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The Wnt family of growth factors regulate many different aspects of embryonic development. Assembly of the complete mouse and human genome sequences, plus expressed sequence tag surveys have established the existence of 19 Wnt genes in mammalian genomes. However, despite the importance of model vertebrates for studies in developmental biology, the complete complement of Wnt genes has not been established for nonmammalian genomes. Using genome sequences for chicken (Gallus gallus), frog (Xenopus tropicalis), and fish (Danio rerio and Tetraodon nigroviridis), we have analyzed gene synteny to identify the orthologues of all 19 human Wnt genes in these species. We find that, in addition to the 19 Wnts observed in humans, chicken contained an additional Wnt gene, Wnt11b, which is orthologous to frog and zebrafish Wnt11 (silberblick). Frog and fish genomes contained orthologues of the 19 mammalian Wnt genes, plus Wnt11b and several duplicated Wnt genes. Specifically, the Xenopus tropicalis genome contained 24 Wnt genes, including additional copies of Wnt7-related genes (Wnt7c) and 3 recent Wnt duplications (Wnt3, Wnt9b, and Wnt11). The Danio rerio genome contained 27 Wnt genes with additional copies of Wnt2, Wnt2b, Wnt4b, Wnt6, Wnt7a, and Wnt8a. The presence of the additional Wnt11 sequence (Wnt11b) in the genomes of all ancestral vertebrates suggests that this gene has been lost during mammalian evolution. Through these studies, we identified the frog orthologues of the previously uncharacterized Wnt2, Wnt3, Wnt9a, Wnt9b, Wnt10a, and Wnt16 genes and their expression has been characterized during early Xenopus development.
Figure 2. In situ hybridization analysis of Wnt2 expression during Xenopus embryonic development. A: Lateral view of stage 20 embryo showing Wnt2 expression in the lateral region just posterior to the head. B: Lateral view of stage 23 embryo showing increased extent of Wnt2 expression domain. C: Transverse section through Wnt2 expressing region of a stage 26 embryo showing expression restricted to deepmesodermtissue adjacent to foregutendoderm. D: Lateral view of stage 28 embryo. The domain of Wnt2 expression is greatest at this stage and extends ventrally toward the heart forming region. E: Transverse section of stage 28 embryo showing Wnt2 expression restricted to mesoderm. F: Lateral view of stage 32 embryo. Residual Wnt2 expression is limited to a small region of the flank (arrowhead) posterior to the pharyngeal arches. Genbank accession number for Xenopus tropicalis Wnt2 is EF447424.
Figure 3. In situ hybridization analysis of Wnt3 expression during Xenopus embryonic development. A: Dorsal view of stage 22 embryo showing Wnt3 transcripts in the anterior region of the brain corresponding to the midbrain. B: Lateral view of stage 28 embryo showing expression of Wnt3 in the midbrain. C: Lateral view of stage 35 embryo showing expression of Wnt3 in the midbrain and extending into the hindbrain. D: Lateral view of the head region of a stage 41 embryo showing expression of Wnt3 in the midbrain and in the eye. GenBank accession number, DQ658158
Figure 4. In situ hybridization analysis of Wnt9a and Wnt9b expression during Xenopus embryonic development. A: Lateral view of stage 32 embryo showing Wnt9a transcripts in the pronephric duct (pd). B: Lateral view of stage 37 embryo showing expression of Wnt9a in the pronephric duct and lung bud (lb) C: Transverse section through the anteriorforegut of a stage 37 embryo showing staining of Wnt9a in the endodermal cells of the lung buds. D: Lateral view of the head region of a stage 40 embryo showing expression of Wnt9a in the eye, lung bud, and pronephric duct. E: Lateral view of stage 22 embryo showing expression of Wnt9b in the branchial arch region. F: Lateral view of stage 26 embryo showing persistent expression of Wnt9b in the branchial arch region. G: Transverse section through the branchial arch region of a stage 26 embryo showing Wnt9b expression confined to the inner ectodermal layer of the epidermis. H: Lateral view of a stage 37 embryo showing expression of Wnt9b in the branchial arches, the eye, and adjacent to the cement gland. GenBank accession numbers for Xenopus tropicalis Wnt9a and Wnt9b are DQ658159 and DQ658160, respectively.
Figure 5. In situ hybridization analysis of Wnt10a during Xenopus development. A: Dorsal view of stage 13 embryo showing expression of Wnt10a in the trunkectoderm of the embryo but not within the neural plate. B: Transverse section through embryo in A showing staining of Wnt10a in the endodermal cells just lateral to the neural plate. For reference, the notochord (nc) is outlined in yellow. C: Lateral view of a stage 25 embryo showing expression of Wnt10a in the ectoderm and dorsal fin keel (dfk). D: Transverse section through the trunk region of a stage 25 embryo showing staining of Wnt10a in the ectodermal cells of the dorsal fin keel. E: Lateral view of a stage 34 embryo showing expression of Wnt10a in the lateralectoderm and persisting in the ectodermtissue of the dorsal fin (df). GenBank accession number for Xenopus laevis Wnt10a is DQ658155.
Figure 6. In situ hybridization analysis of Wnt16 expression during Xenopus embryonic development. A: Lateral view of a stage 35 embryo showing the expression ofWnt16 in the hypochord (hyc). B: Higher magnification of the embryo in A showing hypochord staining. C: Transverse section through the trunk region of a stage 35 embryo showing Wnt16 expression confined to the hypochord. D: Lateral view of a stage 40 embryo showing Wnt16 expression in the eye and a discrete region of the headventral to the fin arches. E: Transverse section through the head region shows Wnt16 expression in the surface ectoderm. GenBank accession number, DQ658157.
wnt3 (Wnt family member 3) gene expression in X. tropicalis embryo, assayed by in situ hybridization, NF stage 27/28, lateral view, anteriorleft, dorsal up.
Fig. 6. Wnt16 expression at stg. 40
wnt3 (Wnt family member 3)gene expression in Xenopus tropicalis embryo, assayed via in situ hybridization, NF stage 41, lateral view, anteriorleft, dorsal up.
