The main goal of our research is to understand the origin of biodiversity. Specifically, we would like to know: (1) how individual mutations alter gene function to change the physical appearance, behavior, or physiology (i.e., the phenotype) of an organism, (2) how these changes impact the ability of organisms to survive and reproduce in nature, (3) how, over time, natural selection and demographic processes (e.g., changes in population size, movement of individuals) shape variation in natural populations, and (4) the circumstances under which these processes lead to the formation of new species. Importantly, we would like to know whether the answers to these questions are predictable/repeatable when examined across different organisms and traits.
Approach and Study Organisms:
To address these questions, we integrate fieldwork, morphological measurements, behavioral observations, molecular work, genetic mapping, population genomics, phylogenomics, and comparative genomics. Our primary study organisms are pine sawflies in the genus Neodiprion (Order: Hymenoptera; Family: Diprionidae). We have chosen these insects for two main reasons. First, they are experimentally tractable: we can find them in nature, rear them in the lab, and make crosses between different species. Second, they harbor tremendous phenotypic variation within and between species and at all stages of their life cycle. Because many species in the genus are economically important pests, this variation is remarkably well described. Some variable traits we are interested in include: host use, overwintering mode, larval color, and larval gregariousness (see specific project descriptions).
Comparative analysis of host-shift speciation
Like many plant-feeding insects, Neodiprion sawflies are intimately associated with their host plant throughout their life cycle (left). It has long been hypothesized that changes in host use drive species formation in plant-feeding insects. To test this hypothesis, we are utilizing the redheaded pine sawfly, a widespread pest of multiple pine species throughout the eastern US. If host shifts predictably initiate speciation, we expect to find increased genetic, ecological, and reproductive divergence between populations collected on different hosts. This project is funded by the NSF and led by Robin Bagley (who is supported by a USDA predoctoral fellowship), with assistance from Carrie Anderson.
Genetic basis of host-use traits and speciation
Whereas Neodiprion pinetum females lay 2-3 eggs per needle and strongly prefer white pine over other hosts, N. lecontei females lay many eggs per needle (left) and generally avoid this thin-needled host. Hybrids tend to prefer white pine, but have a lecontei-like oviposition pattern that causes the needles to dry out and the eggs to die. These two species are interfertile in the lab, and we are taking a QTL-mapping approach to uncover the genes involved in host preference, ovipositor morphology, and oviposition pattern in adult females, a combination of traits that produces a strong postzygotic barrier to reproduction. This project is led by Emily Bendall, with assistance from Brianna Washington and Anna Sosso.
Comparative genomics of host specialization
Comparative genomic studies in fruit flies (Drosophila) suggest that ecological specialization is often accompanied by an accumulation of loss-of-function mutations in chemosensory genes; however, the generality of this model remains unclear. Also, an increasing number of studies suggest that particular genes are repeatedly involved in adaptive evolution. To test the extent to which changes in host use are "predictable," we are characterizing chemosensory genes in multiple generalist/specialist Neodiprion species pairs that also exhibit convergent gains and losses of the same pine hosts. This project is funded by the USDA and led by Kim Vertacnik, in collaboration with Dr. Scott Geib, USDA-ARS in Hilo, HI and Dr. Hugh Robertson at the University of Illinois at Champaign-Urbana. Maya Gershtenson is also participating on this project.
Genetics and evolution of larval gregariousness
Feeding aggregations are one of the most common and taxonomically widespread social behaviors in nature. In Neodiprion, larval behavior varies from solitary to highly gregarious. We are using a comparative approach to uncover the ecological function and genetic mechanisms that generate and maintain this variation. This project is led by John Terbot, with assistance from Layne Gaynor and Adam Douglas.
Genetic basis of larval color
There is substantial variation within and between Neodiprion species in larval body color, head color, and patterning. A major goal of the lab is to find the genes, mutations, and molecular mechanisms underlying this variation. To these ends, we are currently mapping multiple color traits using a cross between two Neodiprion lecontei populations with distinctly different larval color morphs: a white, dark-spotted morph (top) and a yellow light-spotted morph (bottom). This project is led by postdoc Claire O'Quin.
Evolution of larval defense strategies
Neodiprion larvae suffer heavy mortality from avian and insect predators, egg and cocoon parasitoids, viruses, and fungi. Although larvae of most species are chemically defended, larval color ranges from cryptic to aposematic. To explain this variation, we are combining a comparative approach (in which we investigate correlations between defense traits and the environment within and between species) with field experiments (in which we seek to uncover causal relationships between correlated variables). This project is in collaboration with Dr. Carita Lindstedt (University of Jyväskylä), with assistance from John Terbot, Layne Gaynor, and Adam Douglas.
To test hypotheses and accurately describe convergent evolution, we require a clear understanding of historical relationships among Neodiprion species. However, estimating a phylogeny for this group has proved challenging due to their recent and rapid divergence and frequent hybridization. We are tackling these issues with full genome sequences for 20 species and multiple "species tree" methods. This project is led by John Terbot, with assistance from Kim Vertacnik and Emily Bendall, and in collaboration with Dr. Susanne Pfeifer (Arizona State University).
Additional projects and collaborations:
In addition to the projects described above, we are investigating:
Methylation patterns across the Neodiprion genome (in collaboration with Brendan Hunt, University of Georgia).
Neodiprion microbial symbionts (in collaboration with Jen White, UK Dept. of Entomology).
Divergence-with-gene-flow at multiple geographical and temporal scales (in collaboration with Vitor Sousa, University of Bern).
Genetic basis of Neodiprion sex determination (in collaboration with Scott Roy, SFSU).