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Research Project: Host Specificity and Systematics of Insect Biological Control Agents

Location: Beneficial Insects Introduction Research Unit

2017 Annual Report

1a. Objectives (from AD-416):
Objective 1: Determine the physiological, behavioral, ecological, and genetic basis of host ranges of noctuid moths and parasitoids of pest insects, such as soybean aphid, Russian wheat aphid, sugar cane aphid, and spotted-wing Drosophila, with a focus on using molecular genetic methods to elucidate factors responsible for the evolution of host specificity. Subobjective 1.1 – Determine the genetic basis of host ranges of noctuid moths and of parasitoids of pest insects. Subobjective 1.2 – Test whether bacterial endosymbionts affect acceptance and suitability of hosts and determine mechanisms of these effects. Subobjective 1.3 – Test whether the host specificity of Aphelinus species changes with stress or experience. Objective 2: Determine interactions between biological control and host plant resistance in their effects on survival, reproduction, and population dynamics of pest insects, such as soybean aphid, Russian wheat aphid, sugar cane aphid, and spotted-wing Drosophila, in laboratory and field experiments. Objective 3: Determine molecular phylogenetic relationships, test host specificity, and introduce parasitoids for biological control of pest insects, such as soybean aphid, Russian wheat aphid, sugar cane aphid, and spotted-wing Drosophila, and determine the impact of the introduced parasitoids on the abundance and distribution of target and non-target species. Subobjective 3.1 – Determine phylogenetic relationships among parasitoids whose members are candidates for biological control introductions. Subobjective 3.2 – Measure host specificity of parasitoids that are candidates for biological control introductions. Subobjective 3.3 – Introduce parasitoids to control pest insects, such as soybean aphid, Russian wheat aphid, sugar cane aphid, and spotted-wing Drosophila, and measure the impact of the introduced parasitoids on the abundance and distribution of target and non-target species.

1b. Approach (from AD-416):
We will use analysis of genomes for genes that are divergent in sequence or expression, QTL mapping, co-localization of probes for QTL markers and divergent genes with chromosomal fluorescence in-situ hybridization and allele genotyping, analysis of tissue-specific expression (antenna, ovipositor), and gene knock-out with CRISPR/Cas9 and RNAi technology to identify genes involved in host recognition and acceptance. To test whether defensive bacterial endosymbionts affect acceptance and suitability of hosts of parasitoids and to determine mechanisms underlying these effects, we will assay more species of parasitoids on more species of aphids with and without their defensive endosymbionts. To test whether host ranges of Aphelinus species are ever dynamic, we will test the effects of starvation, age, and experience on parasitism of sub-optimal hosts by parasitoid species with broad host ranges. We will do additional experiments on the interactions between host plant resistance and parasitism by Aphelinus species. Continued development of the molecular phylogeny of Aphelinus species will provide a framework for other results. We will conduct host specificity testing of parasitoids for release against D. noxia, M. sacchari and D. suzukii. We will introduce parasitoid species with narrow host ranges and monitor their impact on target and non-target species.

3. Progress Report:
In research on objective 1.1, we identified 4,000 clusters of orthologous genes in 17 species of Aphelinus from annotations of assemblies of their genomes based on high-throughput sequencing. We are testing the association of these genes with the differences in host specificity. Using genetic mapping, we have identified a smaller number of genes that genes associated with differences in host specificity between Aphelinus atriplicis and Aphelinus certus. To test whether they are involved in host recognition we are determining whether they are expressed in sensilla on antennae or ovipositors. The first of these genes we studied is indeed expressed on sensory cells on antennae and ovipositors. A grant to support this research was submitted to NIFA-AFRI Foundational Program and was funded. Together with colleagues at the University of Delaware, the Delaware Biotechnology Institute, the NorthShore University HealthSystem (Evanston, Illinois), and the University of Chicago, we also addressed two issues in genotyping-by-sequencing (GBS) involving missing data across libraries and confounding paralogous loci, while developing and validating a new, comprehensive platform for GBS. Phased GBS in maize revealed the existence of reproducibly spurious genotypes from copy number variants unobservable in the underlying single nucleotide polymorphism data (115 log 337263). Together with colleagues at North Carolina State University and the American Museum of Natural History, we mapped quantitative trait loci (QTL) involved in host plant use herbivores in the genus Chloridea (formerly Heliothis), a group of noctuid moths that contains many damaging crop pests. We found that the underlying genetic architectures of intra- and interspecific variation are very similar, involving a large number of interchangeable QTL with additive effects. We conclude that incremental adaptation to a novel host plant could proceed along many different genetic routes, suggesting that a gradual shift by C. virescens onto a novel host would be possible in the field. With colleagues at Oregon State University, we are investigating adaptation of Tyria jacobaeae, cinnabar moth, which was introduced to control Senecio jacobaea, tansy ragwort, a toxic rangeland weed. Rapid evolution in phenology-related traits and increased fitness on novel host plants has allowed T. jacobaeae to invade mountain habitats in Oregon. We are analyzing differences in genes between valley populations and mountain populations to determine the genetic basis of this adaptation (Agreement number 8010-22000-029-12R). In research on objective 1.2 with colleagues at the University of Georgia, University of Kentucky, and University of Minnesota, and the Institute National de la Recherche Agronomique (Sophia-Antipolis, France), we found moderate support for the hypothesis that the diversity of bacteria) that aphids harbor in their cells and affect traits like host specificity is lower among introduced host populations. We also found that laboratory cultures of aphids are prone to losing these symbiotic bacteria so that such cultures of target and nontarget aphids should be routinely monitored for bacterial symbionts to ensure that laboratory tests of host specificity produce field-relevant results. In research on objective 2 with colleagues at Texas A&M University, Pennsylvania State University, University of Illinois, University of Minnesota, Iowa State University, and Ohio State University, we have begun experiments on the interactions between host plant resistance, aphid virulence, and biological control with parasitoids in the genus Aphelinus. We are studying three aphid-crop systems: soybean aphid (Aphis glycines) on soybean, the sugarcane aphid (Melanaphis sacchari) on sorghum, and the Russian wheat aphid on wheat. In research on 3.1 with colleagues at Texas A&M University, we revised the Aphelinus asychis species complex, described two new species, and developed an identification key for this complex (115 log 333095). We surveyed glandular structures on male antennae of eight species across six species groups in Aphelinus using Scanning Electron Microscope (SEM) imaging. We found differences in the number of pores, the shape of cuticle surrounding pores, the location of pores on the scape, and the shape of the carina delimiting the area around pores. The results indicate that the morphology of the pores is diagnostic for species complexes of Aphelinus. In research on objective 3.2, we prepared and submitted a petition to the North American Plant Protection Organization to introduce Aphelinus hordei for biological control of Diuraphis noxia, the Russian wheat aphid. This petition was based on 10 years of laboratory experiments on host specificity of Eurasian species in the genus Aphelinus (115 log 333430). Aphelinus hordei may not only provide direct control of this aphid, but also may slow the spread of virulent aphid genotypes able to overcome aphid resistance in wheat varieties. In research in an ARS Areawide Project objectives 2 and 3.3 of the project with colleagues at Iowa State University, the University of Minnesota, Ohio State University, and the University of Illinois, ~300,000 Aphelinus glycinis were reared and released in 20 sites in Iowa and 20 sites in Minnesota with resistant and susceptible soybean plants. Parasitism of soybean aphid was not affected by whether the aphids were on resistant versus susceptible soybean, showing that biological control with A. glycinis is compatible with host plant resistance. Furthermore, A. glycinis overwintered successfully and parasitized aphids were found in the leaf litter in soybean fields and under buckthorn this spring.

4. Accomplishments

Review Publications
Hopper, K.R., Kuhn, K.L., Lanier, K., Rhoades, J.H., Oliver, K.M., White, J.A., Asplen, M.K., Heimpel, G.E. 2017. The defensive aphid symbiont Hamiltonella defensa affects host quality differently for Aphelinus glycinis versus Aphelinus atriplicis. Biological Control. doi:10.1016/j.biocontrol.2017.05.008.

Hopper, K.R., Lanier, K., Rhoades, J., Kondos, R., Coutinot, D., Mercadier, G., Ramualde, N., Roche, M., Woolley, J.B., Heraty 2017. Host specificity of Aphelinus species considered for introduction to control Diuraphis noxia. Biological Control. 107:21-32.

Gokhman, V.E., Kuhn, K.L., Woolley, J.B., Hopper, K.R. 2017. Variation in genome size and karyotype among closely related parasitoids of aphids. Comparative Cytogenetics. 11:97-117.

Manching, H., Sengupta, S., Hopper, K.R., Polson, S.W., Ji, Y., Wisser, R.J. 2017. Phased genotyping-by-sequencing enhances analysis of genetic diversity and reveals divergent copy number variants in maize. Genes, Genomes, Genetics. 7(7):2161-2170. doi: 10.1534/g3.117.042036.

Xanthe, S.A., Woolley, J.B., Hopper, K.R. 2017. Revision of the Asychis species group of Aphelinus (Hymenoptera: Aphelinidae). Journal of Hymenoptera Research. 54:1-32.