Epicycle

To function coherently, the pathways that control development in any multicellular organism must be initiated at the appropriate stage of the life cycle, but very little is known about how these two processes - life cycle progression and multicellular development - are integrated and coordinated. Epigenetic regulatory mechanisms are expected to play an important part in this process, providing a means to ensure that only the relevant genetic components are expressed at each stage of the life cycle. Recent genetic analysis of mutants of the brown alga Ectocarpus have identified a major life cycle transition regulator, encoded by the OUROBOROS (ORO) and SAMSARA (SAM) genes, that links life cycle progression to multicellular development. Both ORO and SAM are predicted to encode TALE-class homeodomain transcription factors (TALE HD TFs). Interestingly, TALE HD TFs have also been implicated in life cycle regulation in mosses, indicating that an extremely ancient life cycle regulatory system has been convergently recruited to control sporophyte development in the land plant and brown algal lineages. The objective of this project is to elucidate the genetic and epigenetic processes associated with life cycle progression and the deployment of the sporophyte developmental program in the brown alga Ectocarpus. The project will combine approaches designed to specifically dissect the regulatory pathway that includes the ORO and SAM proteins with approaches aimed at monitoring genome-wide epigenetic modifications (histone post-translational modifications and topologically associating domains detected using Hi-C) during life cycle progression. The subcellular location of the ORO and SAM proteins will be determined at different stages of the life cycle and interacting proteins will be identified and characterised. A broad range of approaches including ChIP-nexus, DAP-seq, protein-binding arrays and RNA-seq will be used to identify both the direct target genes of ORO and SAM and downstream effector genes. Additional components of this regulatory network will be detected using various genetic screens, including the isolation of oro and sam suppressor mutations. Data on the ORO/SAM regulatory pathway will be integrated with genome-wide analysis of transcriptome changes and chromatin modifications and during the life cycle to provide a comprehensive view of the events associated with the alternation of life cycle generations in this brown alga. The project will also explore the role of ORO and SAM in two situations where the sporophyte developmental program is initiated in a haploid context without gamete fusion: parthenogenetic development of gametes and meio-spore heteroblasty Epicycle will associate two highly complementary partners. The Roscoff group has a long history of brown algal research and is currently leading efforts to develop genetic and genomic approaches for this group of organisms, notably through the development of Ectocarpus as a model system and through the Phaeoexplorer project (France Genomique) which is generating complete genome sequences for 45 brown algal species. The Nantes group has extensive experience in the domain of epigenetics and has pioneered the application of epigenetic techniques to stramenopiles (diatoms). Collaboration between these two groups within the context of Epicycle will allow an innovative, interdisciplinary approach to be employed to further our understanding of life cycle regulation and progression, integrating both genetic and epigenetic factors. The results generated by Epicycle are expected to considerably improve our understanding of how developmental programs have been integrated, during evolution, into underlying life cycle processes. In a broader sense, this information is expected to contribute significantly to our understanding of the emergence of complex multicellular development, a major and highly important evolutionary transition.