Mapping gene expression in two Xenopus species: evolutionary constraints and developmental flexibility.
Changes in gene expression are thought to be important for morphological evolution, though little is known about the nature or magnitude of the differences. Here, we examine Xenopus laevis and Xenopus tropicalis, two amphibians with very similar development, and ask how their transcriptomes compare. Despite separation for ~30-90 million years, there is strong conservation in gene expression in the vast majority of the expressed orthologs. Significant changes occur in the level of gene expression but changes in the timing of expression (heterochrony) were much less common. Differences in level were concentrated in the earliest embryonic stages. Changes in timing were prominently found in pathways that respond to selective features of the environment. We propose that different evolutionary rates across developmental stages may be explained by the stabilization of cell fate determination in the later stages.
PubMed ID: 21497761
PMC ID: PMC3118554
Article link: Dev Cell.
Grant support: R01 GM026875-34 NIGMS NIH HHS , R01 GM026875 NIGMS NIH HHS , R01 GM026875 NIGMS NIH HHS , R01 GM026875-34 NIGMS NIH HHS
Genes referenced: c6.1 c6.2 c8a c8b c9 cfd cpn1 cygb fgf8 gli3 hoxb3 krt12 krt24 nog nog2 notch1 orai2 pax6 rdh16 shh wnt11
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|Figure 1. Comparative Transcriptomics of Xenopus Development(A) Developmental stages assayed in this study, taken from Nieuwkoop and Faber (1994).(B) Microarray gene expression data for the eight indicated genes. For each gene, the nine profiles (three clutches across three probes) are shown for both X. laevis (blue) and X. tropicalis (green). The y axis indicates log10 relative concentrations of mRNA abundance (see Experimental Procedures). Also indicated for each gene is its EDi, a metric for divergence explained in the text.(C) Summary of 20 gene expression clusters for X. laevis (left). Clusters were generated using QT clustering (Heyer et al., 1999) with a maximum correlation distance of 0.85. Each summary profile is the mean of the member profiles, normalized by dividing by the maximum value. The average profile of the X. tropicalis orthologs in these same clusters are shown to the right. Orthologous clusters share the same color across plots.|
|Figure 2. Dominant Signal of Gene Expression Conservation between the SpeciesDistributions of correlation coefficients when different classes of transcriptomes are compared. Dark red distributions indicate the median correlations between expression profiles of different probes (within the same clutch). Bright blue distributions indicate distributions of median correlations of expression profiles across clutches (within the same probes). These distributions are shifted toward higher correlations with respect to the distribution of median pairwise correlations between species (black) and randomly paired X. laevis and X. tropicalis genes (dashed line). Since this analysis tests the reproducibility of the data, we limited it to those genes showing dynamic expression in the developmental time course of either species (see Experimental Procedures).|
|Figure 3. Conservation and Divergence across Pathways(A) Expression divergence (EDi) distributions across functional gene sets and indicated by different colors. The y axis indicates the normalized frequency. The black plot indicates the normalized distribution of divergences for all genes. A shift to the left/right indicates enrichment for conservation/divergence, respectively.(B–D) Expression profiles of genes involved in the alternative pathway of the complement system (B), hatching enzymes (C), and oxygen-binding genes (D). See Figure S3B for additional heterochrony in members of the membrane attack complex. Expression profiles are shown in log10 relative concentrations as in Figure 1.|
|Figure 4. Heterochrony and Heterometry in Gene Expression Evolution(A) A sigmoid is defined by b1, t1, h0, and h1 (see text).(B) The ORAI2 gene expression profiles (log10 relative concentrations) in both X. laevis and X. tropicalis is shown fitted by sigmoids.(C) Plots of t1 (time of induction) for pairs of genes with >0.8 goodness of fit in both species. The green line is unity and the red is fitted to the data.(D) Plots of h1 (range of expression). Same format as (C).(E and F) Heterochrony/heterometry phase-plane for families of transcription factors (E) and several signaling pathways (F). The circles and lines indicate the mean and standard deviation of each gene set's heterochronies and heterometries. The transcription factor families are helix-loop-helix (PF00010, 27 genes with sigmoids), Homeobox (IPR001356, 74 genes), Zinc finger (C2H2 type, IPR007087, 24 genes), T-box (IPR001699, 5 genes), Fox head (IPR001766, 16 genes), and HMG (IPR000910, 8 genes). The signaling pathways are Wnt receptor (GO:0016055, 18 genes with sigmoids), transforming growth factor beta receptor (GO:0007179, 13 genes), Hedgehog (GO:0007224, 6 genes), Transmembrane receptor protein tyrosine kinase (GO:0007169, 17 genes), Notch (GO:0007219, 9 genes), Apoptosis (GO:0006917, 40 genes), and G protein-coupled receptor protein (GO:0007186, 73 genes).|
|Figure 5. Global Comparison of the X. laevis and X. tropicalis Developmental Transcriptomes(A–C) The heat maps represent the fraction of genes significantly different between pairs of transcriptomes; the grid separates the replicates across the stages. The color of each square indicates the difference at the specified developmental stages. (A) and (B) represent X. tropicalis and X. laevis plotted against themselves.(C) X. tropicalis plotted against X. laevis. The diagonal squares by definition have zero divergence. The fraction is computed as the number of genes with a difference of at least 1.5 log10 units out of the number of genes with a maximum expression of at least 2.5 log10 units in either transcriptomes.(D) For 2297 dynamically expressed genes, the plot indicates the number of genes with significantly different transcript abundance (>1.5 log10).|
|Figure 6. Comparison of the Maternal Transcriptomes of X. laevis and X. tropicalis(A) For each of four functional gene sets, the plot compares the expression levels (log10 relative concentrations) in stage 2 embryos between the two species.(B) Comparative gene expression profiles of four keratin genes. These genes are less than 80% identical at the protein level allowing good resolution by the microarray data. KRT24 in X. tropicalis exhibits two patterns depending upon the probes examined: one with maternal expression and another with a profile heterochronic to the X. laevis ortholog.(C) Comparative gene expression profiles of four retinol dehydrogenases.|