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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 2

Room: Salon F   4:30 pm–6:00 pm

Moderator(s): G. S. Bever, B.-A. S. Bhullar, & M. R. Sánchez-Villagra
BSI2-1  4:30 pm  Stability and flux in relationships between cranial bones and endocranial structures – paleontology and molecular development. Bhullar B.-A.S*, Yale University
Abstract: There is a general conception that the architecture of the vertebrate cranial roof relates closely to, and perhaps is patterned by, the underlying brain. However, the degree to which there are consistent relationships between brain components and skull components throughout ontogeny, and the nature of the developmental association between the brain and calvarium, have seldom been rigorously explored. Current work in my lab suggests that early ontogenetic relationships between brain and cranial elements are remarkably conserved. This is the case, for instance, throughout the evolution of Reptilia. To a certain extent, relations hold true even in the adult, but there is a considerable amount of decoupling between brain regions and the skull during ontogeny, even as the growing skull compensates for the underlying brain. This suggests coordinated early patterning but an eventual loss of region-specific molecular feedback. Previous work on the development of the brain and calvarium was made difficult by the fact that mouse mutants with very small or very large brains tend to have an unossified cranial roof; but some preliminary data of ours indicate that the skull roof does indeed compensate for changes in brain proportions depending on timing and degree.

BSI2-2  4:45 pm  The heads of the earliest fossil vertebrates: evolution, development and diversity. Ahlberg P E*, Uppsala University
Abstract: Early fossil vertebrates from the Ordovician to Devonian periods illuminate the evolution not only of the jawed vertebrate morphotype, but also of the developmental processes that underlie it. Numerous aspects of head anatomy and early development, conserved between extant cyclostomes and gnathostomes (notably the spatial relationship between pharyngeal slits, cranial nerves and neural crest streams), create a comparative framework for interpreting the anatomical landmarks of fossils. At the same time some early vertebrate morphologies lie well outside the range of extant gnathostomes and cyclostomes, providing invaluable information about evolution and diversity within the gnathostome stem group. All jawless stem gnathostomes ("ostracoderms") lack a discrete shoulder girdle; scapulocoracoids, if present, attach directly to the cranial endoskeleton. Unlike jawed vertebrates, the dermal bone pattern of ostracoderms does not reflect pharyngeal arch architecture. Head-shoulder separation and pharyngeal patterning of the dermal skeleton seem to have evolved simultaneously and may have a shared developmental basis. Different ostracoderm groups show three distinct relationships between pharynx and brain. In heterostracans and galeaspids it is similar to the gnathostome condition, with two pharyngeal pouches anterior to the inner ear; in osteostracans the adult pharynx is strongly displaced anteriorly, a unique condition that contradicts the traditional view of osteostracans as fundamentally lamprey-like; in anaspids the adult pharynx is displaced posteriorly, like in extant cyclostomes, and a rasping tongue may have been present. The most primitive jawed vertebrates such as Brindabellaspis and Romundina are not similar to osteostracans, as has been claimed, but do resemble galeaspids such as Shuyu. Shared specializations of anaspids and cyclostomes suggest that anaspids may be stem cyclostomes, not stem gnathostomes as generally believed.

BSI2-3  5:15 pm  The dinosaur and bird brain: evolution, ontogeny, and function. Balanoff Amy M.*, Stony Brook University
Abstract: With almost 10,000 extant species, birds are one of the most diverse groups of modern vertebrates. Much of their success can be traced to the evolution of a hyper-inflated brain, which is larger than that of any other living reptiles and on par with that of most mammals. The expansion of the bird brain, especially the pallial regions of the cerebrum, has long been considered necessary for their increased cognitive faculties as well as their ability to coordinate the visual, vestibular, somatosensory, and motor components of powered flight. The relationship between the origin of avian flight within non-avian dinosaurs and encephalization, however, has proved to be more complex than previously appreciated, with at least one, and possibly numerous, volumetric increases in brain size occurring well before the evolution of powered flight. It would follow that this region of the avian stem lineage is witness not only to complex volumetric patterns but also to major structural rearrangements of the brain. Such changes, however, can be difficult to assess in the fossil record because the endocasts, on which we must rely for these data, have not traditionally been used explicitly to infer internal morphological structure. In order to more effectively integrate neuroanatomical and fossil endocast data, we need a better understanding of the relationship between the three-dimensional nature of various brain components and the two-dimensional surficial expression of those components as found on an endocast. I will explore these relationships for birds and demonstrate how, once established, these data can serve as a bridge between in vivo functional data and the deep history of character transformation along the avian stem that will afford us a better understanding of the avian brain, skull, and overall body plan.

BSI2-4  5:30 pm  On the interparietal and supraoccipital: the development of the mammalian skull roof and its coevolution with the brain. Koyabu D*, University Museum, University of Tokyo
Abstract: It is known that the brain was expanded during the rise of mammals. Here I highlight the developmental changes which occurred in the skull roof concomitantly with such mammalian encephalizaiton, i.e. changes in ossification and suture closure timing. Sequence of cranial skeletogenesis of 102 mammalian species was investigated and compared to those of non-mammalian amniotes. The developmental timing of the supraoccipital was found to be earlier on an average in mammals than in other amniotes. Squared-change parsimony analysis detected that the timing of the supraoccipital development became considerably accelerated in ancestral mammals and was further accelerated in multiple lineages as primatomorphans, cetaceans, talpids, and dipodid rodents, all of which are known as encephalized. Its relative ossification timing was confirmed to be negatively correlated with encephalization quotient. It was further found that the interparietal, which lays rostral to the supraoccipital, is present in all extant mammalian orders, although it was previously regarded as being lost in various taxa. The fact is that the interparietal is visible during fetal period but later often fuses to the supraoccipital and becomes indistinguishable. The tabulars, which have been thought to be lost in mammals, were also found during fetal period. They constitute the lateral parts of the interparietal, indicating that the so called mammalian "interparietal" is a complex of tabulars and postparietals of other amniotes. The timing of the fusion between the supraoccipital and "interparietal" occurs earlier in larger-brained species. Given this, we infer that the earlier fusion of the tabular and postparietal found in mammals could also be linked to brain size. I suggest that encephalization occurred in concert with earlier ossification and more fusion of the postparietal, tabular, and supraoccipital and point out the possible role of pleiotropic effects of Lmx1b and Dlx5 behind this coevolution.

BSI2-5  5:45 pm  Developing humanized mouse models to study human evolution. Dutrow EV*, Kavli Institute for Neuroscience, Yale School of Medicine; Reilly SK, Broad Institute of MIT and Harvard; Noonan JP, Kavli Institute for Neuroscience, Yale School of Medicine
Abstract: We hypothesize that changes in morphology during human evolution required genetic changes in key developmental regulatory functions. Here, we focus on developmental enhancers marked both by human-specific histone modification gains and an accumulation of sequence changes that may account for these acquired epigenetic signatures. The aim of this study is to functionalize candidate human-gained enhancers involved in the development of major human-specific morphological features: an elaborate cerebral cortex and highly specialized limbs. In this study, we systematically target and edit up to 5.5kb regions of the mouse genome using CRISPR/cas9-mediated replacement in order to successfully produce two humanized mouse models. We are carrying out both molecular and tissue-level functional analysis of brain and limb development in these humanized mouse lines to identify phenotypic correlates of human biological traits. This work was supported by NIH GM094780 and T32 GM007499.

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