Differential deployment of paralogous Wnt genes in the mouse and chick embryo during development

Abstract

SUMMARYGenes encoding Wnt ligands are crucial in body patterning and are highly conserved among metazoans. Given their conservation at the protein-coding level, it is likely that changes in where and when these genes are active are important in generating evolutionary variations. However, we lack detailed knowledge about how their deployment has diverged. Here, we focus on four Wnt subfamilies (Wnt2, Wnt5, Wnt7, and Wnt8) in mammalian and avian species, consisting of a paralogous gene pair in each, believed to have duplicated in the last common ancestor of vertebrates. We use three-dimensional imaging to capture expression patterns in detail and carry out systematic comparisons. We find evidence of greater divergence between these subgroup paralogues than the respective orthologues, consistent with some level of subfunctionalization/neofunctionalization in the common vertebrate ancestor that has been conserved. However, there were exceptions; in the case of chick Wnt2b, individual sites were shared with both mouse Wnt2 and Wnt2b. We also find greater divergence, between paralogues and orthologues, in some subfamilies (Wnt2 and Wnt8) compared to others (Wnt5 and Wnt7) with the more highly similar expression patterns showing more extensive expression in more structures in the embryo. Wnt8 genes were most restricted and most divergent. Major sites of expression for all subfamilies include CNS, limbs, and facial region, and in general there were more similarities in gene deployment in these territories with divergent patterns featuring more in organs such as heart and gut. A detailed comparison of gene expression patterns in the limb showed similarities in overall combined domains across species with notable differences that may relate to lineage-specific morphogenesis.

Wnt evolution and function shuffling in liberal and conservative chordate genomes

“…While in ascidians, Wnt5 seems to be the only ligand determining early posteriority in the primary axis, in amphioxus our study reveals a highly redundant posterior Wnt expression system involving at least eight out of the 13 subfamilies (i.e., Wnt1 , 8 , 11 , and 3 surrounding the blastopore during gastrulation, plus Wnt4 , 5 , 16 , and 6 in the most caudal part of the embryo later during neurulation and larval stages). In vertebrates, interestingly, while some species (similarly to ascidians) use a reduced number of Wnt ligands for determing early posteriority (e.g., Wnt8a in zebrafish [ 76 ], Wnt11 (and Wnt5 ) in Xenopus [ 77 , 78 ], and Wnt3 in mouse [ 79 ] (reviewed in [ 11 ]), other species such as chicken show a redundant posterior Wnt system, more similarly to amphioxus (e.g., [ 80 , 81 ]; Additional file 1 : Figure S3 and associated references). If the ancestral chordate relied on a simple Wnt system for determing posteriority, extensive Wnt function shuffling occurred during the evolution of the cephalochordate lineage as well as some vertebrate species such as chicken, recruiting other Wnt subfamilies for this posterior signaling role.…”

De novo draft assembly of the Botrylloides leachii genome provides further insight into tunicate evolution

“…The most striking was an increase in Wnt5a gene copy number in B. leachii , B. schlosseri and M. oculata . Indeed, most invertebrate genomes, including the basal chordate B. floridae , contain a single Wnt5a gene while most vertebrate genomes have two Wnt5a paralogs, believed to be a result of whole genome duplication 110 . Potentially, these additional genes have been co-opted into novel roles and were retained during tunicate evolution.…”

Divergent Wnt8a Gene Expression in Teleosts

“…Conversely, Wnt8a gene expression in cells of the developing hindbrain rhombomeres seen in various species including zebrafish appears not to be conserved in medaka [17], [28], [29], [33]. Moreover, Wnt8a expression in limbs and branchial arches of mouse and chick embryos is not seen in fish [36]. Intriguingly, expression in the gut, heart and otic vesicles appears to be conserved between medaka and mouse [36]–[38], while only medaka Wnt8a gene expression is found in the caudal hematopoietic tissue, gall- and swim bladder (Table 1).…”

“…Moreover, Wnt8a expression in limbs and branchial arches of mouse and chick embryos is not seen in fish [36]. Intriguingly, expression in the gut, heart and otic vesicles appears to be conserved between medaka and mouse [36]–[38], while only medaka Wnt8a gene expression is found in the caudal hematopoietic tissue, gall- and swim bladder (Table 1). …”

“…This suggests that also during stages following somitogenesis both zebrafish Wnt8a genes are expressed at low levels, which, however, is not detectable by in situ hybridization. In mouse and chick embryos Wnt8a expression is confined to the fore- and midbrain as well as limbs and branchial arches [36] . Furthermore, expression in the gut, heart and otic vesicles has been reported for mouse embryos [36] – [38] .…”