75 research outputs found
Heads or Tails? Karl Ernst von Baer, Robert Remak, and characterization of the primary axis in animal eggs
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Wnt Signaling in the Early Sea Urchin Embryo
No full textWnt signaling regulates a remarkably diverse array of cellular and developmental events during animal embryogenesis and homeostasis. The crucial role that Wnt signaling plays in regulating axial patterning in early embryos has been particularly striking. Recent work has highlighted the conserved role that canonical Wnt signaling plays in patterning the animal-vegetal (A-V) axis in sea urchin and sea anemone embryos. In sea urchin embryos, the canonical Wnt signaling pathway is selectively turned on in vegetal cells as early as the 16-cell stage embryo, and signaling through this pathway is required for activation of the endomesodermal gene regulatory network. Loss of nuclear beta-catenin signaling animalizes the sea urchin embryo and blocks pattern formation along the entire A-V axis. Nuclear entry of beta-catenin into vegetal cells is regulated cell autonomously by maternal information that is present at the vegetal pole of the unfertilized egg. Analysis of Dishevelled (Dsh) regulation along the A-V axis has revealed the presence of a cytoarchitectural domain at the vegetal pole of the unfertilized sea urchin egg. This vegetal cortical domain appears to be crucial for the localized activation of Dsh at the vegetal pole, but the precise mechanisms are unknown. The elucidation of how Dsh is selectively activated at the vegetal cortical domain is likely to provide important insight into how this enigmatic protein is regulated during canonical Wnt signaling. Additionally, this information will shed light on the origins of embryonic polarity during animal evolution. This chapter examines the roles played by the canonical Wnt signaling pathway in the specification and patterning of the A-V axis in the sea urchin. These studies have led to the identification of a novel role for canonical Wnt signaling in regulating protein stability, and continued studies of Wnt signaling in this model system are likely to reveal additional roles for this pathway in regulating early patterning events in embryos
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Functional Roles of Syngap1 in Early Embryonic Development of Zebrafish
No full textDe novo mutations of SYNGAP1 (Synaptic GTPase Activating Protein) have been found in human patients with intellectual disability, epilepsy and autism. Consistent with an important role for SYNGAP1 in nervous system development, mice heterozygous for SYNGAP1 loss-of-function mutations exhibit disrupted behavior and cognition. Studies in mice have focused on the role of SYNGAP1 at excitatory post-synapses even though SYNGAP1 is expressed prior to synaptogenesis. To study the potential roles of SYNGAP1 during early developmental stages, we carried out a syngap1 morpholino study in zebrafish and found that knockdown of syngap1 causes reduced brain size, increased cell death in the brain and spinal cord before synaptogenesis and disrupted behaviors. We also found decreased GABAergic precursors in syngap1b morphants and increased apoptosis in the GABAergic progenitor area. These findings bridge the gap between embryonic mutations and later phenotypes, supporting the emerging view of prenatal origin in etiology of neurodevelopmental disorders associated with autism spectrum disorders.</p
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Identification and Characterization of the Molecular Determinants of the Anterior-Posterior Axis in the Sea Urchin Embryo
No full textHow the embryonic body axis emerges from a seemingly symmetrical egg and is subsequently patterned during embryogenesis is one of the most fundamental questions in developmental biology. It is now clear that localized activation of the Wnt/β-catenin signaling pathway plays a conserved role in primary body axis formation and/or endoderm/endomesoderm specification during embryo development of phylogenetically diverse animal species. In sea urchin embryos, recent experimental evidence has shown that Dishevelled (Dsh), a central hub protein in the Wnt/β-catenin pathway, is asymmetrically enriched and differentially modified in a vegetal cortical domain (VCD) of the unfertilized egg. Experimental evidence has also shown that during embryonic development the VCD-enriched Dsh plays a critical role in locally activating Wnt/β-catenin to specify the primary body axis and to activate the endomesodermal gene regulatory network. Hence, elucidating the molecular mechanisms that regulate the accumulation of Dsh in the VCD, mediate its activity in the Wnt/β-catenin pathway will provide key insight into how the sea urchin egg becomes an embryo. In this dissertation, I successfully used two complementary approaches, RNA-seq and protein co-immunoprecipitation, to identify key candidate molecules that may participate in regulating the spatiotemporal activity of Dsh and the Wnt/β-catenin pathway. Functional analysis of one particular candidate, DIXDC1, has shown that it may play an important role in regulating nuclear β-catenin and endomesoderm formation; two other candidate proteins were shown to be co-localized with Dsh, suggesting that they may have potentially novel functions in interacting with and regulating Dsh in sea urchin embryos. This dissertation has provided a valuable resource of candidate RNAs and proteins to study the regulation of Wnt/β-catenin pathway in early embryo development in sea urchins. Studies on these candidates in other phylogenetically important species will further provide useful insight into the regulation and evolution of the Wnt/β-catenin pathway in metazoans.</p
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Characterization of Changes in Megalagrion Opsin Genes to Detect Signatures of Selection
No full textMegalagrion damselflies have radiated into new breeding habitats independently at least six times in the Hawaiian archipelago, and have evolved bright body coloration numerous times. We hypothesize that these radiations are correlated with specific changes in the opsin proteins. We isolated and characterized two opsin genes from nine different Megalagrion species. The opsin phylogeny is consistent with the phylogeny based on breeding habitat preference of Megalagrion species supporting the correlation between the evolutionary changes of vision and habitat shifts. dN/dS ratios of opsin sequences show that these genes are evolving under purifying selection, though some sites of the opsin genes might be evolving under positive selection. Two terrestrial-breeding Megalagrion species show higher rates of opsin gene evolution that are correlated with a rapid transformation in their breeding habitats from aquatic to terrestrial. These results support the hypothesis that opsin gene evolution has played a role in Megalagrion radiation in Hawaii.</p
Characterization of Changes in Megalagrion Opsin Genes to Detect Signatures of Selection
No full textMegalagrion damselflies have radiated into new breeding habitats independently at least six times in the Hawaiian archipelago, and have evolved bright body coloration numerous times. We hypothesize that these radiations are correlated with specific changes in the opsin proteins. We isolated and characterized two opsin genes from nine different Megalagrion species. The opsin phylogeny is consistent with the phylogeny based on breeding habitat preference of Megalagrion species supporting the correlation between the evolutionary changes of vision and habitat shifts. dN/dS ratios of opsin sequences show that these genes are evolving under purifying selection, though some sites of the opsin genes might be evolving under positive selection. Two terrestrial-breeding Megalagrion species show higher rates of opsin gene evolution that are correlated with a rapid transformation in their breeding habitats from aquatic to terrestrial. These results support the hypothesis that opsin gene evolution has played a role in Megalagrion radiation in Hawaii.</p
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Ecological and Evolutionary Genomics: Differential Variability of Gene Expression and Fine-Scale Adaptive Divergence Among Microhabitats of Large Fundulus heteroclitus Populations
No full textAdaptive evolution on ecological timescales shapes communities. However, adaptation among environments relies on isolation or large selection coefficients that exceed migration effects. This reliance is tempered if adaptation from standing genetic variation is polygenic – does not depend on one allele completely replacing another but instead requires small allele frequency changes at many loci. Thus, whether individuals can evolve adaptation to fine-scale habitat variation is not resolved. To investigate this process, the gene expression and genetic divergence of a teleost fish, Fundulus heteroclitus, was explored. Among nearby (< 200m) microhabitats in three separate saltmarshes, and over the course of a season (spring to fall), thousands of single-nucleotide polymorphisms (SNPs) were analyzed. Among these SNPs, 1.3-2.3% have large and highly significant differences among microhabitats (mean FST = 0.15; false discovery rate (FDR) ≤ 1%), and 1.8-5.7% are significantly different between seasons (mean FST = 0.077 to 0.597; FDR ≤ 1%). The divergence among microhabitats and between seasons for these SNPs is larger than that among populations, exceeds neutral expectation, and indicates surprising population structure among microhabitats. These data suggest that polygenic selection is surprisingly effective in altering allele frequencies over very small geographic distances.</p
Ecological and Evolutionary Genomics: Differential Variability of Gene Expression and Fine-Scale Adaptive Divergence Among Microhabitats of Large Fundulus heteroclitus Populations
No full textAdaptive evolution on ecological timescales shapes communities. However, adaptation among environments relies on isolation or large selection coefficients that exceed migration effects. This reliance is tempered if adaptation from standing genetic variation is polygenic – does not depend on one allele completely replacing another but instead requires small allele frequency changes at many loci. Thus, whether individuals can evolve adaptation to fine-scale habitat variation is not resolved. To investigate this process, the gene expression and genetic divergence of a teleost fish, Fundulus heteroclitus, was explored. Among nearby (< 200m) microhabitats in three separate saltmarshes, and over the course of a season (spring to fall), thousands of single-nucleotide polymorphisms (SNPs) were analyzed. Among these SNPs, 1.3-2.3% have large and highly significant differences among microhabitats (mean FST = 0.15; false discovery rate (FDR) ≤ 1%), and 1.8-5.7% are significantly different between seasons (mean FST = 0.077 to 0.597; FDR ≤ 1%). The divergence among microhabitats and between seasons for these SNPs is larger than that among populations, exceeds neutral expectation, and indicates surprising population structure among microhabitats. These data suggest that polygenic selection is surprisingly effective in altering allele frequencies over very small geographic distances.</p
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Specification and patterning of the animal-vegetal axis in sea urchins by the canonical Wnt signaling pathway
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