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The evolution of the Hippo

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  • The evolution of the Hippo

    No. It's not what you think.

    This is about molecules, but they are molecules that hippos do in fact need. So do we.

    The names of gene/protein pathways, genes, protein domains and so on, do often sound funny. For instance, in a short while you will be introduced to "Hippo", "hpo", "Yap", "Yorkie" and a host of other odd names. But in reality they are often acronyms. Except in the case of "Hippo", where its name is derived from the fact that mutations to the hpo gene can lead to hippopotamus-like tissue overgrowth. However "Yap" is derived from "yes-associated protein". What about "yes"? Well I have not been able to track that one down.

    Anyway, the following paper (which is online):-

    Premetazoan Origin of the Hippo Signaling Pathway

    - deals with the evolution of a signalling pathway called Hippo, which in multicellular organisms like us regulates organ growth by controlling cell proliferation and cell death.

    A block diagram of the pathway can be seen at figure 1 in the above link. (A better view can be had if you download the pdf. This can be done if you examine the top right of the page returned by the link. There you will see the download link.)

    Figure 1 is a good one to study because it neatly describes what the article is about.

    Prior to 2012 researchers had gone looking for the Hippo pathway or something like it in unicellular organisms, coming up with nothing. This lead researchers to conclude that the pathway was unique to multicellular eukaryotes, the metazoa (animals like us, elephants, birds, fish and dinosaurs.).

    However, with better resources (e.g. improved technology) the authors of this paper went looking again, and this time they found what they were looking for - the Hippo pathway or pathways that were close to it. And those that were close to it were found in unicellular organisms, the very thing that had been searched for earlier.

    Figure 1b shows this very well. Look at the graph (of dots) and going from the bottom to the top you are moving from classifications of unicellular towards multicellular organisms. As you move up so you can see the Hippo or Hippo like pathway becoming more complex and additional components are added to it.

    In the paper, the researchers go on and describe what they did to see how well key components of the putative pathway in the unicellular Capsaspora owczarzaki acted in the multicellular Drosophila, demonstrating that these key components, acting together, do indeed behave as normal.

    Figure 1a shows a block diagram of the pathway on the left and the phylogeny of the organisms studied on the right. The colouring in both diagrams shows how the pathway was built up over time as unicellularity evolved into multicellularity. Red first, then green, then grey. And the phylogeny in figure 1a ties to figure 1b, diagramatically showing how the components observed at figure 1b got added to each group in figure 1a as time went by.

    While scientists know what Hippo does in organisms like us, its function in unicellular organisms is unknown. Because it regulates organ size in us, they suspect that it may regulate organism density in unicellular communities. In essence, what started out as a genetic program to control organism abundance in single celled organisms, may have been coopted to control cell abundance in multicellular animals. After all, oranism density and cell abundance are closely related concepts.

    A later paper by a different group, published in February this year, and titled "Molecular Evolution of the Yap/Yorkie Proto-Oncogene and Elucidation of its Core Transcriptional Program" takes the above research one step further and looks at the evolution of a component of this Hippo pathway - the Yorkie gene/protein seen at the bottom of the left diagram in figure 1a. (This gene can, under certain circumstances, give rise to cancer. As a key component of a tissue regulation program, if it gets out of control, so cells can proliferate without bound.)

    What is missing from these studies is I think, a detailed description of the mutational pathways by which this evolution came about. I suspect a lot more research needs to be done on these organisms, determining degrees of variation within each gene and/or protein for the various organisms making up the phylogeny. From there, the following kind of research:-

    Evolutionary Potential of a Duplicated Repressor-Operator Pair: Simulating Pathways Using Mutation Data

    - becomes plausible, establishing detailed putative pathways by which evolution can occur.

The paper which this short post is concerned with, is a broad sweep. The paper linked to just above, is very detailed. It shows step by step, how mutational changes can improve fitness and in the process lead to a disentanglement of a duplicated regulatory system. But to be able to get to this point, an awful lot of mutational and fitness data had to be first collected.

And given the complexity of these pathways, I suspect a lot of computing power would be needed to put all the mutational and fitness data together, to sort through it, looking for evolutionary pathways.

    That kind of thing is perhaps a long way off in the context of the Hippo pathway.

    Another thing is this. It seems that a lot of our complexity derives from genes and groups of genes which are to be found in unicellular organisms.

    Life took a very long time to get to anything like us, some 4 billion years after the earth’s origin before multicellular complexity really got underway. It seems as if a gene repertoire needed to be established first. Single celled organisms would have been ideal for this, given their utter abundance on earth, across all kinds of environments, their reproductive rates, and the many ways by which they throw genes around between themselves. These single celled organisms may well have been the means by which enough complexity became established such that animals like us could then begin to evolve.

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