Behavioral and genetic characteristics of a new species of Nasonia

New species based on the biological species concept within the complex of Lariophagus distinguendus (Hymenoptera, Chalcidoidea, Pteromalidae), a parasitoid of household pests

The pteromalid parasitoid Lariophagus distinguendus (Foerster) belongs to the Hymenoptera, a megadiverse insect order with high cryptic diversity. It attacks stored product pest beetles in human storage facilities. Recently, it has been shown to consist of two separate species. To further study its cryptic diversity, strains were collected to compare their relatedness using barcoding and nuclear genes. Nuclear genes identified two clusters which agree with the known two species, whereas the barcode fragment determined an additional third Clade. Total reproductive isolation (RI) according to the biological species concept (BSC) was investigated in crossing experiments within and between clusters using representative strains. Sexual isolation exists between all studied pairs, increasing from slight to strong with genetic distance. Postzygotic barriers mostly affected hybrid males, pointing to Haldane's rule. Hybrid females were only affected by unidirectional Spiroplasma-induced cytoplasmic incompatibility and behavioural sterility, each in one specific strain combination. RI was virtually absent between strains separated by up to 2.8% COI difference, but strong or complete in three pairs from one Clade each, separated by at least 7.2%. Apparently, each of these clusters represents one separate species according to the BSC, highlighting cryptic diversity in direct vicinity to humans. In addition, these results challenge the recent 'turbo-taxonomy' practice of using 2% COI differences to delimitate species, especially within parasitic Hymenoptera. The gradual increase in number and strength of reproductive barriers between strains with increasing genetic distance also sheds light on the emergence of barriers during the speciation process in L. distinguendus.

The son-killer microbe Arsenophonus nasoniae is a widespread associate of the parasitic wasp Nasonia vitripennis in Europe

Heritable microbes that exhibit reproductive parasitism are common in insects. One class of these are the male-killing bacteria, which are found in a broad range of insect hosts. Commonly, our knowledge of the incidence of these microbes is based on one or a few sampling sites, and the degree and causes of spatial variation are unclear. In this paper, we examine the incidence of the son-killer microbe Arsenophonus nasoniae across European populations of its wasp host, Nasonia vitripennis. In preliminary work, we noticed two female N. vitripennis producing highly female biased sex ratios in a field study from the Netherlands and Germany. When tested, the brood from Germany was revealed to be infected with A. nasoniae. We then completed a broad survey in 2012, in which fly pupal hosts of N. vitripennis were collected from vacated birds' nests from four European populations, N. vitripennis wasps allowed to emerge and then tested for A. nasoniae presence through PCR assay. We then developed a new screening methodology based on direct PCR assays of fly pupae and applied this to ethanol-preserved material collected from great tit (Parus major) nests in Portugal. These data show A. nasoniae is found widely in European N. vitripennis, being present in Germany, the UK, Finland, Switzerland and Portugal. Samples varied in the frequency with which they carry A. nasoniae, from being rare to being present in 50% of the pupae parasitised by N. vitripennis. Direct screening of ethanol-preserved fly pupae was an effective method for revealing both wasp and A. nasoniae infection, and will facilitate sample transport across national boundaries. Future research should examine the causes of variation in frequency, in particular testing the hypothesis that N. vitripennis superparasitism rates drive the variation in A. nasoniae frequency through providing opportunities for infectious transmission.

On the life cycle of Brachymeria podagrica (Fabricius, 1787) (Hymenoptera: Chalcididae) - a parasitoid of sacrophagid, calliphorid, and muscid flies

The lifecycle of Brachymeria podagrica, a parasitic wasp with a worldwide distribution, was studied under laboratory conditions using the flesh fly, Sarcophaga dux, as a host. Two hundred parasite-free 3rd instars of S. dux were exposed for 24 h to 20 female B. podagrica. In daily intervals, maggots and later pupae were examined for developmental stages of the parasitoid. The whole pre-imaginal development at a temperature of 26 °C lasted 21 to 26 days. Three morphologically different instars, followed by a prepupal and a pupal stage, were described using light and scanning electron microscopy. In a second experiment with 100 3rd stage Sarcophaga larvae and 10 parasitoids, a total of 70 wasps emerged 20 to 25 days after exposure. Two fly larvae did not pupate and dried out, while 28 pupae contained a dry or caseous content, dead wasp imagos, or their larval stages. No fly imagines emerged from exposed groups, while all 100 unexposed larvae pupated and adults eclosed between day 12 and day 14 after the start of the experiment, while the imagoes of the parasitoids appeared 8 to 12 days later.

Male sex pheromone in the parasitoid wasp Nasonia longicornis: Chemical and behavioral analyses

The use of sex pheromones for the attraction of potential mating partners is widespread in insects. Species-specificity of these chemical signals is essential, particularly in closely related species with overlapping habitats. In parasitoid wasps of the genus Nasonia, it is the males that produce sex pheromones in their rectal vesicles. The genus consists of four species: N. vitripennis (Nv), N. giraulti (Ng), N. oneida (No), and N. longicornis (Nl). The cosmopolitan species Nv is sympatric with Ng and No in eastern North America and with Nl in the west. Interspecific courtship is common in Nasonia although hybridization is prevented in most combinations by Wolbachia-mediated cytoplasmic incompatibility. The pheromone of Nv males differs from all Nasonia congeners by an additional component that is believed to ensure precopulatory isolation from the sympatric congeners. Detailed investigations on these interactions, however, exist only for the species combination Nv/Ng. Here, we report the results of chemical and behavioral investigations on the sex pheromone of Nl males. The pheromone consists of (4R,5S)-5-hydroxy-4-decanolide (RS) and 4-methylquinazoline (MQ) as a minor component which are produced only after eclosion. Pheromone titers peaked 2 days after eclosion and remained constant on day three. The pheromone is deposited by abdomen dipping which males increasingly exhibited after mating or brief contact with a female. The presence of hosts containing females about to emerge did not increase marking behavior. Site fidelity of males at their own pheromone markings is mediated by MQ. Both natural and synthetic pheromone attracted virgin but not mated females and both RS and MQ are required for female attraction. Females collected during emergence from the host responded likewise to the pheromone and 84% of them produced mixed-sex offspring showing that most of them emerge unmated. Nl females preferred pheromone extract from conspecific males to extract from Nv males, and the addition of the Nv-specific component (4R,5R)-5-hydroxy-4-decanolide (RR) to the Nl pheromone made them unresponsive. The present study demonstrates that Nl uses the male sex pheromone in a similar way as previously shown for Nv and Ng. Furthermore, it shows that Nl females use the Nv-specific pheromone component RR to avoid costly sexual interactions with Nv males in regions of sympatry.

A Margulian View of Symbiosis and Speciation: the Nasonia Wasp System

Species are fundamental units of biology that exemplify lineage diversification, while symbiosis of microbes and macrobial hosts exemplify lineage unification between the domains of life. While these conceptual differences between speciation and symbiosis often dominate the narrative of the respective fields, Lynn Margulis argued for interconnection between these two subdisciplines of biology in a manner that left a legacy for scholars and students alike to pursue, detail, and discover. The Margulian perspective has always been that host evolutionary processes such as speciation are more impacted by microbial symbioses than typically appreciated. In this article, we present and review the case system that she long envisioned, one in which layers of microbial symbiosis reduce species interbreeding and assist species diversification among a closely related group of small, metallic green, parasitoid wasps from the genus Nasonia.

Doublesex regulates male-specific differentiation during distinct developmental time windows in a parasitoid wasp

Sexually dimorphic traits in insects are subject to sexual selection, but our knowledge of the underlying molecular mechanisms is still scarce. Here we investigate how the highly conserved gene, Doublesex (Dsx), is involved in shaping sexual dimorphism in the model parasitoid wasp Nasonia vitripennis (Hymenoptera: Pteromalidae). First, we present the revised Dsx gene structure including an alternative transcription start, and two additional male NvDsx transcript isoforms. We show sex-specific NvDsx expression and splicing throughout development, and demonstrate that transient NvDsx silencing in different male developmental stages shifts two sexually dimorphic traits from male to female morphology, with the effect being dependent on the timing of silencing. In addition, we determined the effect of NvDsx on the development of reproductive organs. Transient silencing of NvDsx in early male larvae affects the growth and differentiation of the internal and external reproductive tissues. We did not observe phenotypic changes in females after NvDsx silencing. Our results indicate that male NvDsx is required to suppress female-specific traits and/or to promote male-specific traits during distinct developmental windows. This provides new insights into the regulatory activity of Dsx during male wasp development in the Hymenoptera.

Males of the parasitoid wasp, Nasonia vitripennis, can identify which fly hosts contain females

The reproductive success of a male is limited by the number of females it can mate with. Thus, males deploy elaborate strategies to maximize access to females. In Nasonia, which are parasitoids of cyclorrhaphous flies, such reproductive strategies are thought to be restricted to competition among males for access to females in the natal patch. This study investigates whether additional strategies are present, especially the capability to identify which fly hosts contain adult females inside. Behavioural assays revealed that only one out of the four species, N. vitripennis, can distinguish which hosts specifically have adult female wasps, indicating a species-specific reproductive strategy. Results of gas chromatography-mass spectrometry analyses and behavioural data suggest that female-signature cuticular hydrocarbons (CHCs) are used as chemical cues, possibly emanating from within the host puparium. Further assays indicated that N. vitripennis males can also detect differences in the intensities of female-signature CHCs, giving them the capability to seek out hosts with maximum number of females. This study uncovers a previously unknown reproductive strategy in one of the most widely studied parasitoid wasps.

Genetic, morphometric, and molecular analyses of interspecies differences in head shape and hybrid developmental defects in the wasp genus Nasonia

Males in the parasitoid wasp genus Nasonia have distinct, species-specific, head shapes. The availability of fertile hybrids among the species, along with obligate haploidy of males, facilitates analysis of complex gene interactions in development and evolution. Previous analyses showed that both the divergence in head shape between Nasonia vitripennis and Nasonia giraulti, and the head-specific developmental defects of F2 haploid hybrid males, are governed by multiple changes in networks of interacting genes. Here, we extend our understanding of the gene interactions that affect morphogenesis in male heads. Use of artificial diploid male hybrids shows that alleles mediating developmental defects are recessive, while there are diverse dominance relationships among other head shape traits. At the molecular level, the sex determination locus doublesex plays a major role in male head shape differences, but it is not the only important factor. Introgression of a giraulti region on chromsome 2 reveals a recessive locus that causes completely penetrant head clefting in both males and females in a vitripennis background. Finally, a third species (N. longicornis) was used to investigate the timing of genetic changes related to head morphology, revealing that most changes causing defects arose after the divergence of N. vitripennis from the other species, but prior to the divergence of N. giraulti and N. longicornis from each other. Our results demonstrate that developmental gene networks can be dissected using interspecies crosses in Nasonia, and set the stage for future fine-scale genetic dissection of both head shape and hybrid developmental defects.

The emergence of ecotypes in a parasitoid wasp: a case of incipient sympatric speciation in Hymenoptera?

Background

To understand which reproductive barriers initiate speciation is a major question in evolutionary research. Despite their high species numbers and specific biology, there are only few studies on speciation in Hymenoptera. This study aims to identify very early reproductive barriers in a local, sympatric population of Nasonia vitripennis (Walker 1836), a hymenopterous parasitoid of fly pupae. We studied ecological barriers, sexual barriers, and the reduction in F1-female offspring as a postmating barrier, as well as the population structure using microsatellites.

Results

We found considerable inbreeding within female strains and a population structure with either three or five subpopulation clusters defined by microsatellites. In addition, there are two ecotypes, one parasitizing fly pupae in bird nests and the other on carrion. The nest ecotype is mainly formed from one of the microsatellite clusters, the two or four remaining microsatellite clusters form the carrion ecotype. There was slight sexual isolation and a reduction in F1-female offspring between inbreeding strains from the same microsatellite clusters and the same ecotypes. Strains from different microsatellite clusters are separated by a reduction in F1-female offspring. Ecotypes are separated only by ecological barriers.

Conclusions

This is the first demonstration of very early reproductive barriers within a sympatric population of Hymenoptera. It demonstrates that sexual and premating barriers can precede ecological separation. This indicates the complexity of ecotype formation and highlights the general need for more studies within homogenous populations for the identification of the earliest barriers in the speciation process.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01938-y.

Courtship behavior and identification of a sex pheromone in Ibalia leucospoides (Hymenoptera: Ibaliidae), a larval parasitoid of Sirex noctilio (Hymenoptera: Siricidae)

BACKGROUND

Ibalia leucospoides (Hymenoptera: Ibaliidae) is a larval parasitoid that has been widely introduced as a biological control agent for the invasive woodwasp,Sirex noctilio (Hymenoptera: Siricidae) in the Southern Hemisphere. In this study, the courtship behavior and identificaion of sex pheromones are described for I. leucospoides under laboratory conditions.

METHODS

For courtship behavior, both sexes were observed in a wire mesh observation cylinder (75 cm length ×10 cm diameter) for 15 minutes. The female body washes were analyzed using Gas Chromatography- Electroantennographic Detection (GC-EAD). Then the EAD-active compounds were tentatively identified using GC-Mass Spectrometry (GC-MS) and examined in olfactometer assays.

RESULTS

The courtship behavior included rhythmic lateral movements, mounting, head-nodding cycles in males, and wing-fanning in females. GC-EAD analysis of female body washes with male antennae revealed seven compounds which elicited antennal responses, four of which are straight-chain alkanes (C23, C25, C26, and C27). The identities of these alkanes were confirmed by matching the retention times, mass spectra, and male antennal activity to those of commercially obtained chemicals. In olfactometer assays, a blend of the four straight-chain alkanes was attractive to I. leucospoides males, and there was no response to blends that lacked any of these four compounds. Female body wash was no more attractive than the four-component blend. The ratios of EAD-active components differ between hydrocarbon profiles from males and females.

CONCLUSION

This study is the first investigation of cuticular hydrocarbons in the family Ibaliidae. It provides evidence that the ubiquitous alkanes (C23, C25, C26, and C27) in sex-specific ratios attract I. leucospoides males.

Acetone application for administration of bioactive substances has no negative effects on longevity, fitness, and sexual communication in a parasitic wasp

Administration of defined amounts of bioactive substances is a perseverative problem in physiological studies on insects. Apart from feeding and injection, topical application of solutions of the chemicals is most commonly used for this purpose. The solvents used should be non-toxic and have least possible effects on the studied parameters. Acetone is widely used for administration of chemical substances to insects, but possible side-effects of acetone application on fitness and behavioral parameters have been rarely investigated. Here we study the effects of acetone application (207 nl) on fitness and sexual communication in the parasitic wasp Nasonia giraulti Darling. Application of acetone had neither negative effects on longevity nor on offspring number and offspring sex ratio of treated wasps. Treatment of females hampered courtship and mating of N. giraulti couples neither directly after application nor one day after. Male sex pheromone titers were not influenced by acetone treatment. Three application examples demonstrate that topical acetone application is capable of bringing active amounts of insect hormones, neuromodulators, and biosynthetic precursors even in tiny insects. We advocate the use of acetone as a convenient, conservative, and broadly applicable vehicle for studying the effects of bioactive substances in insects.

Comparative analysis reveals the expansion of mitochondrial DNA control region containing unusually high G-C tandem repeat arrays in Nasonia vitripennis

Insect mitochondrial DNA (mtDNA) ranges from 14 to 19 kbp, and the size difference is attributed to the AT-rich control region. Jewel wasps have a parasitoid lifestyle, which may affect mitochondria function and evolution. We sequenced, assembled, and annotated mitochondrial genomes in Nasonia and outgroup species. Gene composition and order are conserved within Nasonia, but they differ from other parasitoids by two large inversion events that were not reported before. We observed a much higher substitution rate relative to the nuclear genome and mitochondrial introgression between N. giraulti and N. oneida, which is consistent with previous studies. Most strikingly, N. vitripennis mtDNA has an extremely long control region (7665 bp), containing twenty-nine 217 bp tandem repeats and can fold into a super-cruciform structure. In contrast to tandem repeats commonly found in other mitochondria, these high-copy repeats are highly conserved (98.7% sequence identity), much longer in length (approximately 8 Kb), extremely GC-rich (50.7%), and CpG-rich (percent CpG 19.4% vs. 1.1% in coding region), resulting in a 23 kbp mtDNA beyond the typical size range in insects. These N. vitripennis-specific mitochondrial repeats are not related to any known sequences in insect mitochondria. Their evolutionary origin and functional consequences warrant further investigations.

Genome Report: Whole Genome Sequence and Annotation of the Parasitoid Jewel Wasp Nasonia giraulti Laboratory Strain RV2X[u]

Jewel wasps in the genus of Nasonia are parasitoids with haplodiploidy sex determination, rapid development and are easy to culture in the laboratory. They are excellent models for insect genetics, genomics, epigenetics, development, and evolution. Nasonia vitripennis (Nv) and N. giraulti (Ng) are closely-related species that can be intercrossed, particularly after removal of the intracellular bacterium Wolbachia, which serve as a powerful tool to map and positionally clone morphological, behavioral, expression and methylation phenotypes. The Nv reference genome was assembled using Sanger, PacBio and Nanopore approaches and annotated with extensive RNA-seq data. In contrast, Ng genome is only available through low coverage resequencing. Therefore, de novo Ng assembly is in urgent need to advance this system. In this study, we report a high-quality Ng assembly using 10X Genomics linked-reads with 670X sequencing depth. The current assembly has a genome size of 259,040,977 bp in 3,160 scaffolds with 38.05% G-C and a 98.6% BUSCO completeness score. 97% of the RNA reads are perfectly aligned to the genome, indicating high quality in contiguity and completeness. A total of 14,777 genes are annotated in the Ng genome, and 72% of the annotated genes have a one-to-one ortholog in the Nv genome. We reported 5 million Ng-Nv SNPs which will facility mapping and population genomic studies in Nasonia In addition, 42 Ng-specific genes were identified by comparing with Nv genome and annotation. This is the first de novo assembly for this important species in the Nasonia model system, providing a useful new genomic toolkit.

Phylosymbiosis Impacts Adaptive Traits in Nasonia Wasps

Phylosymbiosis is defined as microbial community relationships that recapitulate the phylogeny of hosts. As evidence for phylosymbiosis rapidly accumulates in different vertebrate and invertebrate holobionts, a central question is what evolutionary forces cause this pattern. We use intra- and interspecific gut microbiota transplants to test for evidence of selective pressures that contribute to phylosymbiosis. We leverage three closely related species of the parasitoid wasp model Nasonia that recently diverged between 0.4 and 1 million years ago: N. vitripennis, N. giraulti, and N. longicornis Upon exposure of germfree larvae to heat-inactivated microbiota from intra- or interspecific larvae, we measure larval growth, pupation rate, and adult reproductive capacity. We report three key findings: (i) larval growth significantly slows when hosts receive an interspecific versus intraspecific gut microbiota, (ii) marked decreases in pupation and resulting adult survival occur from interspecific gut microbiota exposure, and (iii) adult reproductive capacities including male fertility and longevity are unaffected by early life exposure to an interspecific microbiota. Overall, these findings reveal developmental and survival costs to Nasonia upon larval exposures to interspecific microbiota and provide evidence that selective pressures on phenotypes produced by host-microbiota interactions may underpin phylosymbiosis.IMPORTANCE Phylosymbiosis is an ecoevolutionary hypothesis and emerging pattern in animal-microbiota studies whereby the host phylogenetic relationships parallel the community relationships of the host-associated microbiota. A central prediction of phylosymbiosis is that closely related hosts exhibit a lower microbiota beta diversity than distantly related hosts. While phylosymbiosis has emerged as a widespread trend in a field often challenged to find trends across systems, two critical and understudied questions are whether or not phylosymbiosis is consequential to host biology and if adaptive evolutionary forces underpin the pattern. Here, using germfree rearing in the phylosymbiosis model Nasonia, we demonstrate that early life exposure to heat-inactivated microbiota from more distantly related species poses more severe developmental and survival costs than microbiota from closely related or the same species. This study advances a functional understanding of the consequences and potential selective pressures underpinning phylosymbiosis.

Functional characterisation of two Δ12-desaturases demonstrates targeted production of linoleic acid as pheromone precursor in Nasonia

Insect pheromones are often derived from fatty acid metabolism. Fatty acid desaturases, enzymes introducing double bonds into fatty acids, are crucial for the biosynthesis of these chemical signals. Δ12-desaturases catalyse the biosynthesis of linoleic acid by introducing a second double bond into oleic acid, but have been identified in only a few animal species. Here, we report the functional characterisation of two Δ12-desaturases, Nvit_D12a and Nvit_D12b, from the parasitic wasp Nasonia vitripennis. We demonstrate that Nvit_D12a is expressed in the rectal vesicle of males where they produce a linoleic acid-derived sex pheromone to attract virgin females. ¹³C-labelling experiments with Urolepis rufipes, a closely related species belonging to the 'Nasonia group', revealed that females, but not males, are able to synthesise linoleic acid. U. rufipes males produce an isoprenoid sex pheromone in the same gland and do not depend on linoleic acid for pheromone production. This suggests that Δ12-desaturases are common in the 'Nasonia group', but acquired a specialised function in chemical communication of those species that use linoleic acid as a pheromone precursor. Phylogenetic analysis suggests that insect Δ12-desaturases have evolved repeatedly from Δ9-desaturases in different insect taxa. Hence, insects have developed a way to produce linoleic acid independent of the omega desaturase subfamily which harbours all of the eukaryotic Δ12-desaturases known so far.

Mapping key amino acid residues for the epimerase efficiency and stereospecificity of the sex pheromone biosynthetic short-chain dehydrogenases/reductases of Nasonia

Males of the parasitic wasp genus Nasonia use blends of chiral hydroxylactones as sex pheromones to attract conspecific females. Whereas all Nasonia species use a mixture of (4R,5S)-5-hydroxy-4-decanolide (RS) and 4-methylquinazoline (MQ) as sex pheromones, Nasonia vitripennis evolved (4R,5R)-5-hydroxy-4-decanolide (RR) as an extra sex pheromone component. We recently identified and functionally characterized three short-chain dehydrogenases/reductases (SDRs) NV10127, NV10128, and NV10129 that are capable of catalyzing the epimerization of RS to RR via (4R)-5-oxo-4-decanolide (ODL) as intermediate. Despite their very high sequence identities of 88-98%, these proteins differ drastically in their ability to epimerize RS to RR and in their stereoselectivity when reducing ODL to RR/RS. Here, in order to unravel the sequence differences underlying these varying functional properties of NV1027, NV10128 and NV10129, we created chimeras of the three enzymes and monitored their catalytic activities in vitro. The results show that a few amino acid changes at the C-termini and active sites of Nasonia vitripennis SDRs lead to substantially altered RS to RR epimerization and ODL-reduction activities. Thus, our study adds to the understanding of pheromone evolution by showing that subtle mutations in key biosynthetic enzymes can result in drastic effects on the composition of chemical signals.

A framework for estimating the effects of sequential reproductive barriers: Implementation using Bayesian models with field data from cryptic species

Determining how reproductive barriers modulate gene flow between populations represents a major step toward understanding the factors shaping the course of speciation. Although many indices quantifying reproductive isolation (RI) have been proposed, they do not permit the quantification of cross-direction-specific RI under varying species frequencies and over arbitrary sequences of barriers. Furthermore, techniques quantifying associated uncertainties are lacking, and statistical methods unrelated to biological process are still preferred for obtaining confidence intervals and P-values. To address these shortcomings, we provide new RI indices that model changes in gene flow for both directions of hybridization, and we implement them in a Bayesian model. We use this model to quantify RI between two species of the psyllid Cacopsylla pruni based on field genotypic data for mating individuals, inseminated spermatophores and progeny. The results showed that preinsemination isolation was strong, mildly asymmetric, and indistinguishably different between study sites despite large differences in species frequencies; that postinsemination isolation strongly affected the more common hybrid type; and that cumulative isolation was close to complete. In the light of these results, we discuss how these developments can strengthen comparative RI studies.

Parasitoid biology preserved in mineralized fossils

About 50% of all animal species are considered parasites. The linkage of species diversity to a parasitic lifestyle is especially evident in the insect order Hymenoptera. However, fossil evidence for host–parasitoid interactions is extremely rare, rendering hypotheses on the evolution of parasitism assumptive. Here, using high-throughput synchrotron X-ray microtomography, we examine 1510 phosphatized fly pupae from the Paleogene of France and identify 55 parasitation events by four wasp species, providing morphological and ecological data. All species developed as solitary endoparasitoids inside their hosts and exhibit different morphological adaptations for exploiting the same hosts in one habitat. Our results allow systematic and ecological placement of four distinct endoparasitoids in the Paleogene and highlight the need to investigate ecological data preserved in the fossil record.

Evidence for a parasitic lifestyle in extinct species tends to be indirect. Here, the authors provide direct evidence through X-ray examination of approximately 30–40 million year old fossil fly pupae, revealing 55 parasitation events by four newly described wasp species.

The Maternal Effect Gene Wds Controls Wolbachia Titer in Nasonia

Maternal transmission of intracellular microbes is pivotal in establishing long-term, intimate symbioses. For germline microbes that exert negative reproductive effects on their hosts, selection can theoretically favor the spread of host genes that counteract the microbe's harmful effects. Here, we leverage a major difference in bacterial (Wolbachia pipientis) titers between closely related wasp species with forward genetic, transcriptomic, and cytological approaches to map two quantitative trait loci that suppress bacterial titers via a maternal effect. Fine mapping and knockdown experiments identify the gene Wolbachia density suppressor (Wds), which dominantly suppresses bacterial transmission from mother to embryo. Wds evolved by lineage-specific non-synonymous changes driven by positive selection. Collectively, our findings demonstrate that a genetically simple change arose by positive Darwinian selection in less than a million years to regulate maternally transmitted bacteria via a dominant, maternal effect gene.

Differences in the reliance on cuticular hydrocarbons as sexual signaling and species discrimination cues in parasitoid wasps

Background

Cuticular hydrocarbons (CHC) have been documented to play crucial roles as species- and sex-specific cues in the chemical communication systems of a wide variety of insects. However, whether they are sufficient by themselves as the sole cue triggering sexual behavior as well as preference of con- over heterospecific mating partners is rarely assessed. We conducted behavioral assays in three representative species of parasitoid wasps (Hymenoptera: Pteromalidae) to determine their reliance on CHC as species-specific sexual signaling cues.

Results

We found a surprising degree of either unspecific or insufficient sexual signaling when CHC are singled out as recognition cues. Most strikingly, the cosmopolitan species Nasonia vitripennis, expected to experience enhanced selection pressure to discriminate against other co-occurring parasitoids, did not discriminate against CHC of a partially sympatric species from another genus, Trichomalopsis sarcophagae. Focusing on the latter species, in turn, it became apparent that CHC are even insufficient as the sole cue triggering conspecific sexual behavior, hinting at the requirement of additional, synergistic sexual cues particularly important in this species. Finally, in the phylogenetically and chemically most divergent species Muscidifurax uniraptor, we intriguingly found both CHC-based sexual signaling as well as species discrimination behavior intact although this species is naturally parthenogenetic with sexual reproduction only occurring under laboratory conditions.

Conclusions

Our findings implicate a discrepancy in the reliance on and specificity of CHC as sexual cues in our tested parasitioid wasps. CHC profiles were not sufficient for unambiguous discrimination and preference behavior, as demonstrated by clear cross-attraction between some of our tested wasp genera. Moreover, we could show that only in T. sarcophagae, additional behavioral cues need to be present for triggering natural mating behavior, hinting at an interesting shift in signaling hierarchy in this particular species. This demonstrates the importance of integrating multiple, potentially complementary signaling modalities in future studies for a better understanding of their individual contributions to natural sexual communication behavior.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0263-z) contains supplementary material, which is available to authorized users.

The Evolution of Venom by Co-option of Single-Copy Genes

The classic model for the evolution of novel gene function is through gene duplication followed by evolution of a new function by one of the copies (neofunctionalization) [1, 2]. However, other modes have also been found, such as novel genes arising from non-coding DNA, chimeric fusions, and lateral gene transfers from other organisms [3-7]. Here we use the rapid turnover of venom genes in parasitoid wasps to study how new gene functions evolve. In contrast to the classic gene duplication model, we find that a common mode of acquisition of new venom genes in parasitoid wasps is co-option of single-copy genes from non-venom progenitors. Transcriptome and proteome sequencing reveal that recruitment and loss of venom genes occur primarily by rapid cis-regulatory expression evolution in the venom gland. Loss of venom genes is primarily due to downregulation of expression in the gland rather than gene death through coding sequence degradation. While the majority of venom genes have specialized expression in the venom gland, recent losses of venom function occur primarily among genes that show broader expression in development, suggesting that they can more readily switch functional roles. We propose that co-option of single-copy genes may be a common but relatively understudied mechanism of evolution for new gene functions, particularly under conditions of rapid evolutionary change.

Establishment of F1 hybrid mortality in real time

Background

Measuring the evolutionary rate of reproductive isolation is essential to understanding how new species form. Tempo calculations typically rely on fossil records, geological events, and molecular evolution analyses. The speed at which genetically-based hybrid mortality arises, or the “incompatibility clock”, is estimated to be millions of years in various diploid organisms and is poorly understood in general. Owing to these extended timeframes, seldom do biologists observe the evolution of hybrid mortality in real time.

Results

Here we report the very recent spread and fixation of complete asymmetric F₁ hybrid mortality within eight years of laboratory maintenance in the insect model Nasonia. The asymmetric interspecific hybrid mortality evolved in an isogenic stock line of N. longicornis and occurs in crosses to N. vitripennis males. The resulting diploid hybrids exhibit complete failure in dorsal closure during embryogenesis.

Conclusion

These results comprise a unique case whereby a strong asymmetrical isolation barrier evolved in real time. The spread of this reproductive isolation barrier notably occurred in a small laboratory stock subject to recurrent bottlenecks.

Epimerisation of chiral hydroxylactones by short-chain dehydrogenases/reductases accounts for sex pheromone evolution in Nasonia

Males of all species of the parasitic wasp genus Nasonia use (4R,5S)-5-hydroxy-4-decanolide (RS) as component of their sex pheromone while only N. vitripennis (Nv), employs additionally (4R,5R)-5-hydroxy-4-decanolide (RR). Three genes coding for the NAD⁺-dependent short-chain dehydrogenases/reductases (SDRs) NV10127, NV10128, and NV10129 are linked to the ability of Nv to produce RR. Here we show by assaying recombinant enzymes that SDRs from both Nv and N. giraulti (Ng), the latter a species with only RS in the pheromone, epimerise RS into RR and vice versa with (4R)-5-oxo-4-decanolide as an intermediate. Nv-derived SDR orthologues generally had higher epimerisation rates, which were also influenced by NAD⁺ availability. Semiquantitative protein analyses of the pheromone glands by tandem mass spectrometry revealed that NV10127 as well as NV10128 and/or NV10129 were more abundant in Nv compared to Ng. We conclude that the interplay of differential expression patterns and SDR epimerisation rates on the ancestral pheromone component RS accounts for the evolution of a novel pheromone phenotype in Nv.

Disentangling a Holobiont - Recent Advances and Perspectives in Nasonia Wasps

The parasitoid wasp genus Nasonia (Hymenoptera: Chalcidoidea) is a well-established model organism for insect development, evolutionary genetics, speciation, and symbiosis. The host-microbiota assemblage which constitutes the Nasonia holobiont (a host together with all of its associated microbes) consists of viruses, two heritable bacterial symbionts and a bacterial community dominated in abundance by a few taxa in the gut. In the wild, all four Nasonia species are systematically infected with the obligate intracellular bacterium Wolbachia and can additionally be co-infected with Arsenophonus nasoniae. These two reproductive parasites have different transmission modes and host manipulations (cytoplasmic incompatibility vs. male-killing, respectively). Pioneering studies on Wolbachia in Nasonia demonstrated that closely related Nasonia species harbor multiple and mutually incompatible Wolbachia strains, resulting in strong symbiont-mediated reproductive barriers that evolved early in the speciation process. Moreover, research on host-symbiont interactions and speciation has recently broadened from its historical focus on heritable symbionts to the entire microbial community. In this context, each Nasonia species hosts a distinguishable community of gut bacteria that experiences a temporal succession during host development and members of this bacterial community cause strong hybrid lethality during larval development. In this review, we present the Nasonia species complex as a model system to experimentally investigate questions regarding: (i) the impact of different microbes, including (but not limited to) heritable endosymbionts, on the extended phenotype of the holobiont, (ii) the establishment and regulation of a species-specific microbiota, (iii) the role of the microbiota in speciation, and (iv) the resilience and adaptability of the microbiota in wild populations subjected to different environmental pressures. We discuss the potential for easy microbiota manipulations in Nasonia as a promising experimental approach to address these fundamental aspects.

OGS2: genome re-annotation of the jewel wasp Nasonia vitripennis

BACKGROUND

Nasonia vitripennis is an emerging insect model system with haplodiploid genetics. It holds a key position within the insect phylogeny for comparative, evolutionary and behavioral genetic studies. The draft genomes for N. vitripennis and two sibling species were published in 2010, yet a considerable amount of transcriptiome data have since been produced thereby enabling improvements to the original (OGS1.2) annotated gene set. We describe and apply the EvidentialGene method used to produce an updated gene set (OGS2). We also carry out comparative analyses showcasing the usefulness of the revised annotated gene set.

RESULTS

The revised annotation (OGS2) now consists of 24,388 genes with supporting evidence, compared to 18,850 for OGS1.2. Improvements include the nearly complete annotation of untranslated regions (UTR) for 97 % of the genes compared to 28 % of genes for OGS1.2. The fraction of RNA-Seq validated introns also grow from 85 to 98 % in this latest gene set. The EST and RNA-Seq expression data provide support for several non-protein coding loci and 7712 alternative transcripts for 4146 genes. Notably, we report 180 alternative transcripts for the gene lola. Nasonia now has among the most complete insect gene set; only 27 conserved single copy orthologs in arthropods are missing from OGS2. Its genome also contains 2.1-fold more duplicated genes and 1.4-fold more single copy genes than the Drosophila melanogaster genome. The Nasonia gene count is larger than those of other sequenced hymenopteran species, owing both to improvements in the genome annotation and to unique genes in the wasp lineage. We identify 1008 genes and 171 gene families that deviate significantly from other hymenopterans in their rates of protein evolution and duplication history, respectively. We also provide an analysis of alternative splicing that reveals that genes with no annotated isoforms are characterized by shorter transcripts, fewer introns, faster protein evolution and higher probabilities of duplication than genes having alternative transcripts.

CONCLUSIONS

Genome-wide expression data greatly improves the annotation of the N. vitripennis genome, by increasing the gene count, reducing the number of missing genes and providing more comprehensive data on splicing and gene structure. The improved gene set identifies lineage-specific genomic features tied to Nasonia's biology, as well as numerous novel genes. OGS2 and its associated search tools are available at http://arthropods.eugenes.org/EvidentialGene/nasonia/ , www.hymenopteragenome.org/nasonia/ and waspAtlas: www.tinyURL.com/waspAtlas . The EvidentialGene pipeline is available at https://sourceforge.net/projects/evidentialgene/ .

An optimized approach to germ-free rearing in the jewel wasp Nasonia

Development of a Nasonia in vitrogerm-free rearing system in 2012 enabled investigation of Nasonia-microbiota interactions and real-time visualization of parasitoid metamorphosis. However, the use of antibiotics, bleach, and fetal bovine serum introduced artifacts relative to conventional rearing of Nasonia. Here, we optimize the germ-free rearing procedure by using filter sterilization in lieu of antibiotics and by removing residual bleach and fetal bovine serum. Comparison of these methods reveals no influence on larval survival or growth, and a 52% improvement in adult production. Additionally, adult males produced in the new germ-free system are similar in size to conventionally reared males. Experimental implications of these changes are discussed.

Quantitative Trait Locus Analysis of Mating Behavior and Male Sex Pheromones in Nasonia Wasps

A major focus in speciation genetics is to identify the chromosomal regions and genes that reduce hybridization and gene flow. We investigated the genetic architecture of mating behavior in the parasitoid wasp species pair Nasonia giraulti and Nasonia oneida that exhibit strong prezygotic isolation. Behavioral analysis showed that N. oneida females had consistently higher latency times, and broke off the mating sequence more often in the mounting stage when confronted with N. giraulti males compared with males of their own species. N. oneida males produce a lower quantity of the long-range male sex pheromone (4R,5S)-5-hydroxy-4-decanolide (RS-HDL). Crosses between the two species yielded hybrid males with various pheromone quantities, and these males were used in mating trials with females of either species to measure female mate discrimination rates. A quantitative trait locus (QTL) analysis involving 475 recombinant hybrid males (F₂), 2148 reciprocally backcrossed females (F₃), and a linkage map of 52 equally spaced neutral single nucleotide polymorphism (SNP) markers plus SNPs in 40 candidate mating behavior genes revealed four QTL for male pheromone amount, depending on partner species. Our results demonstrate that the RS-HDL pheromone plays a role in the mating system of N. giraulti and N. oneida, but also that additional communication cues are involved in mate choice. No QTL were found for female mate discrimination, which points at a polygenic architecture of female choice with strong environmental influences.

The expanding genetic toolbox of the wasp Nasonia vitripennis and its relatives

The parasitoid wasp Nasonia represents a genus of four species that is emerging as a powerful genetic model system that has made and will continue to make important contributions to our understanding of evolutionary biology, development, ecology, and behavior. Particularly powerful are the haplodiploid genetics of the system, which allow some of the advantages of microbial genetics to be applied to a complex multicellular eukaryote. In addition, fertile, viable hybrids can be made among the four species in the genus. This makes Nasonia exceptionally well suited for evolutionary genetics approaches, especially when combined with its haploid genetics and tractability in the laboratory. These features are complemented by an expanding array of genomic, transcriptomic, and functional resources, the application of which has already made Nasonia an important model system in such emerging fields as evolutionary developmental biology and microbiomics. This article describes the genetic and genomic advantages of Nasonia wasps and the resources available for their genetic analysis.

Pheromone diversification and age-dependent behavioural plasticity decrease interspecific mating costs in Nasonia

Interspecific mating can cause severe fitness costs due to the fact that hybrids are often non-viable or less fit. Thus, theory predicts the selection of traits that lessen reproductive interactions between closely related sympatric species. Males of the parasitic wasp Nasonia vitripennis differ from all other Nasonia species by an additional sex pheromone component, but the ecological selective forces underlying this pheromone diversification are unknown. Here we present data from lab experiments suggesting that costly interspecific sexual interactions with the sympatric species N. giraulti might have been responsible for the pheromone evolution and some courtship-related behavioural adaptations in N. vitripennis. Most N. giraulti females are inseminated already within the host, but N. giraulti males still invest in costly sex pheromones after emergence. Furthermore, they do not discriminate between N. vitripennis females and conspecifics during courtship. Therefore, N. vitripennis females, most of which emerge as virgins, face the risk of mating with N. giraulti resulting in costly all-male broods due to Wolbachia-induced cytoplasmic incompatibility. As a counter adaptation, young N. vitripennis females discriminate against N. giraulti males using the more complex conspecific sex pheromone and reject most of them during courtship. With increasing age, however, N. vitripennis females become less choosy, but often compensate mating errors by re-mating with a conspecific. By doing so, they can principally avoid suboptimal offspring sex ratios, but a microcosm experiment suggests that under more natural conditions N. vitripennis females cannot completely avoid fitness costs due to heterospecific mating. Our study provides support for the hypothesis that communication interference of closely related sympatric species using similar sexual signals can generate selective pressures that lead to their divergence.

Development of a Nasonia vitripennis outbred laboratory population for genetic analysis

The parasitoid wasp genus Nasonia has rapidly become a genetic model system for developmental and evolutionary biology. The release of its genome sequence led to the development of high-resolution genomic tools, for both interspecific and intraspecific research, which has resulted in great advances in understanding Nasonia biology. To further advance the utility of Nasonia vitripennis as a genetic model system and to be able to fully exploit the advantages of its fully sequenced and annotated genome, we developed a genetically variable and well-characterized experimental population. In this study, we describe the establishment of the genetically diverse HVRx laboratory population from strains collected from the field in the Netherlands. We established a maintenance method that retains genetic variation over generations of culturing in the laboratory. As a characterization of its genetic composition, we provide data on the standing genetic variation and estimate the effective population size (N(e)) by microsatellite analysis. A genome-wide description of polymorphism is provided through pooled resequencing, which yielded 417,331 high-quality SNPs spanning all five Nasonia chromosomes. The HVRx population and its characterization are freely available as a community resource for investigators seeking to elucidate the genetic basis of complex trait variation using the Nasonia model system.

Cuticular hydrocarbon divergence in the jewel wasp Nasonia: evolutionary shifts in chemical communication channels?

The evolution and maintenance of intraspecific communication channels constitute a key feature of chemical signalling and sexual communication. However, how divergent chemical communication channels evolve while maintaining their integrity for both sender and receiver is poorly understood. In this study, we compare male and female cuticular hydrocarbon (CHC) profiles in the jewel wasp genus Nasonia, analyse their chemical divergence and investigate their role as species-specific sexual signalling cues. Males and females of all four Nasonia species showed unique, nonoverlapping CHC profiles unambiguously separating them. Surprisingly, male and female phylogenies based on the chemical distances between their CHC profiles differed dramatically, where only male CHC divergence parallels the molecular phylogeny of Nasonia. In particular, N. giraulti female CHC profiles were the most divergent from all other species and very different from its most closely related sibling species N. oneida. Furthermore, although our behavioural assays indicate that female CHC profiles can generally be perceived as sexual cues attracting males in Nasonia, this function has apparently been lost in the highly divergent female N. giraulti CHC profiles. Curiously, N. giraulti males are still attracted to heterospecific, but not to conspecific female CHC profiles. We suggest that this striking discrepancy has been caused by an extensive evolutionary shift in female N. giraulti CHC profiles, which are no longer used as conspecific recognition cues. Our study constitutes the first report of an apparent abandonment of a sexual recognition cue that the receiver did not adapt to.

Circadian rhythms differ between sexes and closely related species of Nasonia wasps

Activity rhythms in 24 h light-dark cycles, constant darkness, and constant light conditions were analyzed in four different Nasonia species for each sex separately. Besides similarities, clear differences are evident among and within Nasonia species as well as between sexes. In all species, activity in a light-dark cycle is concentrated in the photophase, typical for diurnal organisms. Contrary to most diurnal insect species so far studied, Nasonia follows Aschoff's rule by displaying long (>24 h) internal rhythms in constant darkness but short (<24 h) in constant light. In constant light, N. vitripennis males display robust circadian activity rhythms, whereas females are usually arrhythmic. In contrast to other Nasonia species, N. longicornis males display anticipatory activity, i.e. activity shortly before light-on in a light-dark cycle. As expected, N. oneida shows activity patterns similar to those of N. giraulti but with important differences in key circadian parameters. Differences in circadian activity patterns and parameters between species may reflect synchronization of specific life-history traits to environmental conditions. Scheduling mating or dispersion to a specific time of the day could be a strategy to avoid interspecific hybridization in Nasonia species that live in sympatry.

Fine-scale mapping of the Nasonia genome to chromosomes using a high-density genotyping microarray

Nasonia, a genus of four closely related parasitoid insect species, is a model system for genetic research. Their haplodiploid genetics (haploid males and diploid females) and interfertile species are advantageous for the genetic analysis of complex traits and the genetic basis of species differences. A fine-scale genomic map is an important tool for advancing genetic studies in this system. We developed and used a hybrid genotyping microarray to generate a high-resolution genetic map that covers 79% of the sequenced genome of Nasonia vitripennis. The microarray is based on differential hybridization of species-specific oligos between N. vitripennis and Nasonia giraulti at more than 20,000 markers spanning the Nasonia genome. The map places 729 scaffolds onto the five linkage groups of Nasonia, including locating many smaller scaffolds that would be difficult to map by other means. The microarray was used to characterize 26 segmental introgression lines containing chromosomal regions from one species in the genetic background of another. These segmental introgression lines have been used for rapid screening and mapping of quantitative trait loci involved in species differences. Finally, the microarray is extended to bulk-segregant analysis and genotyping of other Nasonia species combinations. These resources should further expand the usefulness of Nasonia for studies of the genetic basis and architecture of complex traits and speciation.

Evolution of shape by multiple regulatory changes to a growth gene

The genetic changes responsible for morphological differences between species are largely unidentified. Such changes can involve modifications of growth that are relevant to understanding evolution, development, and disease. We identified a gene that induces male-specific wing size and shape differences between Nasonia wasp species. Fine-scale mapping and in situ hybridization reveal that changes in at least three regions (two strictly in noncoding sequence) around the gene unpaired-like (upd-like) cause changes in spatial and temporal expression of upd-like in the developing wing and corresponding changes in wing width. Upd-like shows homology to the Drosophila unpaired gene, a well-studied signaling protein that regulates cell proliferation and differentiation. Our results indicate how multiple changes in the regulation of upd-like are involved in microevolution of morphological and sex-specific differences between species.

Hybrid incompatibilities in the parasitic wasp genus Nasonia: negative effects of hemizygosity and the identification of transmission ratio distortion loci

The occurrence of hybrid incompatibilities forms an important stage during the evolution of reproductive isolation. In early stages of speciation, males and females often respond differently to hybridization. Haldane's rule states that the heterogametic sex suffers more from hybridization than the homogametic sex. Although haplodiploid reproduction (haploid males, diploid females) does not involve sex chromosomes, sex-specific incompatibilities are predicted to be prevalent in haplodiploid species. Here, we evaluate the effect of sex/ploidy level on hybrid incompatibilities and locate genomic regions that cause increased mortality rates in hybrid males of the haplodiploid wasps Nasonia vitripennis and Nasonia longicornis. Our data show that diploid F(1) hybrid females suffer less from hybridization than haploid F(2) hybrid males. The latter not only suffer from an increased mortality rate, but also from behavioural and spermatogenic sterility. Genetic mapping in recombinant F(2) male hybrids revealed that the observed hybrid mortality is most likely due to a disruption of cytonuclear interactions. As these sex-specific hybrid incompatibilities follow predictions based on Haldane's rule, our data accentuate the need to broaden the view of Haldane's rule to include species with haplodiploid sex determination, consistent with Haldane's original definition.

Host genotype changes bidirectional to unidirectional cytoplasmic incompatibility in Nasonia longicornis

Wolbachia are the most abundant maternally inherited endosymbionts of insects and cause various reproductive alterations in their hosts. One such manipulation is cytoplasmic incompatibility (CI), which is a sperm-egg incompatibility typically resulting in zygotic death. Nasonia longicornis (Hymenoptera: Pteromalidae) has an A supergroup and two closely related B supergroup Wolbachia infections. The B supergroup bacteria co-diverged in this host genus. Both triple (wNlonAwNlonB1wNlonB2) and double infections (wNlonAwNlonB1, wNlonAwNlonB2) have been obtained from the field. In the present study, CI was determined among the three Wolbachia types in different host genetic backgrounds. Results show that host genetic background determines whether bidirectional CI or unidirectional CI occurs between the two closely related B group Wolbachia. Results show that the wNlonB1-infected males are bidirectionally incompatible with wNlonB2 in their 'native' nuclear genetic background, whereas wNlonB1 males are compatible with wNlonB2 in two other N. longicornis genetic backgrounds, resulting in unidirectional CI. In contrast, wNlonB2-infected males are incompatible with wNlonB1 females in all three host genetic backgrounds. These changes in incompatibility are not due to the loss of the bacteria. We hypothesize that a repressor gene for sperm modification by wNlonB1 is segregating in N. longicornis populations. The relevance of these findings to the potential role of Wolbachia in host-reproductive divergence and speciation is discussed.

Mate preference across the speciation continuum in a clade of mimetic butterflies

Premating behavioral isolation is increasingly recognized as an important part of ecological speciation, where divergent natural selection causes the evolution of reproductive barriers. A number of studies have now demonstrated that traits under divergent natural selection also affect mate preferences. However, studies of single species pairs only capture a snapshot of the speciation process, making it difficult to assess the role of mate preferences throughout the entire process. Heliconius butterflies are well known for their brightly colored mimetic warning patterns, and previous studies have shown that these patterns are also used as mate recognition cues. Here, we present mate preference data for four pairs of sister taxa, representing different stages of divergence, which together allow us to compare diverging mate preferences across the continuum of Heliconius speciation. Using a novel Bayesian approach, our results support a model of ecological speciation in which strong premating isolation arises early, but continues to increase throughout the continuum from polymorphic populations through to "good," sympatric ecologically divergent species.

Genetics of cuticular hydrocarbon differences between males of the parasitoid wasps Nasonia giraulti and Nasonia vitripennis

Many insects rely on cuticular hydrocarbons (CHCs) as major recognition signals between individuals. Previous research on the genetics of CHCs has focused on Drosophila in which the roles of three desaturases and one elongase were highlighted. Comparable studies in other insect taxa have not been conducted so far. Here, we explore the genetics of CHCs in hybrids of the jewel wasps Nasonia giraulti and Nasonia vitripennis. We analyzed the CHC profiles of pure strain and of F(2) hybrid males using gas chromatography coupled with mass spectrometry and distinguished 54 peaks, of which we identified 52 as straight-chain, monounsaturated, or methyl-branched CHCs. The latter compound class proved to be particularly abundant and diverse in Nasonia. Quantitative trait locus (QTL) analysis suggests fixed genetic differences between the two strains in 42 of the 54 studied traits, making Nasonia a promising genetic model for identifying genes involved in CHC biosynthesis. QTL for methyl-branched CHCs partly clustered in genomic regions with high recombination rate: a possible indication for pleiotropic genes that control their biosynthesis, which is largely unexplored so far. Finally, we identified and mapped genes in the Nasonia genome with high similarity to genes that have been implicated in alkene biosynthesis in Drosophila and discuss those that match in their position with predicted QTL for alkenes.

Behavioral and spermatogenic hybrid male breakdown in Nasonia

Several reproductive barriers exist within the Nasonia species complex, including allopatry, premating behavioral isolation, postzygotic inviability and Wolbachia-induced cytoplasmic incompatibility. Here we show that hybrid males suffer two additional reproductive disadvantages, an inability to properly court females and decreased sperm production. Hybrid behavioral sterility, characterized by a reduced ability of hybrids to perform necessary courtship behaviors, occurs in hybrids between two species of Nasonia. Hybrid males produced in crosses between N. vitripennis and N. giraulti courted females at a reduced frequency (23-69%), compared with wild-type N. vitripennis and N. giraulti males (>93%). Reduced courtship frequency was not a simple function of inactivity among hybrids. A strong effect of cytoplasmic (mitochondrial) background was also found in N. vitripennis and N. giraulti crosses; F2 hybrids with giraulti cytoplasm showing reduced ability at most stages of courtship. Hybrids produced between a younger species pair, N. giraulti and N. longicornis, were behaviorally fertile. All males possessed motile sperm, but sperm production is greatly reduced in hybrids between the older species pair, N. vitripennis and N. giraulti. This effect on hybrid males, lowered sperm counts rather than nonfunctional sperm, is different from most described cases of hybrid male sterility, and may represent an earlier stage of hybrid sperm breakdown. The results add to previous studies of F2 hybrid inviability and behavioral sterility, and indicate that Wolbachia-induced hybrid incompatibility has arisen early in species divergence, relative to behavioral sterility and spermatogenic infertility.

Evolution of sex-specific wing shape at the widerwing locus in four species of Nasonia

How do morphological differences between species evolve at the genetic level? This study investigates the genetic basis of recent divergence in male wing size between species of the model parasitoid wasp Nasonia. The forewings of flightless Nasonia vitripennis males are 2.3 times smaller than males of their flighted sister species N. giraulti. We describe a major genetic contributor to this difference: the sex-specific widerwing (wdw) locus, which we have backcrossed from N. giraulti into N. vitripennis and mapped to an 0.9 megabase region of chromosome 1. This introgression of wdw from large-winged N. giraulti into small-winged N. vitripennis increases male but not female forewing width by 30% through wing region-specific size changes. Indirect evidence suggests that cell number changes across the wing explain the majority of the wdw wing-size difference, whereas changes in cell size are important in the center of the wing. Introgressing the same locus from the other species in the genus, N. longicornis and N. oneida, into N. vitripennis produces intermediate and large male wing sizes. To our knowledge, this is the first study to introgress a morphological quantitative trait locus (QTL) from multiple species into a common genetic background. Epistatic interactions between wdw and other QTL are also identified by introgressing wdw from N. vitripennis into N. giraulti. The main findings are (1) the changes at wdw have sex- and region-specific effects and could, therefore, be regulatory, (2) the wdw locus seems to be a co-regulator of cell size and cell number, and (3) the wdw locus has evolved different wing width effects in three species.

The genetic basis of interspecies host preference differences in the model parasitoid Nasonia

The genetic basis of host preference has been investigated in only a few species. It is relevant to important questions in evolutionary biology, including sympatric speciation, generalist versus specialist adaptation, and parasite-host co-evolution. Here we show that a major locus strongly influences host preference in Nasonia. Nasonia are parasitic wasps that utilize fly pupae; Nasonia vitripennis is a generalist that parasitizes a diverse set of hosts, whereas Nasonia giraulti specializes in Protocalliphora (bird blowflies). In laboratory choice experiments using Protocalliphora and Sarcophaga (flesh flies), N. vitripennis shows a preference for Sarcophaga, whereas N. giraulti shows a preference for Protocalliphora. Through a series of interspecies crosses, we have introgressed a major locus affecting host preference from N. giraulti into N. vitripennis. The N. giraulti allele is dominant and greatly increases preference for Protocalliphora pupae in the introgression line relative to the recessive N. vitripennis allele. Through the utilization of a Nasonia genotyping microarray, we have identified the introgressed region as 16 Mb of chromosome 4, although a more complete analysis is necessary to determine the exact genetic architecture of host preference in the genus. To our knowledge, this is the first introgression of the host preference of one parasitoid species into another, as well as one of the few cases of introgression of a behavioral gene between species.