EXAMINING THE ROLE OF HOST USE ON DIVERGENCE IN THE REDHEADED PINE SAWFLY, NEODIPRION LECONTEI , ACROSS MULTIPLE SPATIAL SCALES

OF DISSERTATION EXAMINING THE ROLE OF HOST USE ON DIVERGENCE IN THE REDHEADED PINE SAWFLY, NEODIPRION LECONTEI, ACROSS MULTIPLE SPATIAL SCALES Phytophagous insects make up over one quarter of described species on Earth, and this incredible diversity seems directly linked to feeding on plants. Comparative studies of sister groups have shown shifts to herbivory are consistently associated with increased species diversity in insects, but the reasons for this diversification remain unclear. While other explanations, such as decreased extinction rates or influences on population structure, exist, one prominent hypothesis suggests shifts and subsequent adaptation to novel host plants can lead to the evolution of reproductive barriers. Given their extreme specialization on host plants in the genus Pinus and intimate, life-long association with their host plants, divergent host use has been suspected to drive speciation in the conifer sawfly genus Neodiprion. Previous work showed host shifts coincide with speciation events in the genus; but could not determine if these host shifts initiated speciation or if they occurred after other reproductive barriers arose. Determining the contribution and timing of host shifts relative to speciation will require examination of populations at the earliest stages of divergence, before post-speciation changes amass. If host shifts frequently drive speciation in the genus, there will likely be evidence of host-driven divergence within species occurring on a wide range of host plants. The goal of this dissertation is to examine populations of the red-headed pine sawfly, Neodiprion lecontei, an abundant, well-studied pest species that occurs on multiple hosts throughout its range, for evidence of host-driven divergence. Using a combination of reduced representation genomic sequencing, population genomics, and ecological assays, I specifically look for evidence of 1) genetic differentiation between populations utilizing different host plants, 2) ecological divergence in female oviposition preference, larval performance, and ovipositor morphology between populations on different hosts, and 3) ecologically-driven reproductive isolation between genetically and ecologically divergent populations. Each chapter of this dissertation examines the role of host use in driving ecological, genetic, and/or reproductive divergence within N. lecontei at a different spatial scale. First, I surveyed range-wide patterns of diversity. I identified three genetic clusters, dated the divergence of these clusters to the late Pleistocene, and found evidence that both dispersal limitation (geography) and host use contribute to genetic differentiation within N. lecontei. Next, I looked within one of these genetic clusters for additional evidence of the role of host in driving divergence. Sawflies in this cluster primarily utilize two hosts which differ significantly in needle architecture. Although I found no evidence of neutral genetic differentiation between hosts exists, I did detect spatial and temporal differences in host use, and host-specific differences in ovipositor morphology, a performance-related trait. Finally, I examine a single site where N. lecontei utilizes three structurally divergent species of pine. Although there was little genetic structure, no sexual isolation, and no distinct host preferences, the host types were partially temporally isolated and varied in ovipositor morphology and larval performance across on the three hosts. Overall, although divergent host use consistently resulted in divergent ovipositor morphology, a reduction in gene flow via temporal or geographic isolation may be required before additional forms of ecological and genetic differentiation can develop. Together these results suggest host shifts alone may not be enough to drive population divergence and speciation in Neodiprion.