The Wnt genes produce signalling proteins that play important roles in early development, regulating cell proliferation, differentiation and migration. It’s hugely important, used in everything from early axis specification in the embryo to fine-tuning axon pathfinding in the nervous system. The way they work is that the Wnt proteins are secreted by cells, and they then bind to receptors on other cells (one receptor is named Frizzled, and others are LRP-5 and 6), which then, by a chain of cytoplasmic signalling events, removes β-catenin from a degradation pathway and promotes its import into the nucleus, where it can modify patterns of gene expression. This cascade can also interact with the cytoskeleton and trigger changes in cell migration and cell adhesion. The diagram below illustrates the molecular aspects of its function.
This is greatly simplified, of course. There are different pathways and different roles in different cells under different conditions. Mammals have 19 Wnt genes, so far, and as I mentioned above, have diverse functions. The obvious questions are where all this complexity originated, and what role the original Wnt gene played. One way to answer this question is to examine simpler organisms that separated from our messily complicated lineage long, long ago, and by comparison, try to infer what Wnt genes were present in our last common ancestor. Kusserow et al. (2005) have done this in a sea anemone, Nematostella vectensis, and got a somewhat surprising answer: our last common ancestor with a diploblast also had an elaborate array of Wnt genes.
This particular sea anemone, Nematostella vectensis, is especially interesting because while it is a diploblast from a group best known for radial symmetry, this particular species is bilaterally symmetric; it’s of interest because it may tell us something about the properties of early bilaterians (I’ve commented on Nematostella before.) Compared to us elaborately bilateral mammals, these little cnidarians are just marginally distinguished by a dorso-ventral axis.
Now here’s the thing: mammals have 19 Wnt genes; Nematostella has 12. Those numbers understate the relative complexity, too. Kusserow et al. analyzed all the Wnt genes, and grouped them into 12 distinct subfamilies (those groups with more than 12 have more than one member in a single subfamily, indicative of a relatively recent duplication.) Out of 12 subfamilies of Wnt genes, Nematostella has 11. Here’s the breakdown of the distribution:
*There is an additional Wnt gene from D. melanogaster in the database described as DmWnt8 (accession number Q9VFX1) that shows, however, no orthology to any of the conserved subfamilies and lacks a large set of conserved features of Wnt ligands, including some of the conserved cysteine residues. Similarly, human Wnt16 and three Wnt genes from C. elegans exhibit no orthology to any of the conserved subfamilies. We call these genes ‘orphan Wnts’.
†Sequences cluster within the conserved subfamilies 2,10, but were not used in the phylogenetic analyses in part of the paper.
Now one explanation one might offer is that maybe the original ur-bilaterian had a single Wnt gene that was independently expanded in the cnidarian and vertebrate lineages, but that doesn’t fit the data at all. Each of these Wnt subfamilies are represented in the lineages, and an analysis of the sequences shows that they cluster together. In the tree diagram below, look at Wnt8, the green lines. Wnt8 is found in both Nematostella vectensis (NvWnt8A and NvWnt8B) and in humans (HsWnt8A and HsWnt8B). These genes are more similar to each other than either is to the Wnt6 genes (the dark blue lines) found in the same organism.
What these data tell us is that the primitive condition, the state of the last common ancestor of sea anemones and human beings, was to have all of these Wnt gene subfamilies. The metazoan toolbox was well stocked with at least this class of important signalling/patterning genes at the onset of animal evolution.
Why did such simple animals need so many Wnt genes? The authors suggest that what it is is an ancient blastoporal signaling center, a region that was used to specify body pattern along the anterior-posterior axes, and within the ectoderm and endoderm. Here is their diagram of the pattern of expression of these genes in the planula:
What you see is a staggered array of expression, a pattern that suggests the original (and current in Nematostella) role of these genes was in specifying position along the primary axis, and also in regulating cell movement and differentiation around the blastopore—a key function in the evolution of gastrulation.
In both the title and the conclusion, the authors emphasize the surprising complexity of our ancient ancestors.
Our result also points to an unexpected paradox of genome evolution: the gene diversity in the genomes of simple metazoans is much higher than previously predicted and some derived lineages (flies and nematodes) have an even lower diversity of gene family members. Thus there is no simple relationship between genetic and morphological complexity. We presume that for the successful transition from single cell to multi-cellular animals a whole concert of interacting signalling molecules was required. This led to the formation of a stable signalling centre, which induced the complex cellular machinery of gastrulation, causing the ingression of cells and/or invagination of an outer body layer. This ‘robust’ patterning system was probably the starting point in the rapid generation of more complex animal body plans. An expansion of transcription factor families—as for instance in the case of Hox genes in chordate and of MADS box genes in flower evolution—was probably correlated with the later rise in morphological complexity during the Cambrian evolutionary explosion and more recent evolution.
I don’t know that I’d call it paradoxical. The kernel of this idea is already in evolutionary theory, that evolution may proceed by duplication and expansion of elements of the genome, followed by fine-tuning, specialization, and paring down. In that sense, as a general observation, this result isn’t surprising at all. What is unexpected is how far back in time we have to push the period of expansion of complexity. The original bilaterian was equipped with a fairly elaborate set of molecular tools.
The observation that genomic complexity is not tightly coupled to morphological complexity is also important. What that is saying is that the diversification of complex forms observed in the Cambrian was not going on concurrently with an elaboration of genetic systems, but was instead built on a foundation of rich genomic resources accumulated during the previous few billion years.
Someday we’re going to learn that developmental biology is just a freakishly specialized subset of microbiology, and that many of the really important, critical processes of development evolved first in bacteria and colonial organisms. I’m sure a few of my colleagues are itching for the day they can say, “I told you so.”
Kusserow A, Pang K, Sturm C, Hrouda M, Lentfer J, Schmidt HA, Technau U, von Haeseler A, Hobayer B, Martindale MQ, Holstein TW (2005) Unexpected complexity of the Wnt gene family in a sea anemone. Nature 433:156-160.