Olfactory Sensilla and Olfactory Genes in the Parasitoid Wasp Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae)

Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) is a tiny natural egg parasitoid of several agricultural pest insects, which has been widely used in the biological control for Plutella xylostella, Helicoverpa armigera, Spodoptera frugiperda and Ectomyelois ceratoniae. However, limited studies have been conducted on T. pretiosum olfactory system, which is critical in regulating insect behaviours. In this study, T. pretiosum adult antennae were investigated under ascanning electron microscopy (SEM). Four types of olfactory sensilla were observed, including chaetica sensilla (CS), trichoid sensilla (TS), faleate sensilla (FS) and placoid sensilla (PS). Using T. pretiosum genome, 22 putative odorant binding proteins (OBPs) and 105 odorant receptors (ORs) were identified, which were further compared with olfactory genes of Apis mellifera, Nasonia vitripennis and Diachasma alloeum. The expression patterns of OBPs between T. pretiosum male and female adults were examined by quantitative real time PCR (qRT-PCR) approaches. Three female-specific OBPs (TpreOBP19, TpreOBP15 and TpreOBP3) were identified, which may play crucial roles in T. pretiosum host-seeking and oviposition behaviours. This study enriches our knowledge of T. pretiosum olfactory genes and improves our understanding of its olfactory system.

Bracon brevicornis Genome Showcases the Potential of Linked-Read Sequencing in Identifying a Putative Complementary Sex Determiner Gene

Bracon brevicornis is an ectoparasitoid of a wide range of larval-stage Lepidopterans, including several pests of important crops, such as the corn borer, Ostrinia nubilalis. It is also one of the earliest documented cases of complementary sex determination in Hymenoptera. Here, we present the linked-read-based genome of B. brevicornis, complete with an ab initio-derived annotation and protein comparisons with fellow braconids, Fopius arisanus and Diachasma alloeum. We demonstrate the potential of linked-read assemblies in exploring regions of heterozygosity and search for structural and homology-derived evidence of the complementary sex determiner gene (csd).

Genome of the Parasitoid Wasp Diachasma alloeum, an Emerging Model for Ecological Speciation and Transitions to Asexual Reproduction

Parasitoid wasps are among the most speciose animals, yet have relatively few available genomic resources. We report a draft genome assembly of the wasp Diachasma alloeum (Hymenoptera: Braconidae), a host-specific parasitoid of the apple maggot fly Rhagoletis pomonella (Diptera: Tephritidae), and a developing model for understanding how ecological speciation can “cascade” across trophic levels. Identification of gene content confirmed the overall quality of the draft genome, and we manually annotated ∼400 genes as part of this study, including those involved in oxidative phosphorylation, chemosensation, and reproduction. Through comparisons to model hymenopterans such as the European honeybee Apis mellifera and parasitoid wasp Nasonia vitripennis, as well as a more closely related braconid parasitoid Microplitis demolitor, we identified a proliferation of transposable elements in the genome, an expansion of chemosensory genes in parasitoid wasps, and the maintenance of several key genes with known roles in sexual reproduction and sex determination. The D. alloeum genome will provide a valuable resource for comparative genomics studies in Hymenoptera as well as specific investigations into the genomic changes associated with ecological speciation and transitions to asexuality.

Genome of the parasitoid wasp Diachasma alloeum, an emerging model for ecological speciation and transitions to asexual reproduction

Parasitoid wasps are among the most speciose animals, yet have relatively few available genomic resources. We report a draft genome assembly of the wasp Diachasma alloeum (Hymenoptera: Braconidae), a host-specific parasitoid of the apple maggot fly Rhagoletis pomonella (Diptera: Tephritidae) and a developing model for understanding how ecological speciation can “cascade” across trophic levels. Identification of gene content confirmed the overall quality of the draft genome, and we manually annotated ∼400 genes as part of this study, including those involved in oxidative phosphorylation, chemosensation, and reproduction. Through comparisons to model hymenopterans such as the European honeybee Apis mellifera and parasitoid wasp Nasonia vitripennis, as well as a more closely related braconid parasitoid Microplitis demolitor, we identified a proliferation of transposable elements in the genome, an expansion of chemosensory genes in D. alloeum and other parasitoid wasps, and the maintenance of several key genes with known roles in sexual reproduction and sex determination. The D. alloeum genome will provide a valuable resource for comparative genomics studies in Hymenoptera as well as specific investigations into the genomic changes associated with ecological speciation and transitions to asexuality.

Sequential divergence and the multiplicative origin of community diversity

Phenotypic and genetic variation in one species can influence the composition of interacting organisms within communities and across ecosystems. As a result, the divergence of one species may not be an isolated process, as the origin of one taxon could create new niche opportunities for other species to exploit, leading to the genesis of many new taxa in a process termed “sequential divergence.” Here, we test for such a multiplicative effect of sequential divergence in a community of host-specific parasitoid wasps, Diachasma alloeum, Utetes canaliculatus, and Diachasmimorpha mellea (Hymenoptera: Braconidae), that attack Rhagoletis pomonella fruit flies (Diptera: Tephritidae). Flies in the R. pomonella species complex radiated by sympatrically shifting and ecologically adapting to new host plants, the most recent example being the apple-infesting host race of R. pomonella formed via a host plant shift from hawthorn-infesting flies within the last 160 y. Using population genetics, field-based behavioral observations, host fruit odor discrimination assays, and analyses of life history timing, we show that the same host-related ecological selection pressures that differentially adapt and reproductively isolate Rhagoletis to their respective host plants (host-associated differences in the timing of adult eclosion, host fruit odor preference and avoidance behaviors, and mating site fidelity) cascade through the ecosystem and induce host-associated genetic divergence for each of the three members of the parasitoid community. Thus, divergent selection at lower trophic levels can potentially multiplicatively and rapidly amplify biodiversity at higher levels on an ecological time scale, which may sequentially contribute to the rich diversity of life.

Oviposition-deterring pheromone deposited on blueberry fruit by the parasitic wasp, Diachasma alloeum

Insect parasitoids are known to deposit chemical signals on utilized hosts following oviposition. It is believed that these chemical signals alert future conspecifics of an exploited and thus sub-optimal host alleviating potential suffering among brood that would otherwise compete over a limited resource. Diachasma alloeum (Muesebeck) is a braconid wasp that specifically attacks two species of fruit-parasitic flies in the genus Rhagoletis . Female wasps lay a single egg into a second or third instar fly maggot developing in blueberry, hawthorn, or apple fruit. Following oviposition, female wasps press and drag their ovipositor across the fruit surface depositing a clear liquid; this has been termed 'excreting' behaviour. In this report, we describe excreting behaviour in a field population of D. alloeum attacking the blueberry maggot fly, Rhagoletis mendax Curran. Subsequently, we demonstrate in a series of laboratory assays that D. alloeum females deposit a substance on blueberry fruit directly following egg laying that deters subsequent naïve females from ovipositing into the marked fruit. Marking fruit with this putative oviposition-deterring pheromone without associated egg laying was sufficient to induce the deterring effect, while oviposition alone without subsequent marking had no effect. The oviposition-deterring effect was removed by rinsing fruit with a solution of 50% ethanol in water. Spraying unmarked fruit with an ethanol-water rinsate of previously-marked berries induced the oviposition-deterring effect. Significant deterrence of oviposition lasted up to 7 days after marking. To our knowledge, this is the first demonstration of a hymenopteran oviposition-deterring pheromone that is deposited externally on the surface of the fruit skin rather than on the surface of the parasitized larva. Female D. alloeum maximally parasitize second-instar R. mendax larvae during a short 4-5 d window. The length of activity of the oviposition-deterring pheromone described here should be sufficient to prevent multiple egg-laying into a host that cannot support more than one parasitoid larva, and thus reduce intraspecific competition. Insect parasitoids are known to deposit chemical signals on utilized hosts following oviposition. It is believed that these chemical signals alert future conspecifics of an exploited and thus sub-optimal host alleviating potential suffering among brood that would otherwise compete over a limited resource. Diachasma alloeum (Muesebeck) is a braconid wasp that specifically attacks two species of fruit-parasitic flies in the genus Rhagoletis . Female wasps lay a single egg into a second or third instar fly maggot developing in blueberry, hawthorn, or apple fruit. Following oviposition, female wasps press and drag their ovipositor across the fruit surface depositing a clear liquid; this has been termed 'excreting' behaviour. In this report, we describe excreting behaviour in a field population of D. alloeum attacking the blueberry maggot fly, Rhagoletis mendax Curran. Subsequently, we demonstrate in a series of laboratory assays that D. alloeum females deposit a substance on blueberry fruit directly following egg laying that deters subsequent naïve females from ovipositing into the marked fruit. Marking fruit with this putative oviposition-deterring pheromone without associated egg laying was sufficient to induce the deterring effect, while oviposition alone without subsequent marking had no effect. The oviposition-deterring effect was removed by rinsing fruit with a solution of 50% ethanol in water. Spraying unmarked fruit with an ethanol-water rinsate of previously-marked berries induced the oviposition-deterring effect. Significant deterrence of oviposition lasted up to 7 days after marking. To our knowledge, this is the first demonstration of a hymenopteran oviposition-deterring pheromone that is deposited externally on the surface of the fruit skin rather than on the surface of the parasitized larva. Female D. alloeum maximally parasitize second-instar R. mendax larvae during a short 4-5 d window. The length of activity of the oviposition-deterring pheromone described here should be sufficient to prevent multiple egg-laying into a host that cannot support more than one parasitoid larva, and thus reduce intraspecific competition.