Genomic exploration of adaption and speciation in plant-insect interactions

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The development and maintenance of population differentiation in the face of gene flow is one of the long-standing issues in evolutionary biology. A recent neutral model from my group (CCV# 5) shows that naturally assorting traits, like flowering phenology, synergize Isolation-by-Distance to amplify population genetic structure, even on a continuous landscape. This interesting result is the departure point for much of the work planned over the next 5 years. We will integrate theoretical and genomic approaches to delineate phenology's role in the origin and maintenance population differentiation. Individual-based and analytical models will probe the contribution of phenological assortative mating to the emergence of population genetic structure. Empirical work focuses on the interaction between goldenrod (Solidago spp.) and the gall-inducing insect, Eurosta solidaginis. We will take a genomic approach to examine two distinct hostplant shifts that reflect early stages of ecological speciation. Work is organized around three basic themes. Theme 1. Modeling the Impact of Phenological Assortative Mating on Population Differentiation: This builds on our basic neutral model showing that Isolation-by-Distance is strongly amplified when assortative for mating period induces Isolation-by-Phenology. We will focus on spatial heterogeneity in selection on mating phenology itself, and on the development of linkage disequilibrium between phenology QTL and loci for other traits under local selection. This general approach can inform our understanding the interaction of spatial arrangement with phenological variation to shape population genetic structure, with applications to evolution along clines and ecological speciation. Theme 2. Phenology in Speciation of Phytophagous Insects: Ecological speciation can occur when divergent section on prezygotic isolating traits counter homogenizing gene flow. This divergent selection should produce should "genomic islands" of differentiation-linkage groups cantered on loci promoting reproductive isolation. Studies finding genomic islands have not always determined if they center on adaptive/barrier loci under divergent selection. We will examine two distinct host-shifts by Eurosta, one occurring in sympatry and one in parapatry, to determine genomic islands differ in length, and if they center on loci affecting insect emergence phenology and/or host recognition. Theme 3. Gall Size and Genomic Evidence for Stabilizing Selection: The counter-balancing selection by parasitoids and birds on the goldenrod stem gallmaker is literally a textbook example of stabilizing selection. Quantitative genetic studies indicate the gall-size variation is in part attributable to insect genetic variation, which may in part be due to the timing of attack. Genotype-by-sequencing will locate QTL for gall size and determine if allele frequencies shift in one direction with parasitoid attack, then reverse with bird attack.