Genetics, Development and Evolution Symposium

Much of what we understand about the origins of the vertebrate brain has come from comparative studies within chordates. Invertebrate chordates, particularly cephalochordates, have played pivotal roles in providing key insights. It has generally been assumed that few insights are to be gained from broader comparisons with other deuterostome phyla; hemichordates and echinoderms, as their body plans contrast so markedly with that of chordates. However, we have demonstrated in hemichordates that despite the marked difference in the overt morphological organization of their nervous systems, they share exquisite conservation of gene regulatory network, previously considered to be stem vertebrate innovations. We have focused on ectodermal signaling centers that are characterized by the localized secretion of morphogens, and are key in the early regionalization of the brain. The Zona Limitans Intrathalamica (ZLI) patterns the thalamus and prethalamus in vertebrate brains. We have demonstrated by transient transgenic experiments that the temporal and spatial activation of the ligand that defines the ZLI, Shh, is regulated in hemichordates and vertebrates by a conserved cis-regulatory module. Reciprocal enhancer swap experiments demonstrate the ability of the hemichordate enhancer to drive reporter expression in a pattern similar to the native vertebrate enhancer and vice versa. I discuss the implications of these findings for understanding the early origins of the vertebrate brain, and how morphological and gene regulatory network evolution can become uncoupled over macroevolutionary time frames.

To explain biological diversity, we must understand: (1) how genetic changes act through developmental mechanisms to alter phenotypes, (2) how these phenotypic changes impact the ability of organisms to survive and reproduce in nature, (3) how natural selection and demographic processes shape this variation within and between populations, and (4) how these processes contribute to the formation of new species. Importantly, we must also determine the extent to which the answers to these questions are predictable/repeatable when examined across different organisms and traits. To address these questions, my lab uses insects in the genus Neodiprion (Hymenoptera: Diprionidae), an experimentally tractable and phenotypically variable group of pine-feeding sawflies with convergent gains and losses of multiple traits. In this talk, I introduce this novel system and describe our ongoing efforts to develop genomic resources. Then, using host use as an example, I illustrate how we are identifying the proximate and ultimate mechanisms underlying phenotypic variation and assessing repeatability. Phenotypic, population genomic, and comparative data indicate that variation in needle width among pine species generates divergent selection on host-use traits within and between Neodiprion species; that divergent host use acts as a barrier to gene flow; and that changes in host use are associated with speciation. Ultimately, by combining recently developed molecular and genomic approaches with decades of natural history research, we hope to generate new insights into the origin and maintenance of phenotypic variation and species diversity.