L'origen de la multicel·lularitat a metazous, una aproximació genòmica i funcional / The origin of metazoan multicellularity, a genomics and functional approach

Abstract

The origin of animal multicellularity is a major evolutionary transition and a poorly understood one. In recent years, the sequencing of the genomes of several earlybranching metazoans, such as sponges and cnidarians, has allowed to define a common and exclusive molecular toolkit of animal multicellularity. Most of these genes are involved in cell adhesion, cell-cell communication and control of cell proliferation and differentiation, all of them essential functions for a multicellular organism.

The access to genomic data of close unicellular relatives of animals, such as the choanoflagellates, has revolutionized the comparative genomic studies to understand the origin of animals and the molecular toolkit of the Urmetazoa (the common ancestor of all animals), showing that some genes that were considered exclusive to animals are, in fact, present in their unicellular relatives and were later co-opted to function in a multicellular context. The sequencing of the genome of the unicellular holozoan Capsaspora owczarzaki offers a new opportunity for further studying this topic. Thus, the main objectives of this thesis have been: first, to perform comparative genomic studies to reconstruct the evolutionary history of animal molecular toolkit; second, to study the functional conservation of some of the genes identified in Capsaspora owczarzaki by performing heterologous expression in model systems such as Xenopus laevis and Drosophila melanogaster; and finally, to study the life cycle and cell biology of Capsaspora owczarzaki to further understand the unicellular functional context in which this genetic toolkit of the multicellularity can act and may have evolved first.

One of the main results of our comparative genomic studies is the presence of a complete integrin adhesome in Capsaspora owczarzaki and an almost full adhesome (including integrins) in the more distantly related Thecamonas trahens. This results allowed us to reconstruct the step-wise evolutionary assembly of this key machinery of animal multicellularity. Another important result is the finding of several transcription factors (essential tools for regulating cell differentiation and cell proliferation in animals) in Capsaspora owczarzaki previously considered exclusive to animals, for example NFkappaB, Myc/Max, Mef2, T-box, Runx,... Finally, we found in Capsaspora owczarzaki homologs of the components of the Hippo signalling pathway, an essential mechanism controlling cell proliferation and organ size in animals.

Through heterologous expression studies we could study in detail the functional conservation of two of these machineries discovered in Capsaspora owczarzaki by comparative genomic studies. First, we studied the Capsaspora owczarzaki homolog of Brachyury (CoBra) using Xenopus laevis, the classical model system for studying Brachyury function. Brachyury is a T-box transcription factor involved in gastrulation and mesoderm specification in metazoans. We demonstrated that CoBra is functionally conserved and that it has similar DNA binding sites to those of animal T-box genes.

Second, we used Drosophila melanogaster to demonstrate the functional conservation of three components of Capsaspora owczarzaki's Hippo pathway. One the most striking morphological features of Capsaspora owczarzaki is the presence of long filopodia. Using this organism and the choanoflagellate Salpingoeca rosetta we studied the origin and evolution of metazoan filopodia and microvilli and the associated molecular toolkit. We found that most components are innovations of animals and these two close unicellular lineages and that the microvilli originated in the common ancestor of animals and choanoflagellates.

Finally, we studied the life cycle of Capsaspora owczarzaki using microscopy, flow cytometry and comparative transcriptomics of different life stages. We demonstrated the existence of an aggregative pluricellular stage in Capsaspora owczarzaki and that the transition between this and other stages is tightly regulated at the level of gene expression and alternative splicing. We also found evidence that the integrin adhesome is expressed in the aggregative stage, we an extracellular matrix is produced between the cells.

The study of Capsaspora owczarzaki has provided valuable insights into the origin of animal multicellularity and emphasizes the fact that gene co-option was a major mechanism to generate to multicellular molecular toolkit of animals.

El origen de la multicelularidad animal es una de las mayores transiciones evolutivas de la historia de la vida. La secuenciación, en los últimos años, de genomas de animales basales como esponjas y cnidarios ha permitido establecer la maquinaria genética común a todos los animales.

La mayoría de estos genes son aquellos involucrados en la adhesión celular, la comunicación celular y el control de la proliferación y la diferenciación. El acceso a datos genómicos de organismos unicelulares muy cercanos a los animales, como Capsaspora owczarzaki, es esencial para entender mejor esta transición. Los objetivos principales de esta tesis ha sido estudiar la presencia de genes de multicelularidad y su conservación funcional en Capsaspora owczarzaki, así como su ciclo vital.

Analizando su genoma, descubrimos una completa maquinaria de adhesión por integrinas en C.owczarzaki. Hemos podido reconstruir en detalle la historia evolutiva de este mecanismo crucial de adhesión y comunicación celular. También hemos encontrado un amplio repertorio de factores de transcripción, elementos esenciales para regular la diferenciación y la proliferación en los animales, que se creían exclusivos de animales en el genoma de C.owczarzaki; por ejemplo, genes NFkappaB, T-box o p53. Por último, hemos descrito que una importante vía de señalización en animales, la llamada vía Hippo, también está presente en C.owczarzaki. Este mecanismo es esencial para controlar la proliferación y el tamaño de los órganos en los animales.

Mediante estudios de función heteróloga en Xenopus y Drosophila demostramos la conservación funcional de los homólogos del gen Brachyury (un factor de transcripción de la clase T-box) y de la vía Hippo de C.owczarzaki.

C.owczarzaki y el coanoflagelado Salpingoeca rosetta nos sirvieron para estudiar la evolución y el origen de los filopodios y microvilli animales y su maquinaria molecular.

Por último, el estudio de la biología y el ciclo de C.owczarzaki, mediante el uso de técnicas microscopias, citometría y transcriptómica comparada de los distintos estadios vitales del organismo. Demostramos la existencia de un estadio de pluricelularidad agregativa y que la transición entre éste y otros estadios está finamente regulada a nivel de expresión génica y de splicing alternativo.

El estudio de C.owczarzaki nos ha dado valiosos ejemplos de cómo, más allá de la innovación génica, la co-opción de maquinaria pre-existente en un contexto unicelular fue un mecanismo esencial para el origen de la multicelularidad animal.