RESUMEN
Development of the human pancreas requires the precise temporal control of gene expression via epigenetic mechanisms and the binding of key transcription factors. We quantified genome-wide patterns of DNA methylation in human fetal pancreatic samples from donors aged 6 to 21 post-conception weeks. We found dramatic changes in DNA methylation across pancreas development, with > 21% of sites characterized as developmental differentially methylated positions (dDMPs) including many annotated to genes associated with monogenic diabetes. An analysis of DNA methylation in postnatal pancreas tissue showed that the dramatic temporal changes in DNA methylation occurring in the developing pancreas are largely limited to the prenatal period. Significant differences in DNA methylation were observed between males and females at a number of autosomal sites, with a small proportion of sites showing sex-specific DNA methylation trajectories across pancreas development. Pancreas dDMPs were not distributed equally across the genome and were depleted in regulatory domains characterized by open chromatin and the binding of known pancreatic development transcription factors. Finally, we compared our pancreas dDMPs to previous findings from the human brain, identifying evidence for tissue-specific developmental changes in DNA methylation. This study represents the first systematic exploration of DNA methylation patterns during human fetal pancreas development and confirms the prenatal period as a time of major epigenomic plasticity.
Asunto(s)
Metilación de ADN , Páncreas , Humanos , Páncreas/metabolismo , Páncreas/embriología , Femenino , Masculino , Regulación del Desarrollo de la Expresión Génica , Islas de CpG , Epigénesis Genética , Genoma Humano , Feto/metabolismoRESUMEN
Persistent congenital hyperinsulinism (HI) is a rare genetically heterogeneous condition characterised by dysregulated insulin secretion leading to life-threatening hypoglycaemia. For up to 50% of affected individuals screening of the known HI genes does not identify a disease-causing variant. Large deletions have previously been used to identify novel regulatory regions causing HI. Here, we used genome sequencing to search for novel large (>1 Mb) deletions in 180 probands with HI of unknown cause and replicated our findings in a large cohort of 883 genetically unsolved individuals with HI using off-target copy number variant calling from targeted gene panels. We identified overlapping heterozygous deletions in five individuals (range 3-8 Mb) spanning chromosome 20p11.2. The pancreatic beta-cell transcription factor gene, FOXA2, a known cause of HI was deleted in two of the five individuals. In the remaining three, we found a minimal deleted region of 2.4 Mb adjacent to FOXA2 that encompasses multiple non-coding regulatory elements that are in conformational contact with FOXA2. Our data suggests that the deletions in these three children may cause disease through the dysregulation of FOXA2 expression. These findings provide new insights into the regulation of FOXA2 in the beta-cell and confirm an aetiological role for chromosome 20p11.2 deletions in syndromic HI.
Asunto(s)
Deleción Cromosómica , Cromosomas Humanos Par 20 , Hiperinsulinismo Congénito , Factor Nuclear 3-beta del Hepatocito , Humanos , Factor Nuclear 3-beta del Hepatocito/genética , Factor Nuclear 3-beta del Hepatocito/metabolismo , Hiperinsulinismo Congénito/genética , Hiperinsulinismo Congénito/patología , Cromosomas Humanos Par 20/genética , Femenino , Masculino , Secuencias Reguladoras de Ácidos NucleicosRESUMEN
Identifying genes linked to extreme phenotypes in humans has the potential to highlight biological processes not shared with all other mammals. Here, we report the identification of homozygous loss-of-function variants in the primate-specific gene ZNF808 as a cause of pancreatic agenesis. ZNF808 is a member of the KRAB zinc finger protein family, a large and rapidly evolving group of epigenetic silencers which target transposable elements. We show that loss of ZNF808 in vitro results in aberrant activation of regulatory potential contained in the primate-specific transposable elements it represses during early pancreas development. This leads to inappropriate specification of cell fate with induction of genes associated with liver identity. Our results highlight the essential role of ZNF808 in pancreatic development in humans and the contribution of primate-specific regions of the human genome to congenital developmental disease.
Asunto(s)
Anomalías Congénitas , Elementos Transponibles de ADN , Proteínas de Unión al ADN , Páncreas , Animales , Humanos , Diferenciación Celular , Genoma Humano , Primates/anomalías , Primates/genética , Proteínas de Unión al ADN/genética , Anomalías Congénitas/genética , Páncreas/anomalíasRESUMEN
Transitioning from pluripotency to differentiated cell fates is fundamental to both embryonic development and adult tissue homeostasis. Improving our understanding of this transition would facilitate our ability to manipulate pluripotent cells into tissues for therapeutic use. Here, we show that membrane voltage (Vm) regulates the exit from pluripotency and the onset of germ layer differentiation in the embryo, a process that affects both gastrulation and left-right patterning. By examining candidate genes of congenital heart disease and heterotaxy, we identify KCNH6, a member of the ether-a-go-go class of potassium channels that hyperpolarizes the Vm and thus limits the activation of voltage gated calcium channels, lowering intracellular calcium. In pluripotent embryonic cells, depletion of kcnh6 leads to membrane depolarization, elevation of intracellular calcium levels, and the maintenance of a pluripotent state at the expense of differentiation into ectodermal and myogenic lineages. Using high-resolution temporal transcriptome analysis, we identify the gene regulatory networks downstream of membrane depolarization and calcium signaling and discover that inhibition of the mTOR pathway transitions the pluripotent cell to a differentiated fate. By manipulating Vm using a suite of tools, we establish a bioelectric pathway that regulates pluripotency in vertebrates, including human embryonic stem cells.
Asunto(s)
Células Madre Pluripotentes , Animales , Humanos , Calcio/metabolismo , Potenciales de la Membrana , Diferenciación Celular/genética , Serina-Treonina Quinasas TOR/genética , Serina-Treonina Quinasas TOR/metabolismo , Canales de Potasio Éter-A-Go-Go/metabolismoRESUMEN
Gene expression is tightly regulated, with many genes exhibiting cell-specific silencing when their protein product would disrupt normal cellular function1. This silencing is largely controlled by non-coding elements, and their disruption might cause human disease2. We performed gene-agnostic screening of the non-coding regions to discover new molecular causes of congenital hyperinsulinism. This identified 14 non-coding de novo variants affecting a 42-bp conserved region encompassed by a regulatory element in intron 2 of the hexokinase 1 gene (HK1). HK1 is widely expressed across all tissues except in the liver and pancreatic beta cells and is thus termed a 'disallowed gene' in these specific tissues. We demonstrated that the variants result in a loss of repression of HK1 in pancreatic beta cells, thereby causing insulin secretion and congenital hyperinsulinism. Using epigenomic data accessed from public repositories, we demonstrated that these variants reside within a regulatory region that we determine to be critical for cell-specific silencing. Importantly, this has revealed a disease mechanism for non-coding variants that cause inappropriate expression of a disallowed gene.
Asunto(s)
Hiperinsulinismo Congénito , Células Secretoras de Insulina , Humanos , Hexoquinasa/genética , Hexoquinasa/metabolismo , Hiperinsulinismo Congénito/genética , Hiperinsulinismo Congénito/metabolismo , Secreción de Insulina , Células Secretoras de Insulina/metabolismo , Secuencias Reguladoras de Ácidos Nucleicos/genéticaRESUMEN
Chromatin accessibility is generally perceived as a common property of active regulatory elements where transcription factors are recruited via DNA-specific interactions and other physico-chemical properties to regulate gene transcription. Recent work in the context of mitosis provides less trivial and potentially more interesting relationships than previously anticipated.
Asunto(s)
Cromatina/genética , Mitosis/genética , Factores de Transcripción/genética , Animales , Sitios de Unión/genética , Cromatina/metabolismo , Humanos , Factores de Transcripción/metabolismoRESUMEN
Embryonic development yields many different cell types in response to just a few families of inductive signals. The property of signal-receiving cells that determines how they respond to inductive signals is known as competence, and it differs in different cell types. Here, we explore the ways in which maternal factors modify chromatin to specify initial competence in the frog Xenopus tropicalis. We identify early-engaged regulatory DNA sequences, and infer from them critical activators of the zygotic genome. Of these, we show that the pioneering activity of the maternal pluripotency factors Pou5f3 and Sox3 determines competence for germ layer formation by extensively remodelling compacted chromatin before the onset of inductive signalling. This remodelling includes the opening and marking of thousands of regulatory elements, extensive chromatin looping, and the co-recruitment of signal-mediating transcription factors. Our work identifies significant developmental principles that inform our understanding of how pluripotent stem cells interpret inductive signals.
Asunto(s)
Ensamble y Desensamble de Cromatina/genética , Proteínas de Homeodominio/genética , Células Madre Pluripotentes/citología , Factores de Transcripción SOXB1/genética , Factores de Transcripción/genética , Proteínas de Xenopus/genética , Xenopus/embriología , Animales , Diferenciación Celular/genética , Cromatina/metabolismo , Desarrollo Embrionario/genética , Células Madre Embrionarias/citología , Regulación del Desarrollo de la Expresión Génica/genética , Estratos Germinativos/crecimiento & desarrollo , Secuencias Reguladoras de Ácidos Nucleicos/genética , Xenopus/genéticaRESUMEN
One of the earliest and most significant events in embryonic development is zygotic genome activation (ZGA). In several species, bulk transcription begins at the midblastula transition (MBT) when, after a certain number of cleavages, the embryo attains a particular nuclear-to-cytoplasmic (N/C) ratio, maternal repressors become sufficiently diluted, and the cell cycle slows down. Here we resolve the frog ZGA in time and space by profiling RNA polymerase II (RNAPII) engagement and its transcriptional readout. We detect a gradual increase in both the quantity and the length of RNAPII elongation before the MBT, revealing that >1,000 zygotic genes disregard the N/C timer for their activation and that the sizes of newly transcribed genes are not necessarily constrained by cell cycle duration. We also find that Wnt, Nodal, and BMP signaling together generate most of the spatiotemporal dynamics of regional ZGA, directing the formation of orthogonal body axes and proportionate germ layers.
Asunto(s)
Inmunidad Innata , Oligonucleótidos Antisentido , Animales , Composición de Base , Morfolinos , Xenopus laevisRESUMEN
Here we consider RNA-Seq, used to measure global gene expression through RNA fragmentation, capture, sequencing, and subsequent computational analysis. Xenopus, with its large number of RNA-rich, synchronously developing, and accessible embryos, is an excellent model organism for exploiting the power of high-throughput sequencing to understand gene expression during development. Here we present a standard RNA-Seq protocol for performing two-state differential gene expression analysis (between groups of replicates of control and treated embryos) using Illumina sequencing. Samples contain multiple whole embryos, and polyadenylated mRNA is measured under relative normalization. The protocol is divided into two parts: wet-lab processes to prepare samples for sequencing and downstream computational analysis including quality control, quantification of gene expression, and differential expression.
Asunto(s)
Perfilación de la Expresión Génica/métodos , RNA-Seq/métodos , Animales , Regulación del Desarrollo de la Expresión Génica , Biblioteca de Genes , Manejo de Especímenes , Xenopus/embriologíaRESUMEN
Antisense morpholino oligomers (MOs) have been indispensable tools for developmental biologists to transiently knock down (KD) genes rather than to knock them out (KO). Here we report on the implications of genetic KO versus MO-mediated KD of the mesoderm-specifying Brachyury paralogs in the frog Xenopus tropicalis. While both KO and KD embryos fail to activate the same core gene regulatory network, resulting in virtually identical morphological defects, embryos injected with control or target MOs also show a systemic GC content-dependent immune response and many off-target splicing defects. Optimization of MO dosage and increasing incubation temperatures can mitigate, but not eliminate, these MO side effects, which are consistent with the high affinity measured between MO and off-target sequence in vitro. We conclude that while MOs can be useful to profile loss-of-function phenotypes at a molecular level, careful attention must be paid to their immunogenic and off-target side effects.
Asunto(s)
Empalme Alternativo/efectos de los fármacos , Embrión no Mamífero/inmunología , Regulación del Desarrollo de la Expresión Génica/efectos de los fármacos , Morfolinos/farmacología , Oligonucleótidos Antisentido/farmacología , Proteínas de Xenopus/genética , Xenopus laevis/inmunología , Animales , Embrión no Mamífero/citología , Embrión no Mamífero/metabolismo , Femenino , Regulación del Desarrollo de la Expresión Génica/genética , Técnicas de Silenciamiento del Gen , Mesodermo/citología , Mesodermo/inmunología , Mesodermo/metabolismo , Xenopus laevis/genética , Xenopus laevis/crecimiento & desarrolloRESUMEN
Advances in RNA sequencing technologies have led to the surprising discovery that a vast number of transcripts emanate from regions of the genome that are not part of coding genes. Although some of the smaller ncRNAs such as microRNAs have well-characterized functions, the majority of long ncRNA (lncRNA) functions remain poorly understood. Understanding the significance of lncRNAs is an important challenge facing biology today. A powerful approach to uncovering the function of lncRNAs is to explore temporal and spatial expression profiling. This may be particularly useful for classes of lncRNAs that have developmentally important roles as the expression of such lncRNAs will be expected to be both spatially and temporally regulated during development. Here, we take advantage of our ultra-high frequency (temporal) sampling of Xenopus embryos to analyze gene expression trajectories of lncRNA transcripts over the first 3 days of development. We computationally identify 5689 potential single- and multi-exon lncRNAs. These lncRNAs demonstrate clear dynamic expression patterns. A subset of them displays highly correlative temporal expression profiles with respect to those of the neighboring genes. We also identified spatially localized lncRNAs in the gastrula stage embryo. These results suggest that lncRNAs have regulatory roles during early embryonic development.
Asunto(s)
ARN Largo no Codificante/genética , Xenopus/genética , Animales , Embrión no Mamífero/metabolismo , Exones/genética , Perfilación de la Expresión Génica , Regulación del Desarrollo de la Expresión Génica , Modelos Genéticos , ARN Largo no Codificante/biosíntesis , ARN Largo no Codificante/aislamiento & purificación , Transcriptoma , Xenopus/embriologíaRESUMEN
BACKGROUND: Because ribosomes are ubiquitously required for protein production, it was long assumed that any inherited defect in ribosome manufacture would be embryonically lethal. However, several human congenital diseases have been found to be associated with mutations in ribosome biogenesis factors. Surprisingly, despite the global requirement for ribosomes, these "ribosomopathies" are characterized by distinct and tissue specific phenotypes. The reasons for such tissue proclivity in ribosomopathies remain mysterious but may include differential expression of ribosome biogenesis factors in distinct tissues. METHODS: Here we use in situ hybridization of labeled antisense mRNA probes and ultra high temporal resolution RNA-Seq data to examine and compare expression of 13 disease associated ribosome biogenesis factors at six key stages in Xenopus tropicalis development. RESULTS: Rather than being ubiquitously expressed during development, mRNAs of all examined ribosome biogenesis factors were highly enriched in specific tissues, including the cranial neural crest and ventral blood islands. Interestingly, expression of ribosome biogenesis factors demonstrates clear differences in timing, transcript number and tissue localization. CONCLUSION: Ribosome biogenesis factor expression is more spatiotemporally regulated during embryonic development than previously expected and correlates closely with many of the common ribosomopathy phenotypes. Our findings provide information on the dynamic use of ribosome production machinery components during development and advance our understanding of their roles in disease.
Asunto(s)
Ribosomas/metabolismo , Análisis de Secuencia de ARN/métodos , Proteínas de Xenopus/genética , Xenopus/embriología , Animales , Desarrollo Embrionario , Perfilación de la Expresión Génica , Regulación del Desarrollo de la Expresión Génica , Redes Reguladoras de Genes , Hibridación in Situ , Especificidad de Órganos , Ribosomas/genética , Ribosomas/patología , Xenopus/genética , Xenopus/metabolismo , Proteínas de Xenopus/metabolismoRESUMEN
Transcript regulation is essential for cell function, and misregulation can lead to disease. Despite technologies to survey the transcriptome, we lack a comprehensive understanding of transcript kinetics, which limits quantitative biology. This is an acute challenge in embryonic development, where rapid changes in gene expression dictate cell fate decisions. By ultra-high-frequency sampling of Xenopus embryos and absolute normalization of sequence reads, we present smooth gene expression trajectories in absolute transcript numbers. During a developmental period approximating the first 8 weeks of human gestation, transcript kinetics vary by eight orders of magnitude. Ordering genes by expression dynamics, we find that "temporal synexpression" predicts common gene function. Remarkably, a single parameter, the characteristic timescale, can classify transcript kinetics globally and distinguish genes regulating development from those involved in cellular metabolism. Overall, our analysis provides unprecedented insight into the reorganization of maternal and embryonic transcripts and redefines our ability to perform quantitative biology.
Asunto(s)
ARN/metabolismo , Transcriptoma , Animales , Teorema de Bayes , Embrión no Mamífero/metabolismo , Etiquetas de Secuencia Expresada , Dosificación de Gen , Cinética , MicroARNs/metabolismo , Xenopus/crecimiento & desarrollo , Xenopus/metabolismoRESUMEN
Correct development of the vertebrate body plan requires the early definition of two asymmetric, perpendicular axes. The first axis is established during oocyte maturation, and the second is established by symmetry breaking shortly after fertilization. The physical processes generating the second asymmetric, or dorsal-ventral, axis are well understood, but the specific molecular determinants, presumed to be maternal gene products, are poorly characterized. Whilst enrichment of maternal mRNAs at the animal and vegetal poles in both the oocyte and the early embryo has been studied, little is known about the distribution of maternal mRNAs along either the dorsal-ventral or left-right axes during the early cleavage stages. Here we report an unbiased analysis of the distribution of maternal mRNA on all axes of the Xenopus tropicalis 8-cell stage embryo, based on sequencing of single blastomeres whose positions within the embryo are known. Analysis of pooled data from complete sets of blastomeres from four embryos has identified 908 mRNAs enriched in either the animal or vegetal blastomeres, of which 793 are not previously reported as enriched. In contrast, we find no evidence for asymmetric distribution along either the dorsal-ventral or left-right axes. We confirm that animal pole enrichment is on average distinctly lower than vegetal pole enrichment, and that considerable variation is found between reported enrichment levels in different studies. We use publicly available data to show that there is a significant association between genes with human disease annotation and enrichment at the animal pole. Mutations in the human ortholog of the most animally enriched novel gene, Slc35d1, are causative for Schneckenbecken dysplasia, and we show that a similar phenotype is produced by depletion of the orthologous protein in Xenopus embryos.
Asunto(s)
Blastómeros/metabolismo , Xenopus/embriología , Xenopus/genética , Animales , Tipificación del Cuerpo/genética , Femenino , Técnicas de Silenciamiento del Gen , Humanos , Masculino , Modelos Animales , Proteínas de Transporte de Monosacáridos/antagonistas & inhibidores , Proteínas de Transporte de Monosacáridos/genética , Mutación , ARN Mensajero/genética , ARN Mensajero/metabolismo , Transcripción Genética , Xenopus/metabolismo , Proteínas de Xenopus/antagonistas & inhibidores , Proteínas de Xenopus/genéticaRESUMEN
Environmental stimuli are known to contribute to psoriasis pathogenesis and that of other autoimmune diseases, but the mechanisms are largely unknown. Here we show that the aryl hydrocarbon receptor (AhR), a transcription factor that senses environmental stimuli, modulates pathology in psoriasis. AhR-activating ligands reduced inflammation in the lesional skin of psoriasis patients, whereas AhR antagonists increased inflammation. Similarly, AhR signaling via the endogenous ligand FICZ reduced the inflammatory response in the imiquimod-induced model of skin inflammation and AhR-deficient mice exhibited a substantial exacerbation of the disease, compared to AhR-sufficient controls. Nonhematopoietic cells, in particular keratinocytes, were responsible for this hyperinflammatory response, which involved upregulation of AP-1 family members of transcription factors. Thus, our data suggest a critical role for AhR in the regulation of inflammatory responses and open the possibility for novel therapeutic strategies in chronic inflammatory disorders.
Asunto(s)
Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/inmunología , Inflamación/inmunología , Psoriasis/inmunología , Receptores de Hidrocarburo de Aril/inmunología , Adyuvantes Inmunológicos/farmacología , Aminoquinolinas/farmacología , Animales , Hidrocarburo de Aril Hidroxilasas/biosíntesis , Compuestos Azo/farmacología , Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/agonistas , Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/genética , Carbazoles/farmacología , Citocromo P-450 CYP1A1/biosíntesis , Citocromo P-450 CYP1B1 , Citocinas/farmacología , Exposición a Riesgos Ambientales , Humanos , Imiquimod , Queratinocitos/inmunología , Ratones , Ratones Noqueados , Psoriasis/patología , Pirazoles/farmacología , Receptores de Hidrocarburo de Aril/agonistas , Receptores de Hidrocarburo de Aril/genética , Transducción de Señal/inmunología , Piel/inmunología , Piel/metabolismo , Factores de Transcripción/biosíntesis , Regulación hacia ArribaRESUMEN
The Xenopus mid-blastula transition (MBT) marks the onset of large-scale zygotic transcription, as well as an increase in cell cycle length and a loss of synchronous cell divisions. Little is known about what triggers the activation of transcription or how newly expressed genes interact with each other. Here, we use high-resolution expression profiling to identify three waves of gene activity: a post-fertilisation wave involving polyadenylation of maternal transcripts; a broad wave of zygotic transcription detectable as early as the seventh cleavage and extending beyond the MBT at the twelfth cleavage; and a shorter post-MBT wave of transcription that becomes apparent as development proceeds. Our studies have also allowed us to define a set of maternal mRNAs that are deadenylated shortly after fertilisation, and are likely to be degraded thereafter. Experimental analysis indicates that the polyadenylation of maternal transcripts is necessary for the establishment of proper levels of zygotic transcription at the MBT, and that genes activated in the second wave of expression, including Brachyury and Mixer, contribute to the regulation of genes expressed in the third. Together, our high-resolution time series and experimental studies have yielded a deeper understanding of the temporal organisation of gene regulatory networks in the early Xenopus embryo.
Asunto(s)
Blástula/metabolismo , Regulación del Desarrollo de la Expresión Génica , Xenopus/embriología , Xenopus/genética , Animales , Embrión no Mamífero/metabolismo , Perfilación de la Expresión Génica , Anotación de Secuencia Molecular , Poli A/metabolismo , Poliadenilación/genética , Estabilidad del ARN/genética , ARN Mensajero Almacenado/genética , ARN Mensajero Almacenado/metabolismo , Reproducibilidad de los Resultados , Factores de Transcripción/metabolismo , Transcripción Genética , Proteínas de Xenopus/genética , Proteínas de Xenopus/metabolismo , Pez Cebra/genéticaRESUMEN
The design of effective cell replacement therapies requires detailed knowledge of how embryonic stem cells form primary tissues, such as mesoderm or neurectoderm that later become skeletal muscle or nervous system. Members of the T-box transcription factor family are key in the formation of these primary tissues, but their underlying molecular activities are poorly understood. Here, we define in vivo genome-wide regulatory inputs of the T-box proteins Brachyury, Eomesodermin, and VegT, which together maintain neuromesodermal stem cells and determine their bipotential fates in frog embryos. These T-box proteins are all recruited to the same genomic recognition sites, from where they activate genes involved in stem cell maintenance and mesoderm formation while repressing neurogenic genes. Consequently, their loss causes embryos to form an oversized neural tube with no mesodermal derivatives. This collaboration between T-box family members thus ensures the continuous formation of correctly proportioned neural and mesodermal tissues in vertebrate embryos during axial elongation.
Asunto(s)
Desarrollo Embrionario/fisiología , Proteínas de Dominio T Box/metabolismo , Animales , ADN/genética , ADN/metabolismo , Células Madre Embrionarias/citología , Células Madre Embrionarias/metabolismo , Mesodermo/citología , Mesodermo/metabolismo , Tubo Neural/citología , Tubo Neural/metabolismo , Neuronas/citología , Neuronas/metabolismo , Proteínas de Dominio T Box/genética , XenopusRESUMEN
Sex chromosome divergence has been documented across phylogenetically diverse species, with amphibians typically having cytologically nondiverged ("homomorphic") sex chromosomes. With an aim of further characterizing sex chromosome divergence of an amphibian, we used "RAD-tags" and Sanger sequencing to examine sex specificity and heterozygosity in the Western clawed frog Silurana tropicalis (also known as Xenopus tropicalis). Our findings based on approximately 20 million genotype calls and approximately 200 polymerase chain reaction-amplified regions across multiple male and female genomes failed to identify a substantially sized genomic region with genotypic hallmarks of sex chromosome divergence, including in regions known to be tightly linked to the sex-determining region. We also found that expression and molecular evolution of genes linked to the sex-determining region did not differ substantially from genes in other parts of the genome. This suggests that the pseudoautosomal region, where recombination occurs, comprises a large portion of the sex chromosomes of S. tropicalis. These results may in part explain why African clawed frogs have such a high incidence of polyploidization, shed light on why amphibians have a high rate of sex chromosome turnover, and raise questions about why homomorphic sex chromosomes are so prevalent in amphibians.