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Session Schedule & Abstracts

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Friday 1st July, 2016

Symposium: Fundamental aspects of the spatial associations, development, and birth defects of the muscles and skeleton in non-pentadactyl limbs 2

Room: Salon H   11:30 am–12:45 pm

Moderator(s): V. Abdala, T. Kohlsdorf, & R. Diogo
NPL2-1  11:30 am  Adapting the vertebrate limb neuromuscular system to changes in dactyly. Tschopp P*, Harvard Medical School; Young JJ, Harvard Medical School; Speziale D, University of Basel; Zeller R, University of Basel; Diogo R, Howard University; Tabin CJ, Harvard Medical School
Abstract: The morphology of the vertebrate limb has greatly diversified over the course of evolution, to accommodate for various modes of locomotion, as well as distinct uses of its most distal part, the autopod. In order to provide meaningful mobility, the formation and attachment of distinct limb muscle groups to their corresponding bony elements, as well as the correct innervation of these muscles by motorneurons, has to be tightly coordinated. This task is complicated by the diverse developmental origins of the limb's musculoskeletal system: the somites (muscle), lateral plate mesoderm (bone) and the spinal cord (motorneurons). While a pentadactyl state of the autopod is the norm amongst mammals, there is considerable variation outside of this clade. Moreover, in a number of human and other mammalian congenital disorders, the autopod skeleton deviates from this digit formula. Interestingly enough, however, in many of these cases the extra digits seem to be functionally incorporated into the musculoskeletal apparatus, i.e. they appear movable in a controlled fashion. The goal of our study is to understand how changes in the digit formula of the autopod skeleton are integrated with regards to its neuromuscular system. To do so, we are using mouse genetics, in particular the mutant Gli3-Xt as well as a conditional Prx1-Cre/Gli3 allele, and grafting experiments in chicken and emu embryos. We map out changes in embryonic musculature and muscle innervation patterns, and try to trace back the molecular identity of these additional neuromuscular units. Overall, we find distinct changes in the limb's neuromuscular system that seem to be largely dictated by skeletal topology. We discuss this apparent developmental flexibility and its implication for evolutionary transitions in vertebrate digit formulas.

NPL2-2  12:00 pm  Ecomorphology and biomechanics of digit reduction. Nauwelaerts S*, Univ. Antwerpen; MacLaren J, Univ. Antwerpen; Kaashoek M, Univ. Antwerpen; Aerts P, Univ. Antwerpen
Abstract: Ecomorphology is the study area that correlates anatomical features to ecological variables. A classic example in comparative anatomy is the hand structure of mammals as diverse as humans, cetaceans, bats and horses. Such obvious links with ecology, in this case locomotor modes like walking, swimming, flying and running are easily stated and lead to adaptive story telling. In practice however, these patterns prove to be more difficult to assess. Comparative biomechanics is a mechanistic approach to study how structures are used to perform certain ecological tasks. One of the most notable features in a hand is the number of digits. A wide variety of mammals have diverged from the ancestral pentadactyl condition, with various degrees of digit reduction. Examples include artiodactyls, perissodactyls, xenarthrans and the extinct meridiungulates. We will use a simple theoretical geometrical model of a segmented limb of a running animal to determine the mechanical advantages and constraints on the number of digits. Changing the number of digits has consequences to the compression and bending strength of the hand, and to the inertial properties, which in turn might have consequences to cost of transport. We will demonstrate that even though intuitively digit reduction seems beneficial and straight forward in terms of COT that from a mechanical standpoint several trade-offs and constraints emerge that must have been at play during evolution. An extreme example of digit reduction is monodactyly. Equids (horses, asses and zebras) are the only extant monodactyl mammals. We will discuss the future direction of our projects currently underway, and how the evolution of equids fit into the context of comparative biomechanics.

NPL2-3  12:15 pm  Chameleon hand/foot clefting, a tweak on the pentadactyl design and a challenge to limb congenital malformations. Diaz Raul*, La Sierra University
Abstract: While a handful of vertebrate models have provided the majority of what we know about limb development, the recent wave of evolutionary developmental research has allowed for a seemingly exponential growth in taxa used in the lab to answer a variety of questions within a clinical and evolutionary framework. While squamate reptiles represent >33% of all living amniotes, very little is known about their embryonic development and how body plans within this group have diversified over the course of evolution. Within squamates, the chameleon body plan has been modified for a specialized lifestyle of arboreality and are structurally divergent from other terrestrial tetrapods with respect to the cranial, trunk, tail and limb skeletons. To begin to understand how the body plan of a chameleon develops, we take advantage of the slow development of the Veiled Chameleon (Chamaeleo calyptratus) to study the development of their split-hand/split-feet and ask the following questions. 1) At which developmental time point do we see a break from the typical pentadactyl phenotype? 2) Has early limb genetic patterning been modified? 3) How is hand/foot splitting being controlled? 4) How does our work on the veiled chameleon challenge what we know about congenital cleft formation in humans and other tetrapods?

NPL2-4  12:30 pm  Patterning and post-patterning modes of evolutionary digit loss in mammals. Cooper K. L.*, University of California San Diego; Sears K. E. , University of Illinois Urbana-Champaign; Uygur A., Harvard Medical School; Maier J., University of Illinois Urbana-Champaign; Baczkowski K-S, Ecole Normale Superieure de Lyon; Brosnahan M., Cornell University; Antczak D., Cornell University; Skidmore J. A., The Camel Reproduction Centre; Tabin C. J., Harvard Medical School
Abstract: A reduction in the number of digits has evolved many times in tetrapods, particularly in cursorial mammals that travel over deserts and plains, yet the underlying developmental mechanisms have remained elusive. Here we show that digit loss can occur both during early limb patterning and at later post-patterning stages of chondrogenesis. In the ‘odd-toed’ jerboa (Dipus sagitta ) and horse and the ‘even-toed’ camel, extensive cell death sculpts the tissue around the remaining toes. In contrast, digit loss in the pig is orchestrated by earlier limb patterning mechanisms including downregulation of Ptch1 expression but no increase in cell death. Together these data demonstrate remarkable plasticity in the mechanisms of vertebrate limb evolution and shed light on the complexity of morphological convergence, particularly within the artiodactyl lineage.

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