- Final Discussion
The first question relates to many of the lecturers who have tried to explain development in a mechanistic way, whatever the definition. This was done in historical perspectives, but also related to the present time, when concepts of gene regulatory networks and epigenetic marks were included in mechanistic explanations. My question is, how far have we gotten? Will it be possible in principle, any time soon, to give a mechanistic explanation for development?
I think the way is open to go all the way at this point. And I think that the conceptual challenges are those of really understanding how to deal with the logic-processing functions that are necessary to make development happen by reading out the encoded systems, because the scale is going to be enormous. Even starting with the little piece of one embryo that we’ve been working on, it’s a challenging issue to integrate, mechanistically, the logic operations which are at the root of how that program works. It is an automaton, to use the word Ellen [Rothenberg] mentioned, and it does assess its state at each point and uses it to go to the next point. So if you extrapolate what we now know to the size of a program that would deal with all the different body parts—the program for each phase of development is going to be about equally complex, given GRN hierarchy—we have a really large-scale difficulty in keeping our minds somehow around the logic-processing system. That’s why I think that models of the sort we tried to build are a step in that direction. I think it is important. The other [End Page 172] challenges are getting enough hands and minds and dollars on the problem to solve it. But I don’t think we should worry, because we’re not missing gigantic concepts like [Nils] Roll-Hanson wants us to believe.
François Jacob used to say that when we try to anticipate the future of science, we are always wrong.
We don’t have to agree about that . . .
For the moment, clearly, there are questions asked about development that are well designed. There are no reasons to expect a dramatic change now in our mechanistic conception of the living world. But we can anticipate two future difficulties. Modeling will provide some results, but with the development of more and more precise observations, it may appear that it’s difficult, if not impossible, to model the complexity of the biological processes. The second problem is the risk to consider that one unique study will provide “the” solution to the problem of development. Epigenetics is very important, but I have the feeling that some biologists working on epigenetics have a (too) strong tendency to consider that epigenetics will be the solution of development, and forget other explanations. Mechanisms will probably be very different in different organisms—different to a degree than we do not even imagine today.
I would say, in defense of epigenetic mechanisms, that they are going to modulate these networks eventually. When you look at higher and higher resolution and more and more details, the details will count. But you have to keep in mind that the overall picture almost certainly will dominate everything, and then we will be decorating this tree forever.
I am actually not convinced that we do not need a new revolution. And that is mainly concerned with the question which we always close our eyes in front of, that is the genetics of the adult phenotype, which is not usually a Mendelian genetics. It is usually a quantitative trait genetics, and we don’t know how the genes interact with each other. There are big questions, such as is it adaptive or how much epistasis is in it. Interestingly, epistasis was not a topic at the conference; I followed it, and although it is currently the most difficult thing to understand genetically and approach experimentally, I think we are possibly facing another phase of revolution of how to approach this.
You are talking about the adult phenotype, right?
Of the adult phenotype, which is part...