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Sars International Centre for Marine Molecular Biology

Molecular Genetics of Protochordates

Chourrout Group

With the new model system Oikopleura, we try to understand how simple animals evolved from more complex ancestors.

Oikopleura diocao

We are interested in how macroevolution proceeds to create the diversity of large animal taxons. Our focus is placed on the chordate phylum, in which coexist extremely simple and complex organisms. We have contributed to establish Oikopleura dioica as a model system

Oikopleura is a pelagic tunicate with very short life cycle (6 days at 15°C), and it is bred in the lab for many successive generations. It is highly fecund and transparent at both the embryo and adult stages. Gene expression can be knocked down and knocked out. For these reasons, Oikopleura offers good opportunities for genetic and embryological studies. 

Oikopleura has an extremely compact genome, whose relatively thorough analysis has revealed divergent genome architecture and an important turn over of developmental genes. We have been interested in several features related to genome compaction, such as the very short introns and intergenic sequences, which culminate with the polycistronic transcription of numerous genes. The compaction appears as a lineage-specific secondary event, as suggested by the absence of most entire clades of ancient retrotransposons. We keep strong interest in the mechanisms driving an unusually rapid evolution, and genome changes can be historically traced using several other larvacean genomes. We found original indications in the Oikopleura genome that support two previously poorly documented mechanisms of intron gain. 

Our second and increasing interest is the evolution of tunicate late development. The Oikopleura genome sequence has been an essential piece of information for establishing the new phylogeny of chordates. Tunicates form the sister group of vertebrates, in contrast to older views, and most likely resulted from an anatomic simplification of ancestral chordates. Such simplification occurred multiples times during animal evolution and we are willing to understand how it proceeded at the molecular level, by focusing on conserved key developmental genes and their pathways. The Oikopleura Hox genes are the main focus for this part. Nine of them were found in the genome at nine distinct locations and their expression patterns suggest that the function of several of them has little to do with a role in AP axis patterning.

In parallel to development simplification, the evolution of tunicates has been accompanied by a number of innovations. The most spectacular one for larvaceans, that is definitely lineage specific, is the house, a very sophisticated extracellular structure used by the animals as a filter feeding apparatus. It is repeatedly synthesized (and replaced during the life cycle) by most epithelial cells of the trunk, that are organized in territories that are morphologically and functionally specialized. Our current activity is to identify transcription factors and pathways that are involved in the formation of this extraordinary cell layer. We already identified several candidate transcription factors and showed for two of them their direct involvement.