred on galls with similar structural traits (e.g., toughness, hairiness, stickiness), supporting the Enemy Hypothesis. We also found similar communities on galls that co-occur frequently in time and space; in particular, those occurring on the same oak species and same plant organ (e.g., leaf, bud, seed). Our results suggest that cynipid wasps might escape particular parasitoids via evolutionary shifts in the structure or location of their galls. However, escape may often be transient due to recruitment of new enemies already attacking other host galls with similar trait combinations.

Meiosis genes in Daphnia pulex and the role of parthenogenesis in genome evolution

Background

Thousands of parthenogenetic animal species have been described and cytogenetic manifestations of this reproductive mode are well known. However, little is understood about the molecular determinants of parthenogenesis. The Daphnia pulex genome must contain the molecular machinery for different reproductive modes: sexual (both male and female meiosis) and parthenogenetic (which is either cyclical or obligate). This feature makes D. pulex an ideal model to investigate the genetic basis of parthenogenesis and its consequences for gene and genome evolution. Here we describe the inventory of meiotic genes and their expression patterns during meiotic and parthenogenetic reproduction to help address whether parthenogenesis uses existing meiotic and mitotic machinery, or whether novel processes may be involved.

Results

We report an inventory of 130 homologs representing over 40 genes encoding proteins with diverse roles in meiotic processes in the genome of D. pulex. Many genes involved in cell cycle regulation and sister chromatid cohesion are characterized by expansions in copy number. In contrast, most genes involved in DNA replication and homologous recombination are present as single copies. Notably, RECQ2 (which suppresses homologous recombination) is present in multiple copies while DMC1 is the only gene in our inventory that is absent in the Daphnia genome. Expression patterns for 44 gene copies were similar during meiosis versus parthenogenesis, although several genes displayed marked differences in expression level in germline and somatic tissues.

Conclusion

We propose that expansions in meiotic gene families in D. pulex may be associated with parthenogenesis. Taking into account our findings, we provide a mechanistic model of parthenogenesis, highlighting steps that must differ from meiosis including sister chromatid cohesion and kinetochore attachment.

Fossil oak galls preserve ancient multitrophic interactions

Trace fossils of insect feeding have contributed substantially to our understanding of the evolution of insect-plant interactions. The most complex phenotypes of herbivory are galls, whose diagnostic morphologies often allow the identification of the gall inducer. Although fossil insect-induced galls over 300Myr old are known, most are two-dimensional impressions lacking adequate morphological detail either for the precise identification of the causer or for detection of the communities of specialist parasitoids and inquilines inhabiting modern plant galls. Here, we describe the first evidence for such multitrophic associations in Pleistocene fossil galls from the Eemian interglacial (130000-115000 years ago) of The Netherlands. The exceptionally well-preserved fossils can be attributed to extant species of Andricus gallwasps (Hymenoptera: Cynipidae) galling oaks (Quercus), and provide the first fossil evidence of gall attack by herbivorous inquiline gallwasps. Furthermore, phylogenetic placement of one fossil in a lineage showing obligate host plant alternation implies the presence of a second oak species, Quercus cerris, currently unknown from Eemian fossils in northwestern Europe. This contrasts with the southern European native range of Q. cerris in the current interglacial and suggests that gallwasp invasions following human planting of Q. cerris in northern Europe may represent a return to preglacial distribution limits.