We have seen that Placozoans are very simple animals. They are essentially two sheets of cells that slowly crawl about digesting material they happen to crawl upon (actually, the fiber cells between the two sheets are an interesting story, but we’ll save that for later).
We have also seen that recent research indicates that Placozoans “have passed over sponges and other organisms as an animal that most closely mirrors the root of this tree of life.” This is very significant because it would mean, for example, that the nervous system independently evolved at least twice.
But the real fun comes from surveying the genome of this little creature (specifically, the genome of Trichoplax), as its genome provides some rather striking support for the hypothesis of front-loading.
I’d like to quote extensively from a review paper entitled, Placozoa and the evolution of Metazoa and intrasomatic cell differentiation. It is by Bernd Schierwater, Danielle de Jonga and Rob DeSalleb and was published in The International Journal of Biochemistry & Cell Biology 41 (2009) 370–379.
Trichoplax has a very small genome that only encodes about 11,000 genes. Nevertheless, it contains many genes of great interest. Schierwater et al. write:
In addition, while the genome may be small, the gene content of Trichoplax is relatively complex, in sharp contrast to its low morphological complexity.
Indeed. Consider some examples:
While recognized as having only four (or five) cell types and no axis of symmetry, Trichoplax contains a wide array of transcription factors, both animal- and opistokhont-specific, including those involved in cell type specification and differentiation (e.g. LIM-homeobox genes, bHLH genes and GATA-family zinc finger transcription factors) and embryonic (T-box family) and neuroendocrine development (POU homeodomain family). In addition it contains genes which specify structures in higher animals, which are simply not present in Trichoplax, for examples genes for extracellular matrix production, germline separation and neural signaling.
My goodness. This simple animal seems to be loaded with all kinds of members of the metazoan toolkit. But, as the authors note, we’ve seen this before. Where?
This is similar to the results of the recent genome sequencing of the choanoflagellate Monosiga brevicollis, where many genes previously thought to be metazoan-specific were identified, and in general genetic complexity was higher than expected.
Yet it’s not higher than expected from the perspective of front-loading. On the contrary, this is what we expect from front-loading. And it gets better. What if we turned to the signaling circuits that are needed for the development of embryos in complex animals?
Typically, only a handful of signaling pathways are involved in cell differentiation and/or patterning of embryonic structures in metazoans, including the Hedgehog (Hh), bone morphogenic protein/transforming growth factor-( (BMP/TGF-(), Notch and Wingless (Wnt) pathways.
So what if we turn to the genome of an organism that exists as two sheets of cells?
The Trichoplax genome contains all major components of two highly important pathways, theBMP/TGF-( andWntsignaling pathway. The Wnt pathway is known to be involved in axial patterning in basal animals, in a system that has been compared to the Hox axial patterning system of higher animals…..A search of the Trichoplax genome reveals three wnt genes (belonging to different families), receptors (frizzled and disheveled), and all major downstream intracellular components
In bilaterians, BMP/TGF-( signaling plays a role in the establishment of the embryonic dorsal/ventral axis, and is also thought to play roles in axial patterning in cnidarians and demosponge larvae, despite no discernible homology between the bilaterian, poriferan and cnidarian axes. In the Trichoplax genome, there are representatives of all major genes which constitute the BMP/TGF-( pathway, including ligand, receptors and intracellular components.
These researchers then conclude:
Taken together, a functioning Wnt and BMP/TGF-( pathway alone would give Trichoplax the ability to transducer many complex regulatory and developmental signals, and while not discussed here, the Trichoplax genome also contains many transcription factors, commonly associated in the gastrulation and development of higher animals. However, given the simple cellular morphology, the lack of any type of recognizable symmetry analogous to either the oral/aboral or anterior/posterior-dorsal/ventral axes of cnidarians/bilaterians, and as of yet, incompletely observed embryonic development, the finding Trichoplax contains a repertoire as complex as it does in its genome is intriguing.
What they are saying, folks, is that although this organism is essentially two sheets of cells, it contains the circuitry that is needed to specific tissues, organs, and body plans in complex animals. This does not mean the circuitry is non-functional in Placozoans, as it is doing something for this simple animals, but it does mean the circuitry needed to evolve complex metazoan structures was in place long before those structures appeared – preadaptation. In fact, the authors word this point as follows:
In any case, it is clear that the placozoan/eumetazoan ancestor already possessed a complex repertoire of signaling pathways which were most likely essential for the transition from Protozoa to Metazoa.
While this is truly exciting, what about the other two signaling pathways?
There is no recognizable Hedgehog ligand, receptors (Patched and Smoothened), or downstream GLI-like transcription factor
Ah, but it is likely this is because Trichoplax has lost this circuit. See the analysis by Techne.
And that leaves us with Notch:
although certain Notch pathway components are present, there appears to be no Notch domain-containing gene.
Okay, certain parts of the circuit are present, but not the thing that throws the switch (Notch). Perhaps this is a circuit that has been under decay. The hypothesis of front-loading evolution predicts we’ll find Notch in some protozoan.
In the end, this is all very encouraging. The simplest known animal, that most closely mirrors the root of the multicellular tree, is loaded with circuitry, both signaling and transcription, that were once thought to be specific to complex metazoans precisely because they are needed to carry out essential metazoan functions like differentiation and development. This iswhat we would predict if unicellular life forms were front-loaded to facilitate the evolution of metazoan life. After all, it’s the very kind of thing I predicted years ago, as I shall show in the next installment.