[ca] La transició evolutiva que va donar lloc als animals o metazous és una de les grans qüestions de la biologia evolutiva. Els animals tenen un procés de desenvolupament molt complex, en el qual molts gens controlen que una cèl•lula es converteixi en un ens multicel•lular. Aquests són els gens del desenvolupament, comuns a animals tan llunyans com els corals i els humans. Anàlisis genòmics primerencs van establir que aquests gens eren únics dels animals, ja que no es trobaven en espècies unicel•lulars. Però recents anàlisis filogenètics basats en dades moleculars han mostrat que hi ha certs llinatges de protists unicel•lulars molt estretament emparentats amb els animals. La seqüenciació del genoma del coanoflagel•lat Monosiga brevicollis, pertanyent al grup taxonòmic més proper als animals, va reportar moltes sorpreses, com la presencia de Cadherines, proteïnes claus en l'adhesió cel•lular als animals. El nostre laboratori està involucrat en la iniciativa UNICORN, on s'han seqüenciat més genomes d'espècies també emparentades als animals. Entre aquestes, hi destaca l'ameba Capsaspora owczarzaki, membre dels filasteris, que filogenèticament cau com a grup germà de coanoflagel•lats i animals. El genoma d'aquesta espècie ens ha aportat moltes dades noves sobre l'evolució dels gens del desenvolupament. Gens involucrats en l'adhesió cel•lular, com son les MAGUK o les Integrines es troben al genoma d'aquesta espècie. També hem analitzat els gens importants en el procés de diferenciació cel•lular, ja que aquest és bàsic per a la morfogènesis i la formació de tipus cel•lulars específics de teixits. Entre aquests, els factors de transcripció en son un grup clau. El nostre anàlisis a C. owczarzaki ens ha revelat que molts dels factors de transcripció que es creien únics d'animals de fet tenen orígens molt més antics, ja presents als ancestres unicel•lulars. També hem trobat vies de senyalització claus del desenvolupament presents en el genomes dels holozous unicel•lulars, com serien les tirosina cinases i els receptors associats a proteïnes G (GPCR). Tot plegat hem descobert que la pre-història animal va ser molt més complexa de l'esperat a nivell de contingut genòmic, i que la transició a la multicelul•lartiat va implicar la co-opció de moltes d'aquestes eines ancestrals en noves funcions associades al desenvolupament i no només guany de gens nous.

[en] The origin of multicellular organisms from unicellular species is one of the major transitions of the history of life. Although there have been many independent transitions to multicellularity, only some groups have become surprisingly diverse and complex. Metazoans are one of the most complex multicellular groups, and present an intricate process of embryogenesis, where they develop from a single cell to an adult composed by many different organs and cell types. The genes that govern embryogenesis, the developmental genes, were thought to be exclusive to metazoans, the very core of their multicellular lifestyle. Nevertheless, recent molecular phylogenies have situated some protistan groups sister group to metazoans, allowing to address the question on the evolutionary origins of the developmental toolkit. To reconstruct the evolutionary origin of the developmental toolkit, the genome sequences of diverse unicellular relatives are essential. However, only the genome of the choanoflagellate Monosiga brevicollis had been reported to date. In this thesis my colleagues and me have analysed the complete sequence of the genome of the filasterean Capsaspora owczarzaki, the closest known unicellular relative of metazoans besides choanoflagellates. Analyses of this genome alter our understanding of the molecular complexity of metazoans' unicellular ancestors showing that they had a richer repertoire of proteins than what was thought before. For example MAGUKs, proteins involved in cell-adhesion and signalling scaffolding, evolved prior to the origin of metazoans, as well as many other adhesion proteins, such as Cadherins or Integrins. Not only proteins involved in adhesion where present, also cell differentiation is a very basic process for multicellular metazoans, as it allows differential gene expression and diverse cell behaviours in different cell types with the same genotype. Major regulators in this process are Transcription Factors; most of them key genes in developmental biology. Surveying the genome of C. owczarzaki and other holozoans (unicellular relatives of metazoans) shows that many of the metazoan transcription factors have ancient origins, as they are present and active in today's living protists. In a broader perspective, we have seen that the assembly of the regulatory toolkit in both plants and animals is very similar. Both lineages are the richest in number and diversity of transcription factors among eukaryotes, and both repertoires have evolved by a step-wise process, having a first step of diversification in the unicellular relatives and a second one at the root of the respective lineages. Finally, cell-cell communication is also one of the major hallmarks of multicellularity and development. Here we present our results in two major signalling pathways, Receptor Tyrosine Kinases and G Protein Coupled Receptors. We demonstrate that both receptors where diverse and precede metazoans. Though the receptor types are there, no orthologs can be found between metazoans and unicellular holozoans. Contrastingly, most of the cytoplasmic machinery involved in the signal transduction is widely conserved. This may mean that signalling systems involved in sensing the environment where co-opted into multicellular functions, internalizing the ligand-signal relationships and forming the major signalling pathways of development. In a genome-wide perspective, we also show that pre-metazoan genomes have many pre-adaptations regarding composition of protein domains. Therefore we propose that the co-option of pre-existing genes into new and more complex regulatory networks had a major role in the evolutionary transition from unicellular protists to metazoans.