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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Evo-Devo - Evolution of Developmental Processes 5

Room: Salon C   4:30 pm–6:00 pm

Moderator(s): Ekstrom LJ, Marchini M
EVD5-1  4:30 pm  Divergence and elaboration of skeletal musculature in early vertebrates. Kusakabe R*, Evolutionary Morphology Laboratory, RIKEN; Kuratani S, Evolutionary Morphology Laboratory, RIKEN
Abstract: Jawed vertebrates possess two distinct groups of skeletal muscles in the trunk, epaxial and hypaxial muscles, primarily defined by the pattern of motor innervations. Of these, the hypaxial group includes muscles with highly differentiated morphology and function, such as the muscles associated with paired limbs and girdles and the tongue muscles. During embryogenesis, these muscles are formed by the extensive distal migration of precursor cells from ventral edges of the somites, a developmental process in which the paired box transcription factor Pax3 plays a key role. In order to clarify the evolutionary mechanisms underlying the variety of hypaxial musculature, we have compared the morphology and molecular signature of the skeletal muscles of several species that diverged early in vertebrate evolution. The cyclostome lamprey lacks many of morphological features of the gnathostomes, such as jaws, paired fins and epaxial/hypaxial distinction of the trunk skeletal musculatures, but possess the hypobranchial muscles that are apparently homologous to the gnathostome tongue muscles. On the other hand, the elasmobranchs possess paired fins and other gnathostome-like body plan, yet the myogenetic pathway of each muscle has yet to be clarified. Using these animals, we examined the expression of developmental markers and delineated the temporal order of differentiation of various skeletal muscles, such as the hypobranchial, posterior pharyngeal and cucullaris (trapezius) muscles, all located near the head-trunk interface. Our analysis has provided new insights regarding cellular and molecular characteristics of each musculature and illustrated how they have contributed to the complexity and diversification of vertebrate morphology.

EVD5-2  4:45 pm  A single mutation reveals latent capacity for limb-like development in the zebrafish. Hawkins MB*, Harvard University Department of Organismic and Evolutionary Biology, Harvard Medical School Department of Genetics, Orthopaedic Reseach Boston Children’s Hospital; Henke K, Harvard Medical School Department of Genetics, Orthopaedic Research Boston Children’s Hospital; Harris MP, Harvard Medical School Department of Genetics, Orthopaedic Research Boston Children’s Hospital
Abstract: The diversification and specialization of the paired appendages are hallmarks of vertebrate evolution. In the lineage leading to tetrapods, the appendicular skeleton was elaborated along the proximal-distal (PD) axis by adding articulated skeletal elements to form the stylopod (humerus), zeugopod (radius/ulna), and autopod (wrist/hands) of the limbs. This tripartite skeletal pattern has remained constant during the 360 million years of tetrapod evolution. In contrast, the teleost fish lineage shows a reduced appendicular skeletal pattern with a diminutive endochondral skeleton, consisting of only proximal radials and small, nodular distal radials along the PD axis. This pattern is canalized and has persisted over 250 million years of teleost evolution. Using a forward genetic approach in the zebrafish, we have discovered an adult-viable, dominant mutation that results in the acquisition of supernumerary radial bones located between the proximal and distal radials. Unlike the proximal radials, these extra elements have both proximal and distal growth zones and articulate with proximal and distal radials. Ontogenetic analyses reveal that the new elements develop from the branching and splitting of cartilaginous condensations in a fashion similar to that seen in tetrapod limb development. Unexpectedly, the extra elements are sometimes directly connected to musculature, which is not observed in wild type radials. An analysis of early development shows modification of known limb developmental gene networks in mutant fins. The genetic alteration in this mutant reveals the latent capacity for skeletal elaboration in fins of fishes and may inform our understanding of ‘limbness’ and the fin to limb transition.

EVD5-3  5:00 pm  Establishing proportion: a hypothesis for the role of the vasculature in zebrafish fin length mutants . Ekstrom LJ*, Wheaton College, MA; Fitzgerald E, Wheaton College, MA; Henrikson K, Wheaton College, MA; Shi A, Wheaton College, MA; Harris MP, Harvard Medical School; Children's Hospital Boston; Lanni JS, Wheaton College, MA
Abstract: The zebrafish (Danio rerio) is a unique research model that can be used to explore regulation of proportional growth during vertebrate development. In adult wild-type zebrafish, the relative proportion of fin to body length is constant, even following fin amputation and subsequent regeneration. We studied the Schleierschwanz (Schw) mutant zebrafish, which displays long fins as a dominant Mendelian trait. We found that Schw exhibits overgrowth of all fins and barbs, with an average caudal fin length: body length ratio of 0.49 as compared to 0.21 in wildtype fish. We further characterized the Schw mutant by studying its vasculature. Introduction of the Schw mutation into a transgenic line, in which green fluorescent protein is expressed under the control of a vascular endothelial-specific promoter (fli::GFP), revealed that caudal fins in Schw contain arteries and veins that are over twice the diameter of similarly located vessels in wild type fish. In addition, preliminary particle image velocimetry (PIV) analysis of blood flow in the caudal fin tips revealed that although arterial blood velocity does not differ between mutant and wildtype, the velocity of venous blood is significantly slower in mutant individuals (5.39 mm s-1 and 1.55 mm s-1, respectively). This appears to result in a backlog of blood in the distal tips of the caudal fins in Schw mutants. As a result of our findings, we postulate that greater blood flow to the fins may result in longer fins by increasing exposure of fin tissues to circulating growth factors or by altering signaling from vascular smooth muscle cells. The Schw model may provide insight into underlying causes of fin length differences in natural populations of fish and in proportion systems, like limbs and antlers, in other vertebrates.

EVD5-4  5:15 pm  How is preaxial polarity established in limb development? A comparison of larval and direct developing salamanders to other tetrapods. Triepel S. K. *, Museum für Naturkunde Berlin; Müller H., Friedrich-Schiller-Universität Jena; Mitgutsch C. , Museum für Naturkunde Berlin; Fröbisch N. B.
Abstract: The pattern of limb development in extant tetrapods is highly conservative in a number of aspects, including overall patterns of gene expression as well as skeletal condensation. Tetrapods exhibit “postaxial polarity” – digital cartilages condensate in a sequence of IV-V-III-II-I inside of a preformed “paddle”. This paddle allows expressed genes to diffuse and to establish gradients across the limb field, which produce positional information and regulate the polarity of the developing limb. However, salamanders are the only extant tetrapod clade showing a reversed pattern. Their digital cartilages condensate in a sequence of II-I-III-IV-V, a pattern called “preaxial polarity”. Furthermore, a striking interspecific diversity can be seen in the ontogenetic timing of limb and digit development. Most salamanders with free-swimming larvae, e.g. the Mexican axolotl, bud their digits one by one, whereas in the direct developing plethodontid salamander Desmognathus aeneus limb development undergoes a paddle stage comparable to other tetrapods. Although some classic limb developmental genes have already been investigated in early limb bud stages of axolotls, it remains unclear how the positional information and polarity of the digits are established. Furthermore, limb development via a paddle stage in D. aeneus has only been described morphologically, but gene expression has not been investigated. We morphologically described limb development of four more species of the family Plethodontidae, which all show a paddle stage during limb development regardless of their developmental strategy (larval or direct development). Furthermore, we investigated gene expression patterns of some more limb development genes in the axolotl and compared them to gene expression patterns in plethodontid salamanders. The data show some obvious differences to other tetrapods and provide new insights into mechanisms underlying preaxial polarity in salamander limb development.

EVD5-5  5:30 pm  Probing the cellular and genetic mechanisms involved in producing bone length variation using the Longshanks mouse. Marchini M*, University of Calgary; Rolian C, University of Calgary
Abstract: The main molecular and cellular processes involved in limb development are relatively well known, however, the aspects of these processes that generate continuous, selectable variation in bone length within a population are still poorly understood. To study the cell and molecular mechanisms of variation in this complex trait, our research group selectively bred mice (Longshanks mice) to increase relative tibia length by approximately 15%. We performed histomorphometry and gene expression analyses in the proximal tibial growth plate to study the mechanisms underlying bone length variation. Histomorphometry shows that the proliferative zone is larger and has more cells in Longshanks compared to random-bred Control mice, whereas there is no difference in hypertrophic chondrocyte size and number. These data suggest that strong selection for increased tibia length produces changes in chondrocyte proliferation, with downstream effects on the rates of hypertrophy and apoptosis. Analysis of gene expression between Longshanks and Control proximal tibiae using RNA sequencing identified a few genes that are significantly differentially expressed, and which are known to be involved in growth plate function, such as Ifi202b and Ifi204, Frzb, C1qtnf3 (cartonectin) and the transcription factor Stat1. Surprisingly, there was little to no difference in genes known to be involved in mediating the chondrocyte life cycle and chondrocyte differentiation, such as Ihh, PTHrP, Bmps. Our results suggest that the main mechanism producing longer bones in Longshanks is an increase in chondrocyte proliferation and bone apposition rate. This process seems to be regulated by only a small number of genes previously known to be involved in the growth plate.

EVD5-6  5:45 pm  The role of Hox in pisiform and calcaneus ossification and the nature of the zeugopod/autopod boundary. Reno PL*, Pennsylvania State University; Kjosness KM, Pennsylvania State University; Hines JE, Pennsylvania State University
Abstract: The wrist and ankle, or mesopodium, form at the boundary between the zeugopod and autopod and are composed of short nodular bones that typically lack growth plates. Hoxa11 and Hoxa13 are expressed in mutually exclusive proximal-distal domains that demarcate the zeugopod/autopod boundary. Similarly, Hoxd genes are deployed in two distinct phases during limb development. The early phase corresponds to proximal segments including the zeugopod and a late phase occurs in the digits. This arrangement produces a gap of low Hoxd expression that generally corresponds to the mesopodium. In contrast to the other bones of the wrist and ankle, the mammalian pisiform and calcaneus form true growth plates. We show that these bones develop within the Hoxa11 and Hoxd11 expression domains. We also observe that the pisiform growth plate becomes disorganized with Hoxa11 or Hoxd11 loss-of-function indicating a direct role for Hox11 in its development. Hoxa13 loss-of-function has minimal effect on the pisiform and proximal calcaneus as these bones still form secondary centers and undergo longitudinal growth. Consideration of the phenotypes resulting from hypodactyly (Hd) and synpolydactyly homolog (Spdh) mutations, which result from altered HOXA13 and HOXD13 proteins respectively, confirms that Hox13 plays a limited role in the development of the pisiform and calcaneus and suggests that they lie within the early phase of Hox expression. Therefore, with respect to patterns of ossification and gene expression, these bones share much more in common with the zeugopod than the autopod. Such an interpretation fits with the timing of autopod origins during tetrapod evolution. This work is supported by the NSF (BCS-1540418) and a Hill Fellowship, Department Anthropology, Pennsylvania State University.

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