So this is basically what I want to tell you today what physics in particular software of physics has to do with this question of the evolution of multicellularity which on the surface is a very biological question. I want to tell you how physical changes have play a strong impact in this process and finally I want to tell you why the of the changes that we do observe or observe. And so before we get into all that the evolution of multicellularity is an interesting question because it's one of these events in the history of life on Earth that truly transformed. History as we know it because when you go from being a single cell to being a bunch of cells stuck together your in the doors to all kinds of complexity that were previously impossible Furthermore when you go from being a single cell to being a bunch of cells stuck together you are necessarily probing the universe a new longer length scales and so you're probably going to encounter some forces and challenges that weren't relevant when you're a single cell finally these and these in multicellular clusters of cells stuck together which is a well known of the. Of the building blocks of the complexity of the multicellular organisms we see on Earth today they don't have any regulatory networks so it's unclear how selection acting at the group level can affect changes at the cell level that benefit the group. And so to study all these questions we use a model system and the reason for this is because obviously these transitions took place on time ago and the fossils that we have of them are weak and you can learn a thing from about dynamics and fossils and so this nobody's model system is really good for this because it's so simple you just take some big movies the kind can buy the grocery store you grow it up for a couple for about a day and then if you select it for a large size of the the a settling speed and liquid media. So you just take the bottom ten percent of these these cells that have been sitting on account of one in a centrifuge and let them grow up the next day after about a week you get a single gene mutation which prevents the cells from separating after they reproduce by budding and instead they remain together in these clusters they have a fractal like structure that we call snowflakes. And so young cluster looks like this the numbers here are the generational ages of these cells so the growth of one of these clusters goes something like this we have a base or original cell one generation it's going to have a daughter which then remains attached to it the next generation each of those will have an offspring and this process continues until you run out of space and then the cluster fractures into two or more independently viable proper ules and this is how reproduction of clusters takes place and this is what this looks like under time lapse microscopy over a number of hours of clusters is growing and fracturing and reproducing that way. And so we've done a number of experiments and unfortunately I don't have time to describe in detail that reveal that this fracture process is is a mechanical process and it happens because of spatial constraints basically when you add cells other cells are deformed out of the way and eventually the stress that's stored in the defamation of those bonds exceeds the strength of the bonds themselves and that's what causes the fracture to occur. And so that. Sets up there with this evolution question and if you continue the settling speed selection that I described before where you're just selecting for the largest clusters in the group over the course of seven weeks you get an increase of radius of about seventy percent it's pretty substantial in terms of volume it's about five time increase. And so that sets up the next part of the talk which is how does this happen because remember there's no regulatory network here because there's Kandel learn in a normal sense that it's beneficial to be large there has to be some kind of adaptation allows them to reach these larger sizes. And so to get at that the first thing we did was we measured the volume fraction within the clusters and we see that the volume fraction decreases across the seven weeks of evolution and this is important because remember that fracture is what limits the size of the clusters and so and the fracture occurs as a critical accumulation of internal stress due to cells bumping against each other so it makes sense of your volume for actually decreases you're going to have less cellular contact and therefore the energy will be stored at a slower rate. And so we wanted to know how this volume fraction decrease had occurred and then of course the solicits thoughts of maybe particle packing. And so what we did was we measured the aspect ratio of the individual cells in the cost and we see that they become more long gaited over the seven weeks of evolution. And this goes back to work that's been done before and what sort of packing is but we don't have random only packs of solids here we have this brain thing fractal structure that's dictated by the cluster growth form and so we wrote a simple simulation just stacks ellipsoids our lives oids in the same pattern that snowflake these grow and we seeded this simulation with experimentally measured aspect ratio distributions of the individual cells and so that's the only difference in the different weeks of the simulation with these aspect ratios and we see really remarkable agreement between the experiment and the simulation in this case indicating that this is purely geometric change at the cellular level is really what's driving this phenotypic difference in the costers just changing the aspect ratio of the cells is playing a role. And so to summarize what I've just told you we see this increasing in cluster size that is driven by a change at the cellular level where the cells become more wanted and this allows the cluster packing fraction to decrease which allows them to grow for more generations before they fracture because you have less cells bumping into each other. But why why not just make stronger bonds between cells or pursue any number of other possible routes to large size. It's not obvious nest are priori that this is the way to go when there are other routes to take. But it turns out that there's a geometrical reason that is cellular you on geisha is. The most beneficial and so to explore this we simulate. Did using our simulation on this previously validated we look to the increase in the number of cells per cluster as a function of aspect ratio and as we expected we see the cluster size increases as we increase the aspect ratio of the individual selves. The number of cells in the course that also increases when we increase the bond strength but it does so much more slowly and so we did a full parameter sweep of these two parameters in that data is represented in this heat map where the important. Thing to notice is that the color grading is much steeper in the vertical direction than it is in the horizontal direction as that means that for any point in this program of space if your goal is to increase the number of cells you're much better off increasing aspect ratio than you are increasing the intercellular bond strength. And this turns out to be again a packing question because as you increase the number of cells in the cost or the number the amount of internal stress increases rapidly but this happens much more slowly as the cells become more elliptical. And so the reason for this is just a simple packing question so the cells are. Placed on their parent at a specified angle of attachment and so if you imagine sweeping that angle around the cell it would define what we call a budding reign on the surface of the cell and so if we plot the linear packing fraction of five cells because that's the number that will always fit on that ring we see that the packing fraction decreases as cells become more long gaited and so there's just more space in the coster and this is what allows them to solve your sizes. So again physics plays an important role in the evolution of multicellularity because as when you have a bunch of neighbors stuck to you need to cooperate. Things change and a lot of physical principles come into play and these constraints that arise from this evolution multicellularity are mitigated in this case by an increase in cellular aspect ratio which leads to decreased packing fact in turn allows you to grow larger clusters and while we can't say for sure that this is why this happens because of all the biological complexity that's occurring below the surface we can see that from a geometric perspective it's a much much more efficient to increase your cellular aspect ratio than it is to increase the strength of the bonds between cells. And so thank you for your attention and if you have any more questions I'll be happy to take them and also these papers highlight a lot of the results that I have just silly.