by Victoria Shabardina
It is common in biology to look at little things to learn how the big parts work: we study molecules to understand cells and study organs to know how the body functions. But it is different with Capsaspora, because it is a whole, a complete single-cell organism, and it is actively studied for elucidating how a whole animal functions. Together with choanoflagellates, Capsaspora is being used to study the origin of multicellularity in animals (choanoflagellates were considered on these pages here and here.) It is important to mention "in animals" here, as multicellularity evolved several times independently in different lineages. Although the most elaborate multicellular forms are represented by animals and vascular plants, there are about 20 other examples of multicellularity, including different algal lineages, fungi, and slime molds.
Such frequency in the tree of life means that multicellularity is highly beneficial and… apparently not so difficult to evolve. Yet, we still don't understand all the laws of multicellular harmony and the mechanisms behind it. One of the strict rules for the cells within a macro-organism is to be altruistic. In other words, a cell should know when to die in order to sustain the appropriate body shape. For example, fingers are formed during the embryonic development due to the controlled death of cells in between them. It also should know how to save the organism from the troubles associated with cancer. Indeed, cancerous cells seem to forget the rules of multicellularity and behave exactly opposite to how the obedient cell of an animal should act. Thus, uncovering mechanisms of the establishment of multicellularity in animals can lead to a better understanding how we humans function, and, lastly, help cure diseases.
"Multicellular" Tools of Capsaspora
Increasingly, research groups are becoming interested in how the transition between the unicellular and multicellular lifestyles happened, leading to the discovery of more and more wondrous things. Capsaspora owczaraki, a sister group to Choanoflagellates and Animals has played an essential role in this (Figure 2). Why Capsaspora ? If we want to learn about the origin of animal multicellularity we should look into the transition moment − that is, into the unicellular ancestor of animals that triggered the transition. Since the time of this ancestor has long passed, the only way for researchers to know about the beginning of animals is to compare molecular functions and genomes of animals and their closest unicellular relatives. And Capsaspora is one of them (Figure 1).
Capsaspora is beloved in our lab. We nurture it, talk about it all the time, hang its portraits on the lab walls. No wonder: it is cute − true, it can walk on its filopodia, like, on stilts, above the surface! It can form transient multicellular structures, and it told us incredible genomic stories. The genome of Capsaspora was sequenced and annotated in 2013 by the lab of Iñaki Ruiz-Trillo and made a sensation in the field. The lab members identified in its genome some genes that, until then, were thought to be animal-specific, among them many transcription factors (TF), cell adhesion molecules (integrins, cadherins), and genes responsible for cell communication, such as tyrosine kinases. They also found that Capsaspora has a TF, called Brachyury (from the Greek βραχύς, "short" and ουρά, "tail") that in animals is essential for embryonic development and commands the establishment of anterior-posterior symmetry. Brachyury from Capsaspora proved its "animal identity" in a mesmerizing study with a frog embryo. The researchers of this study injected the mRNA of Capsaspora’s Brachyury into a frog embryo depleted of its own. The TF from Capsaspora rescued the embryo's phenotype just like the endogenous frog Brachyury did (Figure 3.) That means that the active functional site of Brachyury is conserved even in the single-cell organism and likely was so in the ancestor of animals. This finding, supported by several other studies (see, for example, here and here), tells us that the unicellular ancestor of animals was a treasure chest filled with many molecular tools that built up the basics for multicellular cooperation in the subsequent evolution of animals.
Interestingly, Capsaspora displays some of the fundamental processes that are characteristic of animal cells. In the lab, it shows three different life stages: an amoeboid stage (when it walks on the "stilts"), aggregative (when it secretes an extracellular matrix and glues cells together in groups), and cystic. These stages can be considered as temporal cell differentiations, and are reminiscent of cell differentiation in animals. Like in animals, Capsaspora’s cell states are established by genomic and proteomic regulation: differential gene expression, alternative splicing, histone modification, and differential protein phosphorylation. For example, the aggregative stage of Capsaspora (Figure 3), probably the most exciting for researchers, shows increased expression of the proteins involved in cell-cell adhesion and cell communication in animals. Proteins involved in the Hippo signaling pathway (detected so far only in animals and Capsaspora), cytoskeleton rearrangement, cell-cell and cell-ECM (extracellular matrix) contact, signal transduction, and cell communications have elevated levels of phosphorylation during the aggregative state but not in the other two stages. Differential phosphorylation of proteins suggests regulated changes in the activity of the target pathways.
The Tip of the Iceberg
Choanoflagellates and Capsaspora provide many clues and answers to the ongoing studies on the origins multicellularity quest and will always be model organisms. But it is important to not stop here. In the evolutionary research involving the reconstruction of ancestral states, we must take into consideration the taxonomic diversity of the studied organisms. Recently, different research groups started to work on other unicellular relatives of animals, such as Ichthyosporeans and Corallochytreans. At the same time, more species within filasterean and choanoflagellate groups are being incorporated into the studies. All these species have diversified and rich genomic repertoires and promise to be helpful allies in further research. By including more unicellular species, we will be able to close more knowledge gaps in the multicellularity story and discover more details and new genomic specificities of our little relatives and our single-celled ancestor.
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Victoria Shabardina is a postdoc in the Institute of Evolutionary Biology (IBE), Barcelona, Spain. She is interested in protein evolution and, in particular, in functional divergence of protein paralogs. In the lab of Iñaki Ruiz-Trillo, she applies bioinformatics and wet-lab methods to understand evolution of cytoskeletal proteins during the transition to animal multicellularity.