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Session Schedule & Abstracts




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Saturday 2nd July, 2016

SEG2
Symposium: Segmentation and serial homology: surprising new insights for long-standing central questions in vertebrate morphology 2

Room: Salon H   11:30 am–12:45 pm

Moderator(s): R. Diogo, G. Wagner, & F. Galis
SEG2-1  11:30 am  Medial-lateral aspects of anterior-posterior patterning in the vertebrate body plan. Burke A*, Wesleyan University   acburke@wesleyan.edu
Abstract: The regionalization of the anterior-posterior (AP) axis is an important feature of vertebrate diversity and varies widely between taxa. An axial Hox code is strongly correlated with AP patterning of somite-derived tissues, especially the vertebrae. Beyond these axial structures and their intrinsic muscles, variation in morphology along the AP axis is a matter of different interactions between somitic and lateral plate mesoderm in the mediolateral dimension. These interactions result in primaxial and abaxial domains, separated by the lateral somitic frontier. We have examined the frontier in a wide range of vertebrates including chick, mouse, axolotl, shark and lamprey. We hypothesize that developmental patterning is independent on either side of the frontier, facilitating morphological evolution while maintaining integration between axial and appendicular systems.

SEG2-2  12:00 pm  Acquisition of serial patterning and the making of a 'segment' across the evolutionary origin of the vertebrate jaw. Miyashita T*, University of Alberta   tetsuto@ualberta.ca
Abstract: A developing vertebrate head is a whole of many serial structures: hindbrain rhombomeres, neural crest streams, head cavities, pharyngeal arches and pouches, epibranchial placodes, and others. Although the traditional view of a unified segmentation scheme no longer holds, it remains a puzzle whether these serial structures originated at once or independently. A long-standing challenge to differentially test these alternative hypotheses is to identify more than one evolutionary event in which significant additions or modifications occurred to the serial patterns. I provide fossil and developmental evidence for such modifications in the pharyngeal apparatus at the origin of jawed vertebrates. Comparison between cyclostomes (hagfish and lampreys) and gnathostomes (jawed vertebrates) reveals that the mandibular region does not have a typical pharyngeal arch organization in cyclostomes. A diffuse boundary between "premandibular" and "mandibular" anlagen does not persist. In addition the "mandibular" ectomesenchyme occupy posterior positions in the head of cyclostomes that would only emerge as developmental defects in gnathostome embryos. Mapping of musculoskeletal elements in multiple extinct lineages of jawless vertebrates reveals similar patterns to cyclostomes in general, but also suggests that diffuse boundaries around the "mandibular" elements independently evolved in hyoid or hypobranchial positions in some lineages. A synthesis of the evidence indicates that the mandibular arch (typically labeled as PA I or BA I) acquired a pattern serial to the rest of the pharyngeal apparatus only at the origin of the jaw. This Mandibular Confinement Hypothesis leads to an emerging view that (a) in the last common ancestor of all living vertebrates, only hindbrain rhombomeres and pharyngeal pouches were truly segmented in the head; and (b) serial patterns in the living jawed vertebrate head gradually evolved through interdependent tissue interactions facilitated by multipotency of neural crest ectomesenchyme.

SEG2-3  12:15 pm  Serial homology of paired appendages and sexual organs: studies in early gnathostome fossils. Trinajstic K*, Curtin University; Long J, Flinders University; Johanson Z, Natural History Museum   K.Trinajstic@curtin.edu.au
Abstract: Among living early vertebrates, specialized copulatory organs resulting in internal fertilization occur in chondrichthyans (sharks) and osteichthyan (bony fish). For example, the claspers of sharks are a modification of the pelvic fin whereas in bony fishes the gonopodium is a modification of the anal fin. Specialized copulatory organs are absent in the jawless lampreys and hagfishes. This seems to imply that internal fertilization and viviparity are evolutionary novelties in these groups, and primitively absent in early jawed vertebrates. However, recent discoveries in placoderms (extinct armored fishes), currently resolved as a paraphyletic grade along the gnathostome stem, questions this interpretation. When first discovered, the copulatory organs in placoderms were predicted to be similar to sharks (i.e., part of the pelvic fin skeleton), but are now known to be completely independent of the pelvic girdle and fin. In addition, it is now apparent that the pectoral, pelvic, as well as the copulatory structures comprise dermal and perichondral bony elements suggesting that the claspers in placoderms represent a third set of paired appendages serially homologous with pectoral and pelvic fins. Within the fossils from the Gogo Formation in the Canning Basin (Devonian, Western Australia) soft anatomy is preserved in 3D as apatite, including specialized muscles associated with the paired appendages. A unique set of muscles in the abdominal region of placoderms appear morphologically more similar to squamate musculature than fishes and are interpreted as operating the paired copulatory organs. By comparison with other gnathostomes, these observations suggest that placoderms could be more informative regarding the acquisition of not only reproductive structures, but also paired appendages.

SEG2-4  12:30 pm  There may be more to the Hox Code than you thought. The "Distal Phase" HoxA/D expression pattern is an ancient module that is deployed in a variety of novel features in vertebrates. Crow Karen *, San Francisco State University   crow@sfsu.edu
Abstract: Fins and limbs are homologous structures patterned by a shared genetic repertoire of HoxA/D expression, or "the Hox limb building toolkit". A unique inversion of the HoxD expression pattern is associated with the most well characterized example of a novel fin/limb modification to date-the tetrapod autopod, where an inverted collinear HoxD expression pattern specifies digit identity and the origin of the thumb. This pattern also occurs in paddlefish pectoral fins and catshark paired fins, indicating that it arose in the common ancestor of jawed vertebrates. We refer to this pattern as "distal phase" (DP) expression because it occurs in distal structures and is regulated independently. We argue that it may be deployed in a modular fashion, suggesting a greater role in the evolution of morphological diversity in vertebrates than previously recognized. We demonstrated the first evidence for HoxD DP expression in a body plan feature beyond fins and limbs- the paddlefish barbel, and the first evidence for HoxA DP expression in the developing hindgut and vent of ray-finned fishes, suggesting that the limb-building program may have an expanded repertoire. Interestingly, HoxA DP expression is predicted by similar conformational properties between the HoxA/D cis-regulatory landscapes in zebrafish and mice, but has not been reported in vertebrate paired appendages. However, we found evidence suggesting that HoxA DP expression occurs in claspers-modified pelvic fin structures in male cartilaginous fishes. Taken together, these data support the modularity of DP Hox expression pattern, and a greater role for the Hox code in evolution of novel body plan features.



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