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Session Schedule & Abstracts

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Thursday 30th June, 2016

Symposium: Evolution, development, and integration of the vertebrate brain and skull 1

Room: Salon F   2:30 pm–4:00 pm

Moderator(s): G. S. Bever, B.-A. S. Bhullar, & M. R. Sánchez-Villagra
BSI1-1  2:30 pm  Evolution, development, and integration of the vertebrate brain and skull: Frontiers in neuroscience and paleontology. Bever GS*, New York Institute of Technology College of Osteopathic Medicine
Abstract: Comparative neuroscience faces many formidable, yet exciting, challenges. The chordate brain began its remarkable evolutionary history well over 500 million years ago when changes in an increasingly complex and genetically controlled developmental network produced an anterior expansion of the deuterostome dorsal hollow nerve cord. Considering the depth of this history, it is no surprise that we continue to struggle with the structural identity and transformational history of both the brain's component features and the molecular networks that regulate these features during development. The antiquity of the brain exceeds even that of the surrounding cranial skeleton, establishing a temporal relationship paralleled in chordate ontogeny. The brain emerges during late neurulation and then likely serves an under-appreciated developmental role as an early signaling center and organizer of head mesenchyme-directing cells that contribute to a wide diversity of adult cranial structures, including the skull. The developmental and evolutionary influence of the brain on other cranial modules is an area of research just getting underway but one that possesses considerable explanatory potential. I will establish a context for the symposium by using the major chordate crown clades as a framework for reviewing phylogenetic transformations in the morphology and development of the brain. I will also discuss the logical framework for integrating the large amounts of molecular, histological, and functional data flowing from more traditional neuroscience labs with the deep-time perspective afforded by fossil endocasts. Endocasts break up long evolutionary branches, serve as natural experiments for relationships in morphospace, and provide the only access to the early evolution of features along their stem of origin. These early histories often differ markedly from those expressed in the nearest crown clade, thus supplying insight into the changing variability of regulatory networks.

BSI1-2  2:45 pm  Deep deuterostome origins of vertebrate brain regulatory programs. Lowe C.J., Stanford University; Minor P.J. , University of Pennsylvania; Yao Y. , Stanford University; Pani A.M. , University of Chicago; Epstein D.J. *   
Abstract: 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 the 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 networks, 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.

BSI1-3  3:15 pm  Geometric changes in brain and skull, and the integration and interdependence of cranial modules. Marugán-Lobón J.*, Universidad Autónoma de Madrid
Abstract: The skull is the most distinguishing feature of the vertebrate skeleton. Its building blocks are multiple bones with different embryonic origins, yet it is the particular 3D spatial arrangement of the latter what renders identity, functionality and evolvability to the entire system. Classically, the skull is divisible in two inclusive domains, the cranium and the face, the former cradling the brain, and the latter housing the sensory organs. While such parts must be integrated with one another, because they develop, function and evolve jointly, the fact that each skull part serves for different functions lends the rationale that they must evolve semi-independently, as modules. Multiple investigations are beginning to clarify the molecular genetics involved in facial organization, and on the epigenetic influence of the brain upon the organization of the craniofacial system. However, new comparative studies in birds using geometric morphometrics are bringing alternative ideas which add new lines of enquiry to this agenda. For instance, congruent with recent neuroscientists claims, studies have shown that avian brain evolution is modular, entailing that we don't know how does skull morphology respond to this complex source of variation. Whereas the avian beak is tacitly seen as an evolutionary module, recent shape data suggests that craniofacial integration among unrelated clades of birds is much higher than expected (i.e., the facial skeleton and the braincase are not modules). This not only substantiates previous theoretical claims that the correspondence between functional and morphological modules is not straightforward, but also that patterns of skull modularity might be case-specific. Although comparative studies are limited approaches to determine the developmental processes underlying these patterns, they remain a unique heuristic tool to gain key insights and hypothesize which aspects of integration could be examined at the developmental level.

BSI1-4  3:30 pm  Evolution and development of the head in agnathans and fishes. Kuratani S*, RIKEN; Sugahara F, RIKEN; Ota K, Academia Sinica, Yilan Marine Station; Oisi Y, Max Planck Florida Institute for Neuroscience
Abstract: Living cyclostomes consist of hagfish and lamprey, and the monophyly of this group is established by molecular analyses. Comparative morphology of these two cyclostomes, however, has contradicted the monophyly, especially for the embryonic craniofacial pattern of hagfishes: the hagfish adenohypophysis and related structures were suggested to arise from endoderm, unlike that of other vertebrates derived from the ectoderm. Thus, reexamination of hagfish embryology is critical to evaluate the anatomical traits of cyclostomes. By observing staged hagfish embryos, we show that the hagfish adenohypophysis arises ectodermally, as a posterior part of the medial placode, the hypophyseal plate, as in the lamprey larva. This finding allowed us to identify a craniofacial developmental pattern common to cyclostomes, but not to crown gnathostomes. From this cyclostome-specific developmental stage, lamprey and hagfish develop into distinct developmental trajectories, making it difficult to establish morphological homologies in adult anatomy of these animals. We also show that the comparison with gnathostomes, the out group of cyclostomes, implies that many of the hagfish peculiarities can be recognized as hagfish-specific derived traits (autoapomorphies). Thus the lamprey is likely to represent more ancestral state of cyclostomes, possibly reflecting the morphological and developmental pattern of the latest common ancestor of entire vertebrates. Based on the above developmental scheme, we first showed homologies of skeletal elements between lamprey and hagfish chondrocrania.

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