Mechanisms Promoting Speciation with Gene Flow in Host-specialized Haplodiploids

PROJECT SUMMARY Project Overview Evolutionary theory predicts that speciation with gene flow is difficult, yet empirical data indicate such divergence scenarios are common in nature. To reconcile these seemingly contradictory findings, the proposed work takes advantage of an experimentally tractable pair of pine sawfly species (Neodiprion lecontei and N. pinetum) that are adapted to different pines and have exceptional genomic resources, extensive complementary data, and a history of divergence with gene flow. First, larval diet manipulations of the focal species and their reciprocal male and female hybrids will be used to quantify reproductive isolation on all combinations of host plants and evaluate the role of phenotypic plasticity in promoting speciation with gene flow. Second, larval transcriptomes of the two species and their reciprocal F1 hybrids reared on both parental hosts will be used to uncover how divergent transcriptional responses to larval diet contribute to host adaptation and gene misexpression in hybrids. Third, QTL mapping in haploid F2 males reared on both parental hosts will be used to describe the genetic architecture of host-dependent and host- independent hybrid viability, larval performance, and adult male morphology. Because this work provides intuitive examples of the complexity of phenotypic variation and mechanisms through which natural selection produces new species, data from the proposed work will be used to develop a case study and scientist spotlights for use in introductory Biology courses and to fuel student-driven research. Intellectual Merit The overarching goal of the proposed work is to understand genetic, phenotypic, and developmental mechanisms underlying ecological speciation and divergence with gene flow. The data generated will advance our understanding of speciation by comprehensively evaluating proposed mechanisms that facilitate divergence with gene flow, including clustered genetic architectures for barrier traits, genomic features that prevent recombination form breaking up associations among barrier loci, and plastic responses to dissimilar habitats that reduce gene flow between diverging populations. Moreover, the proposed work will make significant progress on two major gaps in modern speciation research: (1) connecting lab-based estimates of reproductive isolation and barrier locus effect sizes to gene flow and genetic differentiation in nature and (2) identifying and functionally characterizing ecological speciation genes. This work also stands apart from other mechanistic speciation studies in three additional respects. First, together with previous mapping experiments, the proposed work will provide a more complete description of the genetic architecture of ecological speciation—including barrier traits expressed in larvae, males, and females—than is available for most taxa. Second, by characterizing gene expression, barrier traits and loci, and reproductive isolation in both parental habitats, the proposed work will provide an unusually comprehensive analysis of how and why phenotypic plasticity affects prezygotic and postzygotic isolation. Third, by focusing on haplodiploid taxa with strong (but incomplete) reproductive isolation, the proposed research fills important taxonomic and temporal gaps in speciation research. Broader Impacts The proposed research has many broader impacts. First, by incorporating a case study and scientist spotlights for use in introductory Biology (~1500 students per year), the proposed work is expected to have significant benefits for student learning, science identity, and belonging, as well as reducing reductionist racist thinking. Second, by disseminating the case study to the broader science education community and training a graduate student in educational research, the proposed work will improve STEM education and contribute to educator development. Third, by providing several different avenues for undergraduate participation in research—including traditional independent research, an evolutionary research CURE course, and paid summer research—the proposed work is expected to improve analytical skills, self-efficacy, persistence in STEM, and evolution understanding and acceptance in at least 24 students per year. Such benefits are expected to be especially pronounced for women and racially minoritized groups. Fourth, the proposed activities will increase participation from women, racially minoritized groups, first-generation students, and students from at-risk Appalachian communities. Finally, graduate students and postdocs involved in this project will receive exceptional training in research, mentoring, and teaching.