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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Evo-Devo - Evolution of Developmental Processes 2

Room: Salon C   9:30 am–11:00 am

Moderator(s): Rehorek SJ, Souza RG
EVD2-1  9:30 am  The development and evolution of cranial nerves and head muscles in two actinopterygian taxa, the longnose gar (Lepisosteus osseus) and the turquoise killifish (Nothobranchius furzeri). Naumann B*, Institute of Special Zoology and Evolutionary Biology; Konstantinidis P; Warth P; Hartmann N; Englert C; Hilton E. J.; Metscher B; Olsson L
Abstract: The vertebrate head has been modified largely during the course of evolution, resulting in enormous craniofacial diversity. These changes did not only include the skull, but also associated structures such as muscles and nerves. Data on the developmental patterning and morphogenesis of these structures are essential for understanding the mechanisms underlying these changes. Recent studies on vertebrate development have mainly focused on axis formation. Ontogenetic data on the head are mostly focused on skeletal elements. Soft tissues of the head are still understudied. Where research on cranial myo- and neurogenesis was performed, mostly tetrapod model organisms like Xenopus, axolotl, chicken and mouse were used. With about 30000 species, the Actinopterygii comprises around half of all extant vertebrates and is the sister group to the Sarcopterygii, including the tetrapods. Understanding the morphological changes of the vertebrate head during evolutionary history therefore inevitably calls for the study of actinopterygians. Insights into head morphogenesis from non-tetrapod vertebrates are mainly based on studies of the zebrafish. The zebrafish belongs to the Teleostei, a derived taxon within the Actinopterygii. To provide an evolutionary view, further representatives of the Actinopterygii, need to be studied and put in comparison. We have examined the development of cranial muscles and nerves in two actinopterygian taxa [the longnose gar (Lepisosteus osseus) and the turquoise killifish (Nothobranchius furzeri)], using µCT scans and whole-mount antibody stainings. The early formation of the head musculature in these taxa is relatively conserved whereas later morphogenetic events, such as the partitioning of the adductor mandibulae complex and the differentiation of the branchial muscles are much more variable. However, the development of the cranial nerves seems to be the most conservative pattern of head morphogenesis within the Actinopterygii.

EVD2-2  9:45 am  Embryonic derivation of the bony skull and cranial musculature in the axolotl (Ambystoma mexicanum). Sefton EM*, Harvard University; Hanken J, Harvard University
Abstract: The vertebrate skull is a complex structure that arises from three embryonic sources—cranial mesoderm, somitic mesoderm and neural crest. Head muscles are also derived from cranial mesoderm and as such are distinct from somite-derived trunk muscles. The contribution of cranial mesoderm to the bony skull and head musculature is well documented in two amniote models, the mouse and chicken. Additional data from anamniotes would facilitate comparisons in a wider phylogenetic context and provide fundamental data regarding features that have been lost in amniotes, including gill musculature. To delineate the fate of cranial mesoderm, we utilize GFP-transgenic axolotls, which permit long-term fate mapping. Several elements in the skull exhibit a dual embryonic origin from both mesoderm and cranial neural crest, including the parasphenoid (the dominant component of the amphibian palate), the squamosal and the stapes. Myogenic unsegmented mesoderm extends posteriorly to the axial level of somite 3 and contributes to both the posterior gill-levator muscles and the cucullaris muscle. These results constitute the first long-term fate map of cranial mesoderm in an anamniote vertebrate.

EVD2-3  10:00 am  Correlation between Hox code and vertebral morphology in archosaurs. Böhmer C*, MNHN Paris; Rauhut O, SNSB-BSPG München; Wörheide G, LMU München   boehmer@vertevo,de
Abstract: The vital importance of the axial column for vertebrate life is clear, because its key functions, the protection of the neural chord and providing a balance between stability and mobility, have remained the same in a huge variety of taxa. However, vertebrae show considerable variation in number and shape across the axial column, resulting in varying degrees of axial regionalization. Nevertheless, functionally equivalent master control genes mediate the embryonic development of the axial column in animals as different as mouse and chicken. The combined expression of Hox genes is a requirement to establish specific vertebral morphologies, indicating that the morphological variation across taxa is likely due to modifications in the pattern of gene expression. In archosaurs, Hox codes have been established for birds, but not yet fully for the crocodilian lineage. First, we analyzed the Hox gene expression in the axial column of the Nile crocodile. Second, by using geometric morphometrics, the present study shows a correlation between Hox code and quantifiable vertebral morphology in living archosaurs, in which the boundaries between morphological subgroups of vertebrae can be linked to Hox gene expression limits. Our results reveal homologous units of vertebrae in modern archosaurs, each with their specific Hox gene pattern. Based on these results, we used the morphological pattern as a proxy to reconstruct the underlying Hox code in fossil taxa where the genetic information is not available. This allows us for the first time to rigorously hypothesize the genetic complexity of an extinct archosaur, the sauropodomorph dinosaur Plateosaurus. By connecting the morphological patterns to developmental processes, inference of the genetic changes that underlie the evolutionary modifications of the axial column appears feasible. This is not only an important case study, but will lead to a better understanding of the origin of morphological disparity in archosaur vertebral columns.

EVD2-4  10:15 am  Comparative anatomy of the nasolacrimal duct: different origins but same end point. Rehorek SJ*, Slippery Rock University; Bly KS, Slippery Rock University; Flethcher QA, Slippery Rock University; Rock JJ, Slippery Rock University; Smith TD, Slippery Rock University; Hillenius WJ, College of Charleston
Abstract: The nasolacrimal duct (NLD) connects the orbital and nasal region in many tetrapods. Caudally, this duct opens into the anterior/ medial orbital region, often in close association with the nictitating membrane or an anterior orbital gland (e.g.: Harderian gland). Rostrally, this duct opens into the nasal cavity, though variation exists: it opens into the nostril region (some mammals), near the vomeronasal Organ (VNO: squamates and amphibians) or onto the lateral nasal wall (other mammals and archosaurs). A similar level of variation is evident in the ontogeny of the nasolacrimal apparatus. There are three different published origins of the NLD in vertebrates: 1) originates from the orbit and grows into the nasal capsule (Mongolian gerbils, rabbits and the common quail), 2) originates from the VNO and grows towards the orbit (squamates) and 3) a groove that sinks into the facial mesenchyme and becomes an enclosed epithelial tube (humans). In most developing vertebrates the rostral part of the NLD becomes longer as the nasal cavity grows. There are at least two major variants. In some archosaurs (Laysan albatross and alligator): 4) The NLD originates as a solid point on the lateral nasal wall, which sinks into the mesoderm, dissociating from the lateral wall and growing to connect the orbital and nasal region. However, the NLD does not elongate, rather the nasal region continues to grow rostral to the NLD. As a result, the NLD remains truncated, and opens in the posterior end of the nasal cavity. Other variants may emerge later in development; for example, in anthropoid primates and tarsiers, the rostral part of the NLD disappears during fetal development. Though the origin of the NLD is variable, its initial connections are evolutionarily conserved. The final structure of this duct is largely determined by the growth of the nasal region.

EVD2-5  10:30 am  An earful of jaw, then and now: using marsupial evo-devo to understand a major evolutionary transition in the paleontological record. Urban DJ*, University of Illinois at Urbana-Champaign; Anthwal N, King's College London; Tucker AS, King's College London; Sears KE, University of Illinois at Urbana-Champaign
Abstract: During synapsid evolution, postdentary elements in the reptilian jaw transitioned into the middle ear of mammals. Though this astounding change is well documented in the fossil record, questions regarding the developmental sequence that drove the ossicular transition still remain. At birth, modern marsupials possess a very reptilian jaw joint with functional articulation between the articular and quadrate. These elements will later become the malleus and incus, respectively, of the middle ear. This entire transition occurs postnatally, and represents a natural system for comparison with the fossil record. We utilized Monodelphis domestica as a model organism, and traced the development of ossicular structures as they separate from the jaw and fully incorporate into the middle ear. Micro-CT scans throughout development and three-dimensional reconstructions show decreasing size and rearward movement of ossicles are illusions created by continued growth and expansion of the surrounding skull elements. Cryosections and immunohistochemistry (IHC) reveal separation of Meckel’s cartilage from the malleus occurs at postnatal day 20 and is facilitated by apoptosis. Additionally, laser capture microscopy and RNA sequencing identify differential gene expression at the time of separation and breakdown of the connecting Meckel’s cartilage. Key gene findings are then verified with fluorescent in situ hybridization (FISH). The morphological changes are facilitated by an upregulation of cartilage resorption genes paired with simultaneous downregulation of proliferative genes. Finally, marsupial developmental stages were compared with the known fossil record of early mammals exhibiting transitional forms of the definitive mammalian middle ear in order to resolve the question, in this instance, of whether ontogeny is truly recapitulating phylogeny.

EVD2-6  10:45 am  Revisiting the homologues hypotheses: are we really testing it? Souza R*, Museu Nacional/UFRJ
Abstract: For Owen the features that are the same under any variety of form and function in different specimens were designated as homologues. The causal explanation of a homologue is sharing the same Archetype, what he called as Homologies. Nowadays, both of these terms are synonymous and its definitions and functions are directly associated to phylogenetic inferences (e.g., Patterson's tests). However, this procedure eliminated the causal explanation for the shared similarities proposed. Moreover, as a consequence, congruence became the main way to test a homologues hypothesis. The zenith of the congruence test is observed in de Pina propositions, where primary homology is the character propositions being tested on a cladogram, and when (and if) is corroborated it became a secondary homology. When primary homology was falsified it is referred as homoplasy. Here, I identify all these assumptions are based on circular reasoning, both homologue and homology definitions as well the congruence tests. In both cases, the data used to construct the cladogram is also used to test them. Popper proposes that a test must be performed based on an independent data set inferred from the data being explained. Homoplasy is an ad hoc hypothesis and there is no empirical reason to treat it as an error. The homologue must be proposed between the same features on different individuals based on arguments, for example the femur bone articulate proximally with pelvic bones. To test this homologue proposition, studies based on ontogeny, molecular, and others anatomical studies must be performed, in this way, if all referred specimens which a homologue is proposed between the bones called femur share the same ontogenetic origin this homologue hypothesis is corroborated. Therefore I conclude that hypotheses and tests must be looked at with care. Also characters as homologues must be more deeply studied and not only seen as “numerical data” for phylogenetic inferences.

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