The first metazoans

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In fact, in this part of the lecture, we're interested in the origin of multicellularity. In fact, the association of cells into multicellular groups represents a major transition in the history of life and the division of labor within an organism. This transition is both important and enigmatic. It has occurred on numerous occasions (algae, descendant land plants, fungi, animals, amoebae), but has not stabilized in all taxa.

Two mechanisms that may lead to multicellularity are discussed by specialists. The first consists in not separating after division. The second consists in bringing together dispersed elements. In the first case, the cells are genetically homogeneous, and heterogeneity can only arise from "accidental" modifications to the genome, such as somatic mutation or infection by a pathogen. As a result, differentiation into different cell types or groupings - a form of organogenesis - is mainly the result of epigenetic mechanisms.

In development by aggregation, as in certain ciliates, myxomycetes or myxobacteria, and in dictyostelium, the individual form predominates, with episodic association. In this system, genetic variety comes from the association of individuals with different genotypes, and if we think in terms of competition, then it will be intra-propagule and not inter-propagule as in the first case, which, according to evolutionists, is probably at the origin of most multicellular organisms.

In either case, we need to look for multicellularity genes that are already present in unicellular eukaryotes and have the dual mission of enabling such gatherings, adhesion molecules for example, and promoting intercellular communication and cellular and spatial differentiation. However, we have to wonder about the function of such "future multicellularity" genes in unicellular beings. Either these genes served another purpose and were reused in a "we gather together" or "we stay together" function, or they were already used for cellular interaction. In this case, the origin of multicellularity would lie in the interaction of two unicellulars, for example in their sexuality.

Multicellularity requires the development of positional coding for cells. Here, we have introduced the notion of positional information, and given some insight into the evolution of the genes that carry this information. On this occasion, we returned to orthology (homology through evolution) and paralogy (homology resulting from the duplication of a gene, often accompanied by the acquisition of new properties by one of the two genes resulting from this duplication). We have taken the example of Otd and Otx2, two orthologs in Drosophila (Otd) and vertebrates (Otx2), and Otx2 and Otx1, two paralogs in vertebrates.