The sporophyte generation of the brown alga Ectocarpus sp. exhibits an unusual pattern of development compared with the majority of brown algae. The first cell division is symmetrical and the apical-basal axis is established late in development. In the immediate upright (imm) mutant, the initial cell undergoes an asymmetric division to immediately establish the apical-basal axis. We provide evidence which suggests that this phenotype corresponds to the ancestral state of the sporophyte. The IMM gene encodes a protein of unknown function that contains a repeated motif also found in the EsV-1-7 gene of the Ectocarpus virus EsV-1. Brown algae possess large families of EsV-1-7 domain genes but these genes are rare in other stramenopiles, suggesting that the expansion of this family might have been linked with the emergence of multicellular complexity. EsV-1-7 domain genes have a patchy distribution across eukaryotic supergroups and occur in several viral genomes, suggesting possible horizontal transfer during eukaryote evolution.
The authors declare no competing or financial interests.
J.M.C. and S.M.C. designed the research. N.M., A.F.P., F.L. and D.S. performed the positional cloning and characterisation of the IMM gene. D.S., S.M.C., M.S. and A.H. developed and applied the RNAi knockdown approach. A.L., M.-M.P. and J.M.C. analysed data. J.M.C. wrote the article.
This work was supported by the Centre National de la Recherche Scientifique; Agence Nationale de la Recherche (project Bi-cycle ANR-10-BLAN-1727 and project Idealg ANR-10-BTBR-04-01); Interreg Program France (Channel)-England (project Marinexus) ; Université Pierre et Marie Curie; and the European Research Council (grant agreement 638240). F.L. and M.-M.P. were supported by grants from the China Scholarship Council and the Conseil Régional de Bretagne (SAD Program), respectively.
RNA-seq data from this article are available at the NCBI Sequence Read Archive under accession number SRP037532 (see Table S8 for details).
Supplementary information available online at http://dev.biologists.org/lookup/doi/10.1242/dev.141523.supplemental
- Received June 24, 2016.
- Accepted December 12, 2016.