Congress Abstracts - Society for Developmental Biology

examples whose Shh expression requires additional explanation: snakes and crocodylians. Emerging in situ hybridization data from
the frog Silurana (Xenopus) tropicalis may increase the need to revisit the odontogenic band, as it provides the first tooth gene
expression data for any amphibian taxon. I present a developmental series of S. tropicalis prometamorphic tadpoles (Nieuwkoop and
Faber stages 55-58), the time period encompassing the odontogenesis of first-generation teeth. This series presents an important
comparative datapoint in representing a vertebrate whose life history has been modified to include an extended, specialized, and
toothless larval stage. I evaluate these comparative data from an evolutionary perspective, identifying phenotypes which indicate that
developmental mechanisms for tooth competence, tooth initiation, and mouth formation may be decoupled in developmental time.
Program/Abstract # 294
Endless pigeons most colorful: genetics and development of feather pigment diversity among domestic rock pigeons
Domyan, Eric; Kronenberg, Zev; Guernsey, Michael; Vickrey, Anna; Cassidy, Pamela; Shapiro, Michael (University of Utah, USA)
Centuries of selective breeding have crafted tremendous phenotypic diversity among breeds of domestic rock pigeon (Columba livia),
providing a rich model to investigate the process of phenotypic variation. To understand the genetic basis of this diversity, we recently
sequenced and interrogated the genomes of 40 pigeons from breeds with distinct sets of derived traits. Here we present the preliminary
identification of several mutations underlying color variation within and among pigeon breeds. Interestingly, one non-synonymous
mutation associated with ash-red feather color is located in the Tyrosinase-related protein 1 (Tyrp1) gene. This dominant mutation
reduces the efficiency of signal peptide cleavage. In addition, we identify putative null alleles of this gene that act recessively to cause
brown feather color; therefore, the dominant ash-red allele may represent a neomorphic allele. Tyrp1 is thus a strong candidate for the
blue/ash-red/brown locus of classical pigeon genetics. We also identify a mutant allele of Slc45a2 that is associated with a reduction
in feather pigmentation and is a strong candidate for the dilute locus of classical pigeon genetics. By determining the genotype of birds
at these two loci, we are able to explain some of the most common feather colors among domestic pigeons.
Program/Abstract # 295
Developmental basis of phallus reduction during bird evolution
Herrera, Ana M; Shuster, Simone (University of Florida, USA); Perriton, Claire (University of Reading, UK); Cohn, Martin (Howard
Hughes Medical Institute, USA)
One of the most puzzling events in evolution is the loss of the phallus in birds. All birds reproduce by internal fertilization, but only
~3% of birds have retained a phallus capable of intromission. Behavioral studies have implicated sexual selection as the evolutionary
mechanism responsible for phallus reduction; however, the underlying developmental mechanism is unknown. We investigated
external genital development in two sister clades, Galliformes (land fowl), which lack an intromittent phallus, and Anseriformes
(waterfowl), which have well developed phalluses, as well as two out groups, Paleognathae (emus) and Crocodilia (alligators).
Galliform embryos undergo cryptic development of a genital tubercle, but later this undergoes apoptosis and regresses. A derived
pattern of Bmp4 expression was identified in galliform genital tubercles. Functional experiments show that Bmp activity is necessary
and sufficient for apoptosis and regression of the genital tubercle. Our results suggest that evolutionary acquisition of a novel Bmp4
domain in the distal genital tubercle underlies loss of an intromittent phallus in galliforms.
Program/Abstract # 296
The evolution of external genitalia: sexual reproduction on dry land
Tschopp, Patrick; Sherratt, Emma; Sanger, Thomas; Groner, Anna; Aspiras, Ariel (Harvard U., USA); Pourquie, Olivier (Strasbourg
Univ. Medical School, France); Gros, Jerome (Institut Pasteur, France); Tabin, Clifford (Harvard U.,USA)
The appearance of internal fertilization has been considered a major step in the emergence of terrestrial vertebrates. In mammals, the
molecular pathways driving the outgrowth of the genital bud highly resemble those that are active during limb development. Here, we
provide a potential explanation for these observed similarities: we show that in squamates (lizards and snakes), the external genitalia
develop as a direct outgrowth of the limb bud, or the remnants thereof. CT scan analyses reveal a relative repositioning of the cloaca,
an important signaling center during genitalia development, towards the limb field in all squamates investigated. Lentiviral lineage
tracing using a novel ex ovo culturing system in anole lizards shows that limb and genitalia share a common embryonic origin in this
species, coming from the lateral plate mesoderm. In contrast, mouse and chicken genital buds seem to originate from the tail bud.
Comparative RNA-seq analysis of embryonic tissues corroborates this notion, showing more similar molecular fingerprints for the
anole genital and limb buds, as opposed to their mouse counterparts. Finally, the importance of cloaca position underlying this
genitalia shift is suggested by a series grafting experiments in chicken and mouse. The transplantation of cloacal tissue into the early
limb bud leads to tissue outgrowth and ectopic activation of genes normally associated with genital bud development. We speculate
that the recruitment of a mesenchymal cell population, namely the early limb bud, by the cloacal signaling center might initially have
fostered the emergence of external genitalia, preserving key ancestral gene circuitries thereafter, regardless of subsequent
repositioning.
Program/Abstract # 297
Moleculars mechanism control cytoskeletal activities during inner ear invagination
Sai, Xiaorei (Riken CDB, Japan)
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