Congress Abstracts - Society for Developmental Biology

length in multiciliated cells of the epidermis, and is associated with disorganization of the basal bodies. Currently, we are working to
determine the mechanisms by which eta-tubulin affects ciliogenesis and basal body trafficking in X. laevis.
Program/Abstract # 305
Misregulation of osteoblast differentiation underlies abnormal skull growth and suture formation in sp7 mutants
Kague, Erika; Fisher, Shannon (University of Pennsylvania, USA)
During skull growth, fibrous sutures unite the edges of the skull bones. They allow expansion and movement of individual bones as
the brain grows, and also regulate the rate of bone growth. Sutures consist of a central region of undifferentiated mesenchymal stem
cells (MSCs) separating the edges of the bones, where osteoblasts differentiate and new bone is deposited. This spatial organization is
conserved across species, yet remains a poorly understood aspect of suture biology. The transcription factor Osterix/Sp7 is essential
for normal osteoblast differentiation and implicated in the maintenance of bone mineral density. Zebrafish sp7 mutants have a
fundamental defect in bone mineralization. The larval skeleton is patterned normally, but older mutants show severe mispatterning of
the sutures and frequent formation of ectopic intrasutural bones. We followed the process of skull growth through sequential imaging
of live fish carrying a transgene labeling early osteoblasts. In mutants, the edges of the bones are irregular, and adjacent areas of bone
formation remain separate, resulting in irregular sutures. At the microscopic level, there is a dramatic increase in MSCs coupled with
abnormal persistence of early osteoblasts at the edges of the bones. At normal sutures, the early osteoblasts at the edges of the bone
are the most highly proliferative cell population, and their rate of proliferation is dramatically increased in the mutants. These
represent non–autonomous functions of sp7, since MSCs and early osteoblasts do not express the gene. The features of the sp7
mutants suggest a feedback signal linking the state of mineralization in the skull bones to the rate of bone formation, through
regulating the behavior of adjacent MSCs and early osteoblasts. Our analysis of skull formation in sp7 mutants has revealed a
previously unknown aspect of suture regulation, and provides evidence for feedback linking the downstream process of mineralization
with the earliest steps of osteoblast induction.
Program/Abstract # 306
Evolution of a tissue-specific silencer underlies diversification of paralogous genes
Haruki, Ochi (Yamagata University, Japan); Kawaguchi, Akane (Nara Institute of Science and Technology, Japan); Ogino, Hajime
(Nagahama Institute of Bio-Science and Technology, Japan)
During the early chordate evolution, whole genome duplications (WGD) have produced many duplicated genes, called paralogs.
These paralogs are often showing overlapping expression, yet distinct expression patterns in modern vertebrate. The evolutionary
mechanisms for divergent expression of paralogous genes have been explained with the duplication-degeneration-complementation
model. This model predicts that parts of duplicated enhancers were lost reciprocally from sibling paralogs because of degenerative
mutations. But at least one enhancer copy remains in either of the paralogs, resulting complementary expressions cover the original
full expression of the progenitor gene. However, involvement of innovative cis-regulatory changes has still remained elusive. pax2
and pax8 arose from a single progenitor following the WGDs. pax8 is mainly expressed in the kidney, ear and thyroid gland during
development. pax2 shows expression not only in the pax8-expressing tissues but also in other tissues such as the eye, pharyngeal
arches, midbrain-hindbrain boundary, hindbrain and spinal cord. We revealed that both pax2 and pax8 retain ancestral enhancers
capable of directing pax2-like, multi-tissue expression. However, a silencer with in the pax8 proximal promoter suppresses pleiotropic
enhancer activity outside the pax8-expressing tissues. These results indicate that the silencer innovation was crucial for the divergent
expression of paralogs with pleiotropic enhancers inherited from their common progenitor.
Program/Abstract # 307
Temporal and Spatial Expression of the Wnt Gene Complement in a Spiral-Cleaving Embryo
Pruitt, Margaret M.; Letcher, Edward; Bastian, Benjamin; Chou, Hsien-chao; Schneider, Stephan (Iowa State Univ, USA)
The Wnt/β-catenin signaling pathway is highly conserved in metazoans and is involved in many developmental processes, such as
cell-fate determination and axis formation. Thirteen distinct wnt subfamilies are common between cnidarians and bilaterally
symmetric animals. To gain insights into conserved functions of this ancient wnt gene complement, we aimed to unravel roles for each
wnt gene within embryos that utilize a broadly conserved mode of development, spiral cleavage. Spiral-cleaving embryos use a series
of stereotyped asymmetric cell divisions that allows for the identification of individual cells by their positions and size and for
prediction of their fate. To study the wnt gene complement in a spiralian model, we made use of the marine annelid Platynereis
dumerilii whose genome retained 12 of the 13 wnt subfamilies, and whose spiral cleavage pattern has been determined. Furthermore, it
was found that in early Platynereis embryos, the β-catenin signaling pathway specifies cell fates in a reiterative binary manner. Here,
we employ transcriptional profiling and in situ hybridization to determine the temporal and spatial regulation of wnt gene expression
in the early Platynereis embryo. RNA-Seq results suggest that only a subset of wnts is expressed in early development. In situ
hybridization further shows that each of these wnts exhibits a similar but distinct cellular pattern of gene expression that can be traced
back to individual cells. This is the first analysis of the expression of all wnt genes encoded by a spiralian genome in early spiral
cleavage stages, and provides the first comprehensive view of Wnt signaling inputs into embryos utilizing a spiral-mode of cell
divisions to segregate cell fates.
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